The present invention relates to an openable panel unit for a structure, such as a building fagade, that separates two air volumes, and to a method for attenuating sound when air flows between a first air volume and second air volume on opening such a panel unit. The openable panel unit may be used for ventilation, particularly natural ventilation, between two internal areas of a building or between the interior of a building and the outside environment. The openable panel unit is preferably a window.

Housing demand means that many homes are built close to sources of noise such as major roads, railways and airports. For buildings in such urban areas, opening windows to effect natural ventilation also increases noise levels and these noise levels can rise to an unacceptable level. It is therefore common for windows not be opened and for mechanical ventilation to be relied on instead.

By way of example, in a city apartment block, the levels of external noise mean that it is not desirable to open any windows. Since natural ventilation is not possible, overheating of the apartments can result. A lack of natural ventilation can be detrimental to health and comfort, and mechanical ventilation systems are expensive, with high energy requirements: they also require space to

accommodate them.

Attempts have been made in the past to design noise-control systems for naturally ventilated buildings. A known method of attenuating sound when naturally ventilating a building is to insert an acoustic louvre having angled blades into a ventilation opening. Another method is to use quarter wave resonators around the edges of a ventilation opening. Lined air vents are also known, such as those disclosed in WO2011/027123 A1. Active noise cancellation is also possible using, for example, noise cancellation speakers.

WO 2016/170274 A1 discloses a sliding window for closing off an open surface in a wall separating the inside from the outside, and comprising an opening panel and a frame. It comprises a passive acoustic silencer comprising two parallel and acoustically absorbent slides, extending in a median plane normal to the open surface, one slide being attached to the opening panel and the other being attached to the frame, the movement of the opening panel relative to the frame creating between the said slides a slit having a thickness equal to the window opening. An active acoustic damping system comprising a speaker placed in one of the slides and a microphone is also disclosed.

WO 98/40598 discloses a frame and casement where the casement adopts three positions relative to the frame: the first position is a closed position; the second position is a separated position where the casement slides outwardly from the frame to form a ventilation duct around the perimeter of the casement; the third position is an open position where the casement hinges away from the frame.

The problem of noise control for natural ventilation openings is difficult to solve because, to obtain a useful airflow for effective ventilation, a relatively large area of a building’s fagade (for example) needs to be opened up and this decreases the sound insulation provided by the fagade.

Another problem is architectural. Louvres, ventilation openings and projecting acoustic absorbers have a dramatic impact on the appearance of a building and are difficult to incorporate into standard fagade systems. Also, since they are much deeper than a typical fagade, they must be incorporated into bulk heads or other architectural features.

The present invention seeks to address the problem of noise ingress by providing a panel unit for a structure, such as a building fagade, that separates two air volumes, and that is openable to provide ventilation while attenuating sound.

The present invention also seeks to provide an openable panel unit which can be manufactured in a cost effective and straightforward manner (eg using extrudable frame and panel profiles), and which can provide a facade that is weather tight and that is readily operable by user or a building management system. Architectural aesthetics, ventilation (eg to prevent overheating) and acoustics have all been considerations of the inventor of the openable panel unit of the present invention.

The present invention therefore provides the openable panel unit for a structure of independent claim 1 and the method of independent claim 15. The dependent claims specify preferred but optional features.

The present invention provides an openable panel unit for a structure which separates two air volumes, the panel unit comprising a frame and a panel mounted for movement relative to the frame, the panel being movable into an open position to define an air path for ventilation between a first air volume and a second air volume, the frame comprising at least one first sound-attenuating member and the panel comprising at least one second sound-attenuating member; wherein the first sound-attenuating member is mounted on an outer surface of the frame to provide an externally-exposed, sound-absorbing surface; and the second sound- attenuating member is mounted on an outer surface of the panel to provide an externally-exposed, sound-absorbing surface; wherein the first sound-attenuating member has its externally-exposed, sound-absorbing surface provided in at least two planes for attenuating sound in the air path; and the second sound-attenuating member has its externally-exposed, sound-absorbing surface provided in at least two planes for attenuating sound in the air path; wherein the externally-exposed, sound-absorbing surface of the first sound-attenuating member comprises a primary sound-absorbing surface and a secondary sound-absorbing surface, these primary and secondary sound-absorbing surfaces lying in different planes; and the externally-exposed, sound-absorbing surface of the second sound-attenuating member comprises a primary sound-absorbing surface and a secondary sound- absorbing surface, these primary and secondary sound-absorbing surfaces lying in different planes; wherein the frame has a first sound-attenuating depth of at least 30mm provided by at least the thickness of the first sound-attenuating member measured in a direction perpendicular to its primary sound-absorbing surface and has a second sound-attenuating depth of at least 30mm provided by at least the thickness of the first sound-attenuating member measured in a direction

perpendicular to its secondary sound-absorbing surface; and wherein the panel has a first sound-attenuating depth of at least 30mm provided by at least the thickness of the second sound-attenuating member measured in a direction perpendicular to its primary sound-absorbing surface and has a second sound- attenuating depth of at least 30mm provided by at least the thickness of the second sound-attenuating member measured in a direction perpendicular to its secondary sound-absorbing surface. By‘sound-attenuating depth’ is meant the overall depth of a system provided in the panel or the frame that allows for the transmission and attenuation of sound. It is measured in a direction which is perpendicular to the relevant sound-absorbing surface of the sound-attenuating member of the panel or the frame, as appropriate.

In a preferred embodiment, the first sound-attenuating member comprises a sound-absorbing porous material which is located adjacent to the externally- exposed, sound-absorbing surface of the first sound-attenuating member; and the second sound-attenuating member comprises a sound-absorbing porous material which is located adjacent to the externally-exposed, sound-absorbing surface of the second sound-attenuating member.

In one embodiment, the first and/or the second sound-attenuating depths of the frame and/or the panel are further provided by at least one air cavity located in a direction perpendicular to the corresponding sound-absorbent surface; and wherein the air cavity is spaced from the corresponding sound-absorbing surface by at least the thickness of the porous material in a direction perpendicular to the corresponding sound-absorbent surface.

The frame and/or the panel may comprise a profile on which the

corresponding sound-attenuating member is mounted, the air cavity is contained within the profile, and perforations are provided in a region of the profile located between the porous material and the air cavity.

In one embodiment, a sound-attenuating depth of at least 30mm is provided by the combined thicknesses of the relevant sound-absorbing surface and the porous material measured in the direction perpendicular to the sound-absorbing surface.

In another embodiment, a sound-attenuating depth of at least 30mm is provided by the combined thicknesses of the relevant sound-absorbing surface, the porous material and the air cavity contained within the profile measured in the direction perpendicular to the sound-absorbing surface.

In the absence of an air cavity contributing to the sound-attenuating depth of the frame and/or the panel, the porous material preferably has an approximate thickness of at least 30mm and more preferably an approximate thickness of 30mm to 50mm.

When an air cavity contained within in the profile contributes to the sound- attenuating depth of the frame and/or the panel, the porous material preferably has an approximate thickness of at least 20mm (for example) and preferably an approximate thickness of 20mm to 40mm and the air cavity has an approximate thickness of at least 10mm (for example) and preferably an approximate thickness of 10mm to 30mm.

The present invention is able to provide a panel unit which attenuates sound in an open position when the first and second sound-attenuating members are distanced from one another to provide a minimum distance of 80 to 120 mm between the frame and the panel: at this distance, an improvement in sound attenuation of from 5 to 12 dB(A) is achievable in the air path whilst providing good ventilation (the improvement in sound attenuation being compared to an identical panel unit but with the first and second sound-attenuating members removed).

The panel unit is preferably sized so that in a room provided with one or more openable panel units of the present invention, a total opening area of the panel units defining the air path is 5% of the internal floor area of the room.

Hence, if the internal floor area of the room is 10 to 12 m ² , the opening area of a single panel unit is 0.5 to 0.6 m ² .

The panel unit may have height of between 0.5 m and 2.5 m and a width of between 0.5 m and 2.5 m.

In one embodiment, the panel unit has a height that is approximately three times greater than its width. For example the panel unit may be 200 cm high and 70 cm wide.

When an air cavity is contained within the profile of the frame and/or panel, and perforations are provided in a region of the profile located between the porous material and the air cavity, the number and/or size of the perforations is balanced to ensure there is minimal loss of structural integrity of the profile while providing the desired noise attenuation.

Utilising the air cavity contained within the profile of the frame and/or panel realises an extra depth for sound attenuation, particularly at lower frequencies. No porous material or other sound-absorbing material, other than air, is used to fill the air cavity.

In one embodiment, each sound-attenuating member has an externally- exposed, sound-absorbing surface provided in two or three planes for attenuating sound in the air path.

By‘externally-exposed’ is meant that these sound-absorbing surfaces of the sound-attenuating members come into direct contact with the air path, and therefore they come into direct contact with sound waves carried by air in the air path.

The resulting air path is therefore defined, at least in part, by the externally- exposed surface of the first sound-attenuating member and the externally-exposed surface of the second sound-attenuating member.

Preferably, each sound-attenuating member has an externally-exposed surface consisting of two or three sound-absorbing faces: these faces are offset from one another so that they are not in alignment and each face is generally or substantially planar.

The present invention allows passive sound attenuation. In a preferred embodiment, no active sound or noise attenuation is used.

Whilst the openable panel unit may be used in a building fagade, it may also be used for an internal wall, for example, to ventilate adjacent internal spaces whilst providing sound attenuation. In such embodiments, the panel unit is typically configured for vertical, or substantially vertical, use. The panel unit may also be used in a roof or a ceiling. There is therefore no limitation on the orientation of the panel unit in use.

The panel is movable into an open position relative to the frame to define an air path: the panel may be mounted on the frame or it may be mounted on an adjacent sill or structure (eg a wall or a roof), for example. The frame, in one embodiment, is adapted to support the panel which is moveably mounted thereon.

The frame defines at least one aperture which is covered or closed by the panel. Internal frame sections may be present, to divide the main aperture defined by the frame into a plurality of apertures, each being covered by the same or a different panel. For example, the frame may support an upper panel and a lower panel which is positioned below the upper panel and lies in the same plane as the upper panel.

The frame preferably has a substantially square or rectangular shape. With the frame oriented in a vertical position (by way of example), any internal frame sections present are preferably aligned either horizontally or vertically within the frame.

The panel is preferably of a similar shape and size to the shape and size of the corresponding aperture of the frame.

The panel may comprise one or more glazing units and/or one or more opaque sections, depending on the selected design.

In embodiments where the frame supports a plurality of planar panels, the individual panels form a composite panel when the panel unit is closed against ventilation. References to a‘panel’ in this specification encompass both an individual panel and a composite panel.

When the panel unit is mounted in a structure that separates an internal and an external air volume, such as the fagade of a building, it has a face directed towards the inside (an inner face) and a face directed towards the outside (an outer face). The panel may be mounted on the outer face of the panel unit, but may alternatively be mounted on the inner face of the panel unit. In one embodiment, the panel is mounted on the outer face of the panel unit in a parallel-opening design. In another embodiment, the panel is mounted on the inner face of the panel unit in a tilt and turn design. In further embodiment, the panel is mounted on the outer face of the panel unit in a sliding design or in a hung design. It is noted that the outer face of the panel unit is normally required to be weather proof and watertight when mounted in the fagade of a building or the roof of a building.

The panel may be in the form of a planar opaque fagade surface unit, possibly matching adjacent cladding units or panel units of the fagade. The panel may have its own frame structure, in addition to the frame of the panel unit. In one embodiment, the panel is a window capable of being opened and shut, preferably comprising a sash supporting one or more glazing units.

The panel is moveably mounted on the frame in one embodiment. For example, it may be mounted on the frame using one or more hinges in a hung design. In another example, it may be mounted on at least two opposing edges of the frame such that, in its open position, the panel is substantially parallel to the frame: the edges of the panel are preferably spaced from the corresponding edges of the frame by an approximately equal distance. As a result, the panel may be designed to open in parallel from the plane defined by the frame.

The panel may open inwards or outwards with respect to the frame to create a ventilation opening. In addition or alternatively, the panel may slide or tilt with respect to the frame to create a ventilation opening.

The panel may therefore be mounted to slide with respect to the frame or to be hinged along one edge of the frame to open inwardly or outwardly. The panel may be designed to tilt with respect to the frame, with the panel being adapted to open inwardly or outwardly, for example in a tilt and turn design.

When the panel is moved into an open position relative to the frame it defines an air path for ventilation between adjacent air volumes: these air volumes may both be internal airspaces or they may be an external and an internal airspace. The air may be driven from a first air volume to a second air volume through the ventilation opening by buoyancy, by cross-ventilation or by mechanical means, for example.

Whilst the present invention is designed for passive natural ventilation, it may be used in combination with an active (eg mechanical) ventilation system to increase airflow: for example, a fan unit may be used inside a building to draw in outside air through the ventilation opening. It may be desirable to use both passive and active ventilation systems due to a difference between the external

temperature and the desired internal temperature, or because of a high level of external or internal pollutants.

The present invention therefore provides a passive noise reduction system. This is in contrast to the active noise reduction systems used in the prior art, where electric or mechanical noise reduction apparatus is necessary.

In one embodiment, the panel opens substantially in parallel from the frame such that a ventilation opening is created, and an air path is defined, around the majority of, or all of, the edges of the panel: the panel therefore‘pops-out’ from the frame. This achieves good natural air ventilation. However a balance has to be reached between increasing the airflow rates and minimising the incoming noise levels. Thus the panel may be spaced from the frame by a relatively small minimum distance, for example from 80 to 120 mm. For a panel area of 0.5 m in width and 2 m in height, this results in a maximum free air path around the perimeter of the panel of 0.600 m ² and a perimeter distance measuring 5 m (by way of example only).

In pop-out type panel units, according to the present invention, the panel is movable into an open position such that it lies in a plane which is as close to parallel as possible to the frame, given the panel opening trajectory, which may be dictated by the panel mount mechanism.

Since an openable panel unit is provided by the present invention, it is not necessary to provide a separate air vent in the panel unit. Moreover, no integral barrier or screen is necessary, for noise or ventilation reasons, across the ventilation opening that is created when the panel unit is opened. Screens and/or filters may be necessary for other reasons, for example to prevent incoming particulates or to prevent out-falling objects.

The frame and the panel each comprise at least one sound-attenuating member to absorb sound and to reduce noise levels within the ventilated space. These sound-attenuating members are mounted to the frame and the panel to provide externally-exposed sound-absorbing surfaces. This means that the sound- attenuating members are provided, at least in part, on an outer surface of the frame and on an outer surface of the panel, meaning that the sound-attenuating members are not solely provided within an internal cavity of the frame or the panel as this would not allow any of their surface to be externally-exposed. Thus the present invention does not provide for the air path to pass through an isolatable or internal sound-absorbing chamber.

The term‘outer surface’ in the context of the frame and the panel refers to the structure of the frame and of the panel and not to the orientation of these surfaces relative to the outside or inside of a building, for example. Therefore the term‘externally-exposed’ does not mean that the sound-absorbing surface has to be located on the outside of a structure: an externally-exposed, sound-absorbing surface can be on the inside or outside of a building, for example. In a preferred embodiment, the frame and/or the panel have their structures formed at least in part by one or more profiles (which may be extruded profiles) and the sound-attenuating members are mounted on the profile(s) of the frame and/or the panel to provide externally-exposed sound-absorbing surfaces. This means that the sound-attenuating members are provided, at least in part, on an outer surface of the frame profile(s) and on an outer surface of the panel profile(s), meaning that the sound-attenuating members are not solely provided within an internal cavity of the frame profile(s) or the panel profile(s) as this would not allow any of their surface to be externally-exposed.

When the panel is moved into an open position to create a ventilation opening, it defines an air path. The air path passes around at least some of the edges of the panel in its open position. The air path passes between the panel and the frame. The air path is defined, at least in part, by some or all of the externally- exposed surfaces of the first and second sound-attenuating members. No sound- absorbing cavity is provided along the air path. Moreover, the air path does not form a ventilation duct or trickle vent.

To ensure an adequate flow of air, the present invention excludes additional panels being mounted in parallel to the claimed panel. Therefore, the air path does not travel between two panels (for example, the air path does not travel between two openable windows). The panel unit of the present invention is preferably a single window or a single fagade surface unit.

Whilst the presence of two panels positioned in parallel, supported by a frame, is not part of the present invention, the present invention does encompass a window of the panel being a double or triple glazed unit, for example.

In one embodiment, the panel unit consists of the frame and the panel. The panel may include a frame structure.

When the panel unit is mounted in a building fagade, the panel preferably presents an exterior surface to the fagade such that if forms part of the fagade: no interior panel (for example, a window) is additionally present to provide an interior surface.

At least a portion of the externally-exposed, sound-absorbing surface of the first sound-attenuating member may be positioned to substantially oppose at least a portion of the externally-exposed, sound-absorbing surface of the second sound- attenuating member, at least when the panel unit is open. Thus the air path passes between opposing sound-attenuating members to achieve good levels of sound reduction.

These opposing externally-exposed surfaces of the sound-attenuating members preferably abut one another, at least in part, when the panel unit is closed against ventilation: however this depends on factors such as the design of the panel unit, how the panel is mounted relative to the frame and how the sound- attenuating members are mounted on or fitted into the profile of the frame and/or panel.

The externally-exposed, sound-absorbing surface of the first sound- attenuating member has a primary sound-absorbing surface and a secondary sound-absorbing surface which lie in different planes; and the externally-exposed, sound-absorbing surface of the second sound-attenuating member has a primary sound-absorbing surface and a secondary sound-absorbing surface which lie in different planes.

The terms‘primary’ and‘secondary’ do not indicate the relative importance of the surfaces in terms of the effectiveness of sound-absorption or in terms of any other effect.

Each sound-attenuating member has an externally-exposed, sound- absorbing surface provided in at least two planes, meaning that the primary and secondary sound-absorbing surfaces are each generally or substantially planar.

In one embodiment, the primary sound-absorbing surface of the first sound- attenuating member lies in a plane which is generally or substantially parallel to a plane defined by the frame, at least when the panel unit is open and preferably when the panel unit is open and closed. In the same or a different embodiment, the primary sound-absorbing surface of the second sound-attenuating member lies in a plane which is generally or substantially parallel to a plane defined by the frame, at least when the panel unit is open and preferably when the panel unit is open and closed. These primary sound-absorbing surfaces therefore extend in a direction which is generally parallel to the building fagade or other surface structure which has the panel unit therein.

Preferably, the primary sound-absorbing surface of the first sound- attenuating member lies in a plane which generally faces in the direction of the first air volume, and the primary sound-absorbing surface of the second sound- attenuating member lies in a plane which generally faces in the direction of the second air volume, at least when the panel unit is open and preferably when the panel unit is open and closed.

The primary sound-absorbing surface of the first sound-attenuating member may lie in a plane which is generally or substantially parallel to the plane of the primary sound-absorbing surface of the second sound-attenuating member, at least when the panel unit is open and preferably when the panel unit is open and closed.

In one embodiment, the first and second sound-attenuating members are positioned such that, when the panel is in an open position relative to the frame, the resulting air path is provided with at least one bend which may be a dog-leg.

By a‘dog-leg’ is meant that the air path bends relatively sharply. In comparison, a substantially straight air path from a first air volume (eg externally of a building) to a second air volume (eg internally of a building) does not have a bend. In a preferred embodiment, the first and second sound-attenuating members are positioned such that, when the panel is in an open position relative to the frame, the resulting air path is approximately S-shaped (the air path may comprise more than one S-shape).

Preferably, the first and second sound-attenuating members are positioned such that, when the panel is in an open position relative to the frame, the resulting air path does not follow a reverse direction at any point as this is not thermally efficient. Hence the air path is not substantially n-shaped, u-shaped, v-shaped, w- shaped or o-shaped at any point. Although the air path created by the open panel unit of the present invention may be approximately S-shaped (for example), the air path does not follow a reverse direction at any point: the air path is loosely S- shaped in this example. In the same or other embodiments, the first and second sound-attenuating members are positioned to create a baffle when the panel is in an open position relative to the frame. A baffle in this context is an arrangement of sound- attenuating members that restrains or re-directs the flow of air in the air path: this prevents the spread of air and sound in a particular direction.

The panel unit separates a first air volume from a second air volume (eg in a building fagade). When the panel opens towards the second air volume with respect to the frame, the second sound-attenuating member is preferably positioned on a region of the panel facing the first air volume, with the first sound- attenuating member on the frame substantially opposing at least a portion of the second sound-attenuating member, at least when the panel unit is open. At least part of the first sound-attenuating member is preferably positioned on a region of the frame facing the second air volume.

The second sound-attenuating member or members may substantially cover a face of the panel or they may cover a part of the face of the panel (without covering any window glazing units, for example).

Irrespective of the shape of the second sound-attenuating member, at least part of it is preferably positioned on an edge region or regions of the panel. In one embodiment, substantially all of the edge region of the panel has the second sound-attenuating member(s) positioned thereon. Similarly, at least part of the first sound-attenuating member is preferably positioned on an edge region or regions of the frame. In one embodiment, substantially all of the edge region of the frame has the first sound-attenuating member(s) positioned thereon.

One or more first sound-attenuating members are preferably positioned around the edge region(s) of the frame to provide a sound-absorbing volume.

Preferably they are positioned continuously around the edge region(s) of the frame such that either a single continuous sound-attenuating member is provided or a series of adjacent sound-attenuating members are provided, preferably abutting one another to provide a continuous sound-absorbing volume. The single continuous sound-attenuating member or the series of adjacent (and optionally abutting) sound-attenuating members may take the shape of a rectangular or square frame, although the present invention is not limited to such a shape. One or more second sound-attenuating members are preferably positioned around the edge region(s) of the panel to provide a sound-absorbing volume.

Preferably they are positioned continuously around the edge region(s) of the panel such that either a single continuous sound-attenuating member is provided or a series of adjacent sound-attenuating members are provided, preferably abutting one another to provide a continuous sound-absorbing volume. The single continuous sound-attenuating member or the series of adjacent (and optionally abutting) sound-attenuating members may take the shape of a rectangular or square frame, although the present invention is not limited to such a shape.

When a series of adjacent (and optionally abutting) sound-attenuating members is used, each sound-attenuating member is preferably elongate in shape or may comprise a series of sound-attenuating member units to form an elongate shape. In one embodiment, the longest dimension of the sound-attenuating member generally extends along the length of the corresponding edge region of the frame or the panel, as appropriate. Each sound-attenuating member preferably has a cross-section which is substantially polygonal in shape, the cross-section being across the width of the member in a direction which is perpendicular to the plane defined by the frame or the panel, as appropriate. This width-wise cross- sectional direction is the cross-sectional direction mentioned below in terms of the cross-sectional shape of a sound-attenuating member, even when the sound- attenuating member is not elongate, in the sense that its longest dimension does not extend along the length of the relevant edge region of the frame or the panel, as appropriate.

Adjacent sound-attenuating members may lie along a common axis (for example, to define an edge of a frame shape) or may lie along axes which are perpendicular to one another (for example, to define the corner of a frame shape). Alternatively, adjacent sound-attenuating members may lie parallel to one another (for example, to define an inner surface and an edge of a frame respectively).

The first and/or second sound-attenuating members may have a shape which is a quadrilateral in cross-section (the cross-section being in a direction which is perpendicular to the plane defined by the frame or the panel, as

appropriate). This quadrilateral shape may be a parallelogram, or a rectangle. Such sound-attenuating members with a quadrilateral cross-sectional shape preferably consist of two adjacent surfaces which are non-sound-absorbing and meet at a corner and may define an approximate L-shape, and two adjacent surfaces which are sound-absorbing and meet at a corner and may define an approximate L-shape. A non-sound absorbing surface and a sound-absorbing surface meet at the other two corners. The externally-exposed, sound absorbing surface of one or both sound-attenuating members may extend beyond the quadrilateral shape to define an approximate Z-shape in cross-section (for example): in this respect, two sound-absorbing surfaces meet at one corner and two sound-absorbing surfaces meet at another corner, with one sound-absorbing surface being common to both of these corners.

The first and second sound-attenuating member or members are therefore not planar sheets. However planar sheets may be used to provide one or more surfaces (faces) of the sound-attenuating members.

The sound-attenuating members each define a sound-absorbing volume.

By way of example, this volume may have the shape of a cube, a rectangular prism, a triangular prism, a pyramid or a combination thereof. The volume may be regular, irregular or semi-regular in shape. A suitable combination may be two adjacent rectangular prisms having their respective longitudinal axes arranged perpendicularly to one another, to provide an approximate L-shaped member when viewed in cross-section.

In a preferred embodiment, the first sound-attenuating member defines a sound-absorbing volume in the shape of at least two adjacent rectangular prisms having their respective longitudinal axes arranged perpendicularly to one another, to provide an approximate L-shaped or Z-shaped member when viewed in cross- section.

In a preferred embodiment, the second sound-attenuating member defines a sound-absorbing volume in the shape of a cube or a rectangular prism.

These preferred embodiments may relate to pop-out design panel units, sliding design panel units or hung design panel units (including tilt and turn designs). In an embodiment of the openable panel unit of the present invention where the panel is movable into an open position such that it lies in a plane which is substantially parallel to a plane defined by the frame, the second sound-attenuating member defines a sound-absorbing volume in the shape of a cube or a rectangular prism and the first sound-attenuating member defines a sound-absorbing volume in the shape of three adjacent rectangular prisms having their respective longitudinal axes arranged perpendicularly to one another, to provide an approximate Z-shaped member when viewed in cross-section. This embodiment may also be used for a hung window design.

In an embodiment of the openable panel unit of the present invention where the panel is movable into an open position such that it lies in substantially the same plane as the plane defined by the frame, the second sound-attenuating member defines a sound-absorbing volume in the shape of a cube or a rectangular prism and the first sound-attenuating member defines a sound-absorbing volume in the shape of two adjacent rectangular prisms having their respective longitudinal axes arranged perpendicularly to one another, to provide an approximate L-shaped member when viewed in cross-section. The frame may also have an additional sound-attenuating member defining a sound-absorbing volume in the shape of a cube or a rectangular prism. This embodiment may also be used for a hung window design.

Within the boundary of the sound-absorbing volume of the sound- attenuating member, an air cavity may be provided to increase the level of sound- absorption.

More than one sound-attenuating member may be provided on the panel and/or the frame. When more than one-sound attenuating member is provided on the panel and/or the frame, the other sound-attenuating member(s) may have an externally-exposed sound-absorbing surface provided in a single plane only or provided in at least two planes (for example, two or three planes).

In one embodiment, which may be used for a panel unit where the panel opens substantially in parallel from the frame, the frame has a first sound- attenuating member which has an externally-exposed sound-absorbing surface provided in at least three planes and that may have an approximate Z-shape when viewed in cross-section. The panel may have a second sound-attenuating member which has an externally-exposed sound-absorbing surface provided in at least two planes and may have a quadrilateral shape when viewed in cross-section.

In another embodiment, which may be used for a panel unit where the panel opens by sliding to lie in substantially the same plane as the plane defined by the frame, the frame has a first sound-attenuating member which has an externally- exposed sound-absorbing surface provided in at least three planes and that may have an approximate L-shape when viewed in cross-section. The panel has a second sound-attenuating member which has an externally-exposed sound- absorbing surface provided in at least two planes and may have a quadrilateral shape when viewed in cross-section. The frame may be provided with an additional sound-attenuating member which has an externally-exposed sound- absorbing surface provided in at least one plane and may have a quadrilateral shape when viewed in cross-section. The panel may comprise two sashes that are mounted to slide towards each other and thus allow ventilation on both sides of the frame.

In one embodiment which is applicable to all designs of panel unit (eg parallel-opening, sliding and hung), the sound-attenuating member of the frame and/or the panel comprise a sound-absorbing porous material; the frame and/or the panel comprises a profile on which the corresponding sound-attenuating member is mounted; an air cavity is contained within the profile; and perforations are provided in a region of the profile located between the porous material and the air cavity.

The externally-exposed, sound-absorbing surface of the first and/or the second sound-attenuating members preferably comprise primary and secondary sound-absorbing surfaces to define at least part of the sound-absorbing volume.

The air path is preferably defined, at least in part, by the primary and secondary surfaces of the first and second sound-attenuating members.

In one embodiment which is applicable to all designs of panel unit (eg parallel-opening, sliding and hung), the ratio of the length of the secondary sound- absorbing surface to the length of the primary sound-absorbing surface of the second sound-attenuating member on the panel is greater than or equal to 0.5 and less than or equal to 2.0, when viewed in cross-section across the width of the second sound-attenuating member in a direction which is perpendicular to the plane defined by the panel. In this embodiment, the second sound-attenuating member is preferably a rectangle when viewed in cross-section across the width of the second sound-attenuating member in a direction which is perpendicular to the plane defined by the panel. Preferably the first sound-attenuating member on the frame has the same or a similar ratio between the length of the secondary sound- absorbing surface and the length of the primary sound-absorbing surface when viewed in cross-section across the width of the first sound-attenuating member in a direction which is perpendicular to the plane defined by the frame.

The angle between the plane of the primary sound-absorbing surface and the plane of the secondary sound-absorbing surface of the first sound-attenuating member may be 50 to 130 degrees, preferably 60 to 120 degrees and more preferably 60 to 1 10 degrees. The angle between the plane of the primary sound- absorbing surface and the plane of the secondary sound-absorbing surface of the second sound-attenuating member may be 50 to 130 degrees, preferably 60 to 120 degrees and more preferably 60 to 1 10 degrees. In one embodiment, both angles are substantially 90 degrees. In another embodiment, both angles are about 70 degrees. In yet another embodiment, both angles are about 105 degrees.

The primary sound-absorbing surface of the first sound-attenuating member may have a greater surface area than that of its secondary sound-absorbing surface; or the primary sound-absorbing surface of the first sound-attenuating member may have substantially the same surface area as that of its secondary sound-absorbing surface; and/or the primary sound-absorbing surface of the second sound-attenuating member may have a greater surface area than that of its secondary sound-absorbing surface; or the primary sound-absorbing surface of the second sound-attenuating member may have substantially the same surface area as that of its secondary sound-absorbing surface.

Alternatively, the surface areas of the respective primary sound-absorbing surfaces may be smaller than the surface areas of the respective secondary sound-absorbing surfaces. Moreover, in one embodiment, the primary sound- absorbing surface of the first sound-attenuating member has a smaller surface area than the surface area of its secondary sound-absorbing surface, whilst the primary sound-absorbing surface of the second sound-attenuating member has a greater surface area than the surface area of its secondary sound-absorbing surfaces. The opposite may also be true such that the primary sound-absorbing surface of the first sound-attenuating member has a greater surface area than the surface area of its secondary sound-absorbing surface, whilst the primary sound- absorbing surface of the second sound-attenuating member has a smaller surface area than the surface area of its secondary sound-absorbing surfaces.

The relative surface areas of the primary and secondary sound-absorbing surfaces depend on the design of the panel unit and the desired levels of sound attenuation.

In one embodiment, the secondary sound-absorbing surface of the externally-exposed surface of the first sound-attenuating member(s) is adjacent to the aperture defined by the frame: it therefore defines part of the air path when the ventilation opening is created.

In the same or a different embodiment, the secondary sound-absorbing surface of the externally-exposed surface of the second sound-attenuating member(s) is located generally at the edge of the panel: it therefore defines part of the air path when the ventilation opening is created.

In one embodiment, since the sound-attenuating members on the frame and the panel are arranged to have a frame shape, their primary surfaces are also arranged to have a frame shape.

In a preferred embodiment, the primary sound-absorbing surface of the first sound-attenuating member lies in a plane which is as parallel as possible (dictated for example by the method of hinging/ sliding/ parallel opening, rotating etc) to the plane of the primary sound-absorbing surface of the second sound-attenuating member. The secondary sound-absorbing surface of the externally-exposed surface of the first sound-attenuating member(s) may be adjacent to the aperture defined by the frame. When the panel unit is closed, the secondary sound- absorbing surface of the externally-exposed surface of the second sound- attenuating member may be located generally adjacent to the frame rather than adjacent to its aperture. The externally-exposed, sound-absorbing surface of the first and/or second sound-attenuating member(s) is preferably at least partially perforated. Sound absorption is then assisted via the perforations on the externally-exposed surface of these members. A micro perforated absorber may be provided.

In one embodiment, the externally-exposed surface of the first and/or the second sound-attenuating member(s) is 5 to10 % open as a result of the

perforations. The perforated externally-exposed surface may be in the form of a perforated sheet. The perforation diameter is preferably greater than the thickness of the sheet. The perforated sheet may be made from metal or plastic, for example.

The sound-attenuating members may comprise a porous material for absorbing sound. By‘sound-absorbing’ is meant that the total reflected sound energy by the sound-absorbing material is less than the incident sound energy.

The sound-absorbing material may be an open cell foam (for example, melamine foam) or a fibrous material (for example, mineral wool). The material may be injectable in liquid form to form a volume of porous material. The porous material is selected to absorb sound efficiently.

When the perforated externally-exposed surface is in the form of a

perforated sheet, additional low frequency sound attenuation may be provided in the absence of porous material. The first and/or the second sound-attenuating member(s) may comprise one or more ribs, each rib preferably having a profile which is straight, curved or angled in cross-section (the cross-section being in a direction which is perpendicular to the length of the rib). Each rib preferably has a cross-sectional shape substantially being an arc or a parabola. When a rib has a straight profile it preferably extends in a direction which is perpendicular (in cross- section) to a non-sound-absorbing surface and/or a sound-absorbing surface (for example a primary sound-absorbing surface).

Each rib is preferably positioned in the sound-attenuating member such that, in the panel unit, its length runs substantially along the length of the corresponding edge of the panel or frame, as appropriate. The length of each rib therefore preferably extends along the length of its sound-attenuating member.

The presence of one or more ribs may enhance sound attenuation. The ribs may also provide additional structural integrity to the sound-attenuating member and therefore to the panel unit. The ribs are used to seek to prevent shortcuts being taken by the sound to be attenuated.

If more than one rib is present, the ribs are preferably positioned so as not to contact one other.

The ribs may be used to define the boundaries of the planes of the externally-exposed surface of the first and/or second sound-attenuating members.

In this respect, the planes of the externally-exposed, sound-absorbing surface of the first and/or second sound-attenuating members preferably meet at a corner when viewed in cross-section (the cross-section being in a direction which is perpendicular to the plane defined by the frame or the panel, as appropriate).

Hence the primary and secondary sound-absorbing surfaces of each sound- attenuating member preferably meet at a corner. As a result, this corner is porous to sound in two dimensions (when viewed in cross-section). The first and/or second sound-attenuating members are therefore provided with one or more edges where two sound-absorbing surfaces meet, these edges being porous to sound from two directions.

One or more other corners may be provided where a sound-absorbing surface meets a non-sound-absorbing surface, these corners being porous to sound in one dimension (when viewed in cross-section). The first and/or second sound-attenuating members may therefore be provided with one or more edges where a sound-absorbing surface meets a non-sound-absorbing surface, these edges being porous to sound from one direction.

One or more other corners may be provided where two non-sound absorbing surfaces meet, these corners not being porous to sound. The first and/or second sound-attenuating members may therefore be provided with one or more edges where two non-sound absorbing surfaces meet, these edges not being porous to sound. These edges are typically located internally of the panel unit, so that they are not directly exposed to air.

Each rib defines at least two longitudinally-extending edges (‘longitudinal edges’) which extend to (and may touch or not) different surfaces or edges of its sound-attenuating member: these edges of the ribs are adjacent to or abut the relevant surface or edge of the sound-attenuating member. Preferably, one longitudinally-extending edge of the rib extends to a non- sound-absorbing surface or non-sound-porous edge of the sound-attenuating member and is therefore distal the air path: most preferably, this distal longitudinal edge of the rib extends to a non-sound-absorbing surface of the sound-attenuating member. This edge of the rib preferably touches this surface. This edge of the rib is preferably spaced from an adjacent edge of the sound-attenuating member (for example, a non-sound-porous edge and/or an edge which is porous to sound in one dimension (when viewed in cross-section)).

Preferably, another longitudinally-extending edge of the rib extends to a sound-absorbing surface or sound-porous edge of the sound-attenuating member and is therefore proximate the air path: most preferably, this proximal longitudinal edge extends to a sound-porous edge. This edge of the rib preferably touches this edge. The edge of the rib is preferably located at an edge which is porous to sound in two dimensions (when viewed in cross-section) although it may instead be located at an edge which is porous to sound in one dimension (when viewed in cross-section).

In one embodiment, the or each rib is configured such that the longitudinal edge of the rib proximate the air path is adjacent to or abuts a longitudinally- extending boundary edge (ie at a corner which is porous to sound in two

dimensions) of two planes of the externally-exposed, sound-absorbing surface of the first and/or second sound-attenuating members defining the air path (eg. the longitudinally-extending boundary edge where the primary sound-absorbing surface meets the secondary sound-absorbing surface of each sound-attenuating member).

The longitudinal edge of the rib distal from the air path (and therefore distant from the externally-exposed surfaces of the sound-attenuating members) may be adjacent to or abut a surface of the sound-attenuating member that is remote from the air path. If two or more ribs are present, the distal longitudinal edges of the ribs are preferably positioned close to one another along these remote surface(s) of the sound-attenuating member, without making contact with one another. This means that the distance between adjacent ribs is relatively small at their distal longitudinal edges compared to the distance between adjacent ribs at their proximal longitudinal edges.

In one embodiment, the first and/or second sound-attenuating members each have one, two, three or four ribs which have their distal longitudinal edges adjacent to or abutting a non-sound-absorbing surface of the sound-attenuating member, and have their respective proximal longitudinal edges adjacent to or abutting different sound-porous edges of the sound-attenuating member (these sound-porous edges being porous to sound from one or two directions): the distance between adjacent ribs is relatively small at their distal longitudinal edges compared to the distance between adjacent ribs at their proximal longitudinal edges.

By extending to an edge which is porous to sound, a rib presents an obstacle to sound taking a short-cut through the sound-attenuating member. The geometry of the sound-attenuating member will help to dictate the most effective profile of the rib. If the first and/or second sound-attenuating members have externally-exposed, sound-absorbing surfaces in three planes then rib(s) with a curved profile provide a less erratic sound absorption spectra than rib(s) with a straight profile.

In one embodiment, the first and/or second sound-attenuating members each comprise one, two or three ribs. Preferably, the first sound-attenuating member has three ribs and the second sound-attenuating member has two ribs.

In one example, the sound-attenuating members are polygonal in cross- section and have at least one edge which is porous to sound from two directions, at least one edge which is porous to sound from one direction and optionally one edge which is not porous to sound.

If ribs are present in this example, one or both sound-attenuating members has a rib that extends in profile from a surface which is non-sound-absorbing to an edge which is porous to sound from two directions. Any additional ribs may extend in profile from the same non-sound-absorbing surface to either an edge which is porous to sound from one direction or to an externally-exposed, sound-absorbing surface. In one example, the sound-attenuating members are quadrilateral in cross- section and have one edge which is porous to sound from two directions, two edges which are porous to sound from one direction and one edge which is not porous to sound.

If ribs are present in this example, each sound-attenuating members has a rib that extends in profile from a surface which is non-sound-absorbing to an edge which is porous to sound from two directions. The first-sound attenuating member preferably has two additional ribs which extend in profile from the same non-sound- absorbing surface as its first rib to one or both edges (but not the same edge) which are porous to sound from one direction or to an externally-exposed, sound- absorbing surface. This non-sound-absorbing surface may be located opposite to the secondary sound-absorbing surface of the first sound-attenuating member.

The second-sound attenuating member preferably has one additional rib which extends in profile from the same non-sound-absorbing surface as its first rib to one edge which is porous to sound from one direction or to an externally-exposed, sound-absorbing surface. This non-sound-absorbing surface may be located opposite to the primary sound-absorbing surface of the second sound-attenuating member.

The profiles of the ribs, when viewed in cross-section, preferably form an expansion pattern starting from their distal longitudinal edges, such that the spacing between neighbouring ribs gradually increases towards their longitudinal edges proximate the air path. Thus, in cross-section, tapering shapes are formed by the ribs with the resulting shapes tapering towards a surface (or surfaces) remote from the air path (ie towards a non-sound-absorbing surface), and therefore tapering outwards towards surfaces which form part of the externally-exposed, sound-absorbing surface of each sound-attenuating member.

In one embodiment, where the sound-attenuating members may be quadrilateral in cross-section, the primary sound-absorbing surface of the first and/or second sound-attenuating member has a longitudinal edge of a rib at one or both longitudinally-extending boundary edges of the primary sound-absorbing surface, each boundary edge being sound-porous from two directions or from one direction. Preferably there is a longitudinal edge of a respective rib at both longitudinally-extending boundary edges of the primary sound-absorbing surface on the first and the second sound-attenuating members, one boundary edge preferably being sound-porous from two directions and one boundary edge preferably being sound-porous from one direction.

In the same or a different embodiment, the secondary sound-absorbing surface of the first and/or second sound-attenuating member has a longitudinal edge of a respective rib at one or both longitudinally-extending boundary edges of the secondary sound-absorbing surface, each boundary edge being sound-porous from two directions or from one direction. Preferably there is a longitudinal edge of a respective rib at both longitudinally-extending boundary edges of the secondary sound-absorbing surface on the first sound-attenuating member (on the frame), one boundary edge preferably being sound-porous from two directions and one boundary edge preferably being sound-porous from one direction: also there is a longitudinal edge of a rib at only one longitudinally-extending boundary edge of the secondary sound-absorbing surface on the second sound-attenuating member (on the panel), this edge preferably being sound-porous from two directions.

In a preferred embodiment, each sound-attenuating member comprises a perforated housing defining a sound-absorbing volume which comprises a porous material and optionally one or more ribs. The perforated housing may be a housing formed from one or more perforated sheets.

In one embodiment, the ribs and/or the perforated sheet are formed from materials that exhibit a low thermal conductivity, for example plastic. In another embodiment, the ribs and/or perforated sheet are made of metal (eg steel). The ribs and the perforated sheet need not be of the same material.

The first and/or the second sound-attenuating member(s) may comprise one or more air cavities. Preferably any air cavity is not adjacent to the externally exposed, sound-absorbing surface of the sound-attenuating member in question, such that it is preferably distanced from the primary and/or secondary sound- absorbing surfaces. The provision of an air cavity may extend the sound absorption to a lower frequency.

The air cavity may have a quadrilateral or triangular shape in cross-section (the cross-section being in a width-wise direction of the sound-attenuating member, this direction being perpendicular to the plane defined by the frame or the panel, as appropriate). For example, the air cavity may have the shape of a parallelogram (preferably a rectangle) in cross-section. Also, any air cavity in the sound- attenuating member of the frame may have the same or a different shape to any air cavity in the sound-attenuating member of the panel.

An air cavity in the first and/or second sound-attenuating member may extend along its length in the same general direction as the length of the respective sound-attenuating member. In cross-section (the cross-section being in a direction which is perpendicular to the plane defined by the frame or the panel, as

appropriate), the longest dimension of the air cavity may be generally parallel to the primary sound-absorbing surface of the respective sound-attenuating member and therefore generally parallel to a plane defined by the frame or the panel (as appropriate), at least when the panel unit is open.

In one embodiment, one or both sound-attenuating members comprise a porous material which is located adjacent to the externally exposed, sound- absorbing surface of the sound-attenuating member. One or more air cavities may be provided adjacent to this porous material and remote from the externally exposed, sound-absorbing surface of the sound-attenuating member. One or both of the sound-attenuating members may comprise a perforated housing and/or one or more ribs.

According to the present invention the panel and the frame each have a first sound-attenuating depth of at least 30mm in a direction perpendicular to their primary sound-absorbing surface and a second sound-attenuating depth of at least 30mm in a direction perpendicular to their secondary sound-absorbing surface. This sound-attenuating depth allows absorption of low frequency sound such as traffic noise.

The sound-attenuating depth is the space through which a sound wave can propagate: the sound-attenuating depth is normally limited by solid walls which surround this space. The sound-attenuating depth of the frame or panel is distinct from the physical thickness of the sound-attenuating member mounted on the outer surface of the frame or panel; this is because the sound-attenuating depth may be provided by an air cavity contained within the profile of the frame or panel and not within the sound-absorbing volume of the corresponding sound-attenuating member.

In some embodiments, the sound-absorbing volume is defined by a housing having at least some perforated surfaces to form sound-absorbing surfaces; in some embodiments, the sound-absorbing volume is defined by a sound-absorbing material; and in other embodiments, the sound-absorbing volume is defined by perforated surfaces and a sound-absorbing material; these embodiments are not limiting and depend on the design of the panel unit.

The first and the second sound-attenuating depths may comprise either the corresponding sound-absorbing surface (for example, a perforated sheet) and a sound-absorbing porous material; or the corresponding sound-absorbing surface (for example, a perforated sheet), a sound-absorbing porous material and an air cavity, either contained within the sound-attenuating member or contained within the profile of the frame or panel.

When he first and/or the second sound-attenuating depths of the frame and/or the panel comprise an air cavity which is spaced from the corresponding primary and/or secondary sound-absorbing surface by at least the sound- absorbing porous material, this has the advantage of being able to provide a porous material which has a thickness of less than 30mm in a direction

perpendicular to the relevant sound-absorbent surface.

In one embodiment, the frame and/or the panel comprises a profile (for example, an extruded profile) and the corresponding sound-attenuating member is mounted on the profile adjacent a perforated wall or perforated walls of the profile. A perforated profile wall may lie parallel to the corresponding primary sound- absorbing surface and/or parallel to the corresponding secondary sound-absorbing surface. An air cavity is provided within the profile and is separated from the sound-absorbing material by the perforated profile wall.

Therefore the frame and/or the panel preferably comprises a profile which has at least one region provided with perforations and the cavity is formed within the profile adjacent at least part of the perforated region; the perforated region is preferably in a surface of the profile. The profile of the frame and/or the panel may be substantially hollow. The sound-absorbing porous material is not contained within a profile of a frame and/or panel, irrespective of whether the profile has at least one region provided with perforations to allow sound to pass therethrough.

The frame is preferably designed such that, when fitted, its internal elevation can be covered with a room lining such as plasterboard, therefore limiting the visual impact of the frame for an occupant.

Sound-absorbing material may be used elsewhere on the panel unit to increase sound attenuation whilst also providing thermal insulation. For example, where thermal insulation is required on an exposed air path, a porous material such as acoustic foam can be used.

In one embodiment, the present invention provides a parallel-opening panel in a building fagade that guides air through an air path having at least one bend (eg a dog-leg shape or an S-shape), the air path being substantially lined with sound- attenuating members.

In another embodiment, the present invention provides a tilt and turn panel in a building fagade that guides air through an air path having at least one bend (eg a dog-leg shape or an S-shape), the air path being substantially lined with sound- attenuating members.

In a further embodiment, the present invention provides a sliding panel in a building fagade that guides air through an air path having at least one bend (eg a dog-leg shape or an S-shape), the air path being substantially lined with sound- attenuating members.

The dimensions of the sound-attenuating members, their material make-up and the air path created between them are some of the factors that determine the level of sound attenuation they provide.

The first and/or the second sound-attenuating member(s) may be partly integrated (but not fully contained) within the profiles of the respective frame or panel.

The panel may be movable into an open position such that it lies in a plane which is substantially parallel to a plane defined by the frame, or such that it lies in substantially the same plane as the plane defined by the frame, or such that it lies in a plane which is at a variable angle from the plane defined by the frame. The sound-attenuating members may be formed integrally with the panel unit. Alternatively, the sound-attenuating members may be retro-fitted to existing panel units, for example to existing window sashes and frames.

By using the openable panel unit of the present invention, it is possible to provide sufficient ventilation to meet building regulations whilst offering significant noise reduction compared to a standard open window. The use of the sound- attenuating members on a window may provide additional sound attenuation of around 10 dB(A) (for example) compared to the same window without the sound- attenuating members. This roughly translates to an additional halving in loudness. The present invention may therefore be used in situations where the use of natural ventilation has previously been considered unfeasible due to external noise levels.

An internal noise level of 35 dB(A) is required for a comfortable night’s sleep and a noise level of 45 dB(A) is required for comfort during the day. For buildings in noisy urban areas, mechanical ventilation is relied upon to keep internal noise levels acceptable.

By using an openable panel unit of the present invention having a total free area of 0.6 m ² , initial experiments have shown that attenuation of 23 dB(A) is achievable whilst maintaining a free area equivalent to 1 /20 ^th floor area of a typical bedroom. This is a rule of thumb used in the industry to guide designers on the sizing of natural ventilation openings. Thus the openable panel unit may allow natural ventilation at acceptable noise levels in areas where the external noise level is up to 58 dB(A) at night and 68 dB(A) during the day.

The publication from the UK Association of Noise Consultants, 2018:

Acoustics Ventilation and Overheating: Residential Design Guide describes the nominal 12dB(A) sound level difference from outside to inside for a typical fagade with openings appropriate for control of overheating (roughly 5% of the internal floor area).

Rw is an abbreviation for weighted sound reduction, as defined by British Standard BS717-1 : 2013 relating to Acoustics: sound insulation in buildings and of building elements. The Sound Reduction Index is used to measure the level of sound insulation provided by a structure such as a wall, window, door, or ventilator. The Sound Reduction Index is expressed in decibels (dB).

British Standard BS EN ISO 10140-2:2010 Acoustics: Laboratory measurement of sound insulation of building elements. Measurement of airborne sound insulation describes the methodology to test the sound insulation of a panel unit such as a glazing system.

In pop-out (ie parallel-opening) type panel units according to the present invention, and based on an opening of 100mm, the dimensions of the sound- attenuating members can be determined for a given acoustic requirement which is preferably a 5 to 12 dB(A) improvement, preferably an 6 to 10 dB(A) improvement, compared to the same panel unit, open the same amount, having the first and second sound-attenuating members absent from the frame and the panel respectively. Such an improvement would allow the majority of urban residences to be naturally ventilated. This is because about 50% of the urban population are exposed to night-time noise levels of 50-60 dB(A).

In a room having a floor area of 9 to 12 m ² , a parallel-opening panel unit of the present invention can be opened 80 to 120mm to allow sufficient ventilation whilst offering sound attenuation of around R _w 5 to R _w 15 more than the same panel unit, open the same amount, having the first and second sound-attenuating members absent from the frame and the panel respectively.

In sliding type panel units according to the present invention, and based on an opening of 80mm, the dimensions of the sound-attenuating members can be determined for a given acoustic requirement which is preferably at least a R _w 5 improvement over the same panel unit, open the same amount, having the first and second sound-attenuating members absent from the frame and the panel respectively.

The openable panel unit of the present invention is able to simultaneously provide ventilation sufficient to control overheating and attenuation of noise ingress that is preferably R _w 5 to R _w 15 greater than the same panel unit, open the same amount, having the first and second sound-attenuating members absent from the frame and the panel respectively. A standard open panel unit such as a window can offer sufficient ventilation, but little control on noise ingress. A trickle vent is a suitable solution for high sound attenuation and low ventilation rates but provides a fraction of the air ventilation offered by the openable panel unit of the present invention. By way of example, in order for a trickle vent to be directly comparable to a pop-out type panel unit according to the present invention, over fifty trickle vents would be required.

Opening the panel unit of the present invention to provide a minimum distance of 80 to 120 mm between the frame and the panel is able to provide an improvement in sound attenuation in the air path of from 5 to 12 dB(A) compared to an identical panel unit, opened the same distance, having the first and second sound-attenuating members absent from the frame and the panel respectively. In this comparison, the first and second sound-attenuating members may be removed from, or never mounted on, the frame and the panel respectively

This distance of 80 to 120 mm is the minimum distance between the frame and the panel (ie including the sound-attenuating members): an opening of this distance provides a clear air path for ventilation to prevent overheating, particularly where the opening area of the panel unit is 5% of the internal floor area of the room served by the panel unit. Also, opening a panel unit beyond 120mm in a high rise structure is not advisable for safety reasons.

The minimum distance between the frame and the panel is straightforward to measure for a parallel-opening design or a sliding design: when the panel unit has a hung design (eg a tilt and turn design or a simple hinged design), the minimum distance between the frame and the panel is measured at the free edge of the panel that is opposite the hinged edge of the panel (ie where the hinge or hinges are provided to mount the panel for movement relative to the frame).

The openable panel unit of the present invention is able to provide an integrated design which attenuates sound and that works thermally, aesthetically and structurally. The openable panel unit, in a preferred embodiment, is of a parallel-opening design or a sliding design.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a partial perspective view of a panel unit in a closed position, showing cross-sections of a frame and a panel, in a first embodiment;

Figure 2 is a partial planar cross-sectional view of the closed panel unit of figure 1 ;

Figure 3 is a partial perspective view of the panel unit of the first

embodiment in an open position;

Figure 4 is a partial planar cross-sectional view of the open panel unit of figure 3;

Figure 5 is a partial planar cross-sectional view of a panel unit in a closed position, showing cross-sections of a frame and a panel, in a second embodiment;

Figure 6 is a partial planar cross-sectional view of the panel unit of the second embodiment in an open position;

Figure 7 is a partial planar cross-sectional view of a panel unit in a closed position, showing cross-sections of a frame and a panel, in a third embodiment;

Figure 8 is a partial planar cross-sectional view of the panel unit of the third embodiment in an open position;

Figure 9 is a partial planar cross-sectional view of a panel unit in a closed position, showing cross-sections of a frame and a panel, in a fourth embodiment;

Figure 10 is a partial planar cross-sectional view of the panel unit of the fourth embodiment in an open position;

Figure 11 is a partial planar cross-sectional view of a panel unit in an open position, showing cross-sections of a frame and a panel, in a fifth embodiment;

Figure 12 is a partial planar cross-sectional view of a panel unit in an open position, showing cross-sections of a frame and a panel, in a sixth embodiment;

Figure 13 is a partial planar cross-sectional view of a panel unit in an open position, showing cross-sections of a frame and a panel, in a seventh embodiment;

Figure 14 is a partial planar cross-sectional view of a panel unit in a closed position, showing cross-sections of a frame and a panel, in an eighth embodiment;

Figure 15 is a partial planar cross-sectional view of the panel unit of the eighth embodiment in an open position; and

Figure 16 is a graph of frequency against composite sound reduction. Referring to figures 1 to 15, an openable panel unit comprises a frame 4 and a panel 6, the frame being mounted on adjacent support structure 44 such as a wall or building facade. The panel is shown in an open position in figures 3, 4, 6, 8, 10, 11 , 12, 13 and 15, therefore creating a ventilation opening. This open position defines an air path 8 for ventilation between a first air volume and a second air volume.

Frame 4 comprises a first sound-attenuating member 10. Panel 6

comprises a second sound-attenuating member 12. These sound-attenuating members are positioned around the edge regions of the frame and the panel, respectively. They may be formed as a single continuous sound-attenuating member or they may comprise a series of sound-attenuating members that are preferably aligned to form a continuous sound-attenuating member. In practice, a length of the first or second sound-attenuating member may be segmented and fitted to form a frame shape to be fitted along the edge regions of the frame or the panel, as appropriate.

In this respect, first sound-attenuating member 10 is attached to surface(s) 14 of frame 4 and second sound-attenuating member 12 is attached to surface(s) 16 of panel 6.

Each sound-attenuating member has a primary sound-absorbing surface, being primary surface 18 of the first sound-attenuating member 10 and primary surface 20 of the second sound-attenuating member 12. In the first to eighth embodiments, one primary surface generally faces in the direction of the first air volume, whilst the other primary surface generally faces in the direction of the second air volume. The primary surfaces of the sound-attenuating members may be aligned substantially in parallel to one another, preferably with opposing primary surfaces, at least when the panel unit is in an open position.

In the first to eighth embodiments, the primary surfaces of the sound- attenuating members are aligned substantially in parallel to one another and also opposing one another when the panel unit is in a closed position.

The first to fifth and eighth embodiments are parallel-opening panel units, meaning that the primary surfaces of the sound-attenuating members are aligned substantially in parallel to one another and also oppose one another when the panel unit is in an open position.

The double-headed unlabelled arrows in figures 11 to 13 show the direction of opening and of closing of the panel unit.

The sixth embodiment (figure 12) is a sliding window, so the primary surfaces of the sound-attenuating members are aligned substantially in parallel to one another and either partially oppose one another or do not oppose one another when the panel unit is in an open position.

The seventh embodiment (figures 13) is a hinged (or hung) window (for example a tilt and turn window which may open inwardly or outwardly), so the panel opens at a variable angle to the frame. The smaller the angle between the frame and the panel, the better the sound-attenuation: accordingly, for a small angle (eg 20 to 40 degrees) between the plane defined by the frame and the plane defined by the panel, the primary surfaces of the sound-attenuating members are not aligned substantially in parallel to one another: however, they do generally oppose one another when the panel unit is in an open position, as shown in figure 13.

In an arrangement where the air path 8 flows from a first air volume to a second air volume, such as shown in the embodiments of figures 3, 4, 6, 8, 10, 11 , 12, 13 and 15, the primary surface 18 of the sound-attenuating member on the frame faces in the direction of the first air volume while the primary surface 20 of the sound-attenuating member on the panel faces in the direction of the second air volume. However, the air path may flow in the opposite direction. The air path shown in the figures has a general or substantial S-shape.

In the embodiment of figure 15, the secondary surface 24 of the sound- attenuating member 12 on the panel and the primary surface 18 of the sound- attenuating member on the frame have a greater effect on sound attenuation than the respective primary surface 20 and secondary surface 22, as a result of the likely route taken by the air path.

In the embodiments shown, at least a portion of the primary surface 20 of the sound-attenuating member 12 on the panel opposes at least a portion of the primary surface 18 of the sound-attenuating member 10 on the frame. Also, in these embodiments, since the sound-attenuating members on the frame and the panel are arranged to have a frame shape, their primary surfaces are also arranged to have a frame shape.

From the figures, it can be appreciated that when the panel unit is in a closed position, the first and second sound-attenuating members 10 and 12 may abut one another, at least in part. This assists in sealing the panel unit and also provides for sound attenuation when the panel unit is closed.

In the first to eighth embodiments, the sound-attenuating members each have a secondary sound-absorbing surface. Hence, the sound-attenuating member 10 on the frame has a secondary sound-absorbing surface 22, whereas the sound-attenuating member 12 on the panel has a secondary sound-absorbing surface 24. The air path is defined at least in part by the primary and secondary sound-absorbing surfaces of the first and second sound-attenuating members.

In all but the second embodiment, the angle between the plane of the primary sound-absorbing surface 18 and the plane of the secondary sound- absorbing surface 22 of the first sound-attenuating member 10 is substantially 90 degrees at a corner 34. Also, the angle between the plane of the primary sound- absorbing surface 20 and the plane of the secondary sound-absorbing surface 24 of the second sound-attenuating member 12 is substantially 90 degrees at corner 34.

In the second embodiment (see figures 5 and 6), the angle between the plane of the primary sound-absorbing surface 18 and the plane of the secondary sound-absorbing surface 22 of the first sound-attenuating member 10 is substantially 68 degrees at corner 34. Also, the angle between the plane of the primary sound-absorbing surface 20 and the plane of the secondary sound- absorbing surface 24 of the second sound-attenuating member 12 is substantially 68 degrees at corner 34.

Since the primary and secondary sound-absorbing surfaces of each sound- attenuating member preferably meet at corner 34, this corner is porous to sound in two dimensions (when viewed in cross-section). The first and/or second sound- attenuating members are therefore provided with one or more edges where two sound-absorbing surfaces meet, these edges being porous to sound from two directions.

One or more other corners 38 may be provided where a sound-absorbing surface meets a non-sound-absorbing surface, these corners being porous to sound in one dimension (when viewed in cross-section). The first and/or second sound-attenuating members may therefore be provided with one or more edges where a sound-absorbing surface meets a non-sound-absorbing surface, these edges being porous to sound from one direction.

One or more other corners 42 may be provided where two non-sound absorbing surfaces meet, these corners not being porous to sound. The first and/or second sound-attenuating members may therefore be provided with one or more edges where two non-sound absorbing surfaces meet, these edges not being porous to sound. These edges are typically located internally of the panel unit, so that they are not directly exposed to air.

In the present embodiments, each sound-attenuating member is elongate in shape with a width-wise cross-section which is substantially polygonal in shape, the cross-section being in a direction which is perpendicular to the plane defined by the frame or the panel, as appropriate. In all but the sixth and eighth

embodiments, the first and second sound-attenuating members define a sound- absorbing volume having a shape which is substantially quadrilateral in cross- section.

In the second embodiment (see figures 5 and 6), the externally-exposed, sound absorbing surface of the first sound-attenuating member 10 extends beyond the quadrilateral shape of the sound-absorbing volume to define an approximate Z- shape in cross-section: in this respect, the primary and secondary sound-absorbing surfaces meet at one corner 34 and the primary sound-absorbing surface meets a tertiary sound-absorbing surface 36 at another corner 34’, so that the primary sound-absorbing surface 18 is common to both corners. The secondary sound- absorbing surface 22 is substantially parallel to the tertiary sound-absorbing surface 36.

In the sixth embodiment (see figure 12), the first sound-attenuating member 10 has a sound-absorbing volume which is substantially L-shaped (step-shaped) in cross-section. The externally-exposed, sound absorbing surface of the first sound- attenuating member 10 defines an approximate Z-shape (step-shape) in cross- section: in this respect, the primary and secondary sound-absorbing surfaces meet at one corner 34 and the primary sound-absorbing surface meets a tertiary sound- absorbing surface 36 at another corner 34’, so that the primary sound-absorbing surface is common to both corners. The secondary sound-absorbing surface 22 is substantially parallel to the tertiary sound-absorbing surface 36. The second sound-attenuating member 12 defines a sound-absorbing volume having a shape which is substantially quadrilateral in cross-section.

In this embodiment of figure 12, the first sound-attenuating member has a sound-absorbing volume which is a combination of two adjacent rectangular prisms having their respective longitudinal axes arranged perpendicularly to one another, to provide an approximate L-shaped member.

In the eighth embodiment of figures 14 and 15, the first sound-attenuating member 10 has a sound-absorbing volume which is substantially Z-shaped (step- shaped) in cross-section. The externally-exposed, sound absorbing surface of the first sound-attenuating member 10 defines an approximate Z-shape (step-shape) in cross-section: in this respect, the primary and secondary sound-absorbing surfaces meet at one corner 34 and the primary sound-absorbing surface meets a tertiary sound-absorbing surface 36 at another corner 34’, so that the primary sound- absorbing surface is common to both corners: these corners may define

approximate right angles. The secondary sound-absorbing surface 22 is substantially parallel to the tertiary sound-absorbing surface 36. The second sound-attenuating member 12 defines a sound-absorbing volume having a shape which is substantially quadrilateral in cross-section. The ratio of the length of the secondary sound-absorbing surface 24 of the second sound-attenuating member 12 to the length of the primary sound-absorbing surface 20 of the second sound- attenuating member is greater than or equal to 0.5 and less than or equal to 2.0, when viewed in cross-section across the width of the second sound-attenuating member in a direction which is perpendicular to the plane defined by the panel.

In this embodiment of figures 14 and 15, the first sound-attenuating member has a sound-absorbing volume which is a combination of three adjacent rectangular prisms having their respective longitudinal axes arranged

perpendicularly to one another, to provide an approximate Z-shaped member.

The sound-absorbing volumes of any of the embodiments of the invention may be regular, irregular or semi-regular when viewed in cross-section.

Also, in any of these embodiments, the angle between the plane of the primary sound-absorbing surface and the plane of the secondary sound-absorbing surface of the first sound-attenuating member may be 50 to 130 degrees, preferably 60 to 120 degrees and more preferably 60 to 110 degrees. The angle between the plane of the primary sound-absorbing surface and the plane of the secondary sound-absorbing surface of the second sound-attenuating member may be 50 to 130 degrees, preferably 60 to 120 degrees and more preferably 60 to 110 degrees.

Adjacent sound-attenuating members lie along a common axis (for example, to define an edge of a frame shape) or lie along axes which are perpendicular to one another (for example, to define the corner of a frame shape).

More than one sound-attenuating member may be provided on the panel and/or the frame. In the sixth embodiment of figure 12, an additional sound- attenuating member 40 is provided on the frame: the additional sound-attenuating member 40 has an externally-exposed sound-absorbing surface provided in a single plane only.

In the seventh embodiment of figure 13, an additional sound-attenuating member 40 is provided on the panel: the additional sound-attenuating member 40 has an externally-exposed sound-absorbing surface provided in two perpendicular planes (and a tertiary externally-exposed sound-absorbing surface may be provided opposite the secondary sound-absorbing surface): it therefore has primary and secondary sound-absorbing surfaces.

As shown in the figures, the sound-attenuating members of these

embodiments each contain a porous material 26. In the third embodiment (figures 7 and 8) and the fifth embodiment (figure 11 ) both sound-attenuating members also contain an air cavity 32 located within the boundary of the sound-absorbing volume of the sound-attenuating member. However, the porous material and/or the air cavity may be omitted from one or more sound-attenuating members. The eighth embodiment (figures 14 and 15) has an air cavity 32 located within the first sound-attenuating member 10. This air cavity is optional.

The air cavities of the third embodiment each have a rectangular cross- section (the cross-section being in a width-wise direction of the sound-attenuating member, this direction being perpendicular to the plane defined by the frame or the panel, as appropriate). They may however have an alternative shape in cross- section such as a parallelogram or a square (as in the eighth embodiment) or a triangle or an L-shape.

In the fifth embodiment, the air cavity on the frame 4 has a rectangular cross-section (the cross-section being in a direction which is perpendicular to the plane defined by the frame). However the air cavity on the panel 6 has an L-shape in cross-section (the cross-section being in a direction which is perpendicular to the plane defined by the panel). They may each however have an alternative shape in cross-section, as mentioned above.

In the third, fifth and eighth embodiments, each air cavity extends in a direction which is generally parallel to the length of the relevant sound-attenuating member.

In these embodiments, the frame 4 has a first sound-attenuating depth of at least 30mm in a direction perpendicular to its primary sound-absorbing surface 18 and has a second sound-attenuating depth of at least 30mm in a direction perpendicular to its secondary sound-absorbing surface 22; also the panel 6 has a first sound-attenuating depth of at least 30mm in a direction perpendicular to its primary sound-absorbing surface 20 and has a second sound-attenuating depth of at least 30mm in a direction perpendicular to its secondary sound-absorbing surface 24.

In an embodiment where the sound-attenuating member has a perforated housing (which provides a sound-absorbing surface), the sound-attenuating depth comprises the thickness of the housing (eg a 1 mm thick sheet) and the thickness of a sound-absorbing porous material contained within the housing.

The frame and/or the panel may comprise a profile (for example, an extruded profile) and the corresponding sound-attenuating member is mounted on the profile. The sound-absorbing porous material is not contained within the profile of a frame and/or panel.

In the eighth embodiment of figures 14 and 15, the profile of the frame optionally has a surface 46 which is at least partly perforated. The first sound- attenuating member 10 is mounted adjacent to this surface such that the

secondary sound-absorbing surface 22 is parallel to the perforated surface 46.

The perforated surface 46 of the profile extends the volume through which soundwaves can propagate into an adjacent profile cavity 48 and thereby extends the sound-attenuating depth of the frame in a direction perpendicular to the secondary sound-absorbing surface 22.

Similarly, the profile of the panel optionally has a surface 46 which is at least partly perforated. The second sound-attenuating member 12 is mounted adjacent to this surface such that the primary sound-absorbing surface 20 is parallel to the perforated surface 46. The perforated surface 46 of the profile extends the volume through which soundwaves can propagate into an adjacent profile cavity 48 and extends the sound-attenuating depth of the panel in a direction perpendicular to the primary sound-absorbing surface 20.

In the sixth embodiment of figure 12, the profile of the panel optionally has a surface 46 which is at least partly perforated. The second sound-attenuating member 12 is mounted adjacent to this surface such that the primary sound- absorbing surface is parallel to the perforated surface 46. The perforated surface 46 of the profile extends the volume through which soundwaves can propagate into an adjacent profile cavity 48 and extends the sound-attenuating depth of the panel in a direction perpendicular to the primary sound-absorbing surface.

In the seventh embodiment of figure 13, the profile of the frame optionally has a surface 46 which is at least partly perforated. The first sound-attenuating member 10 is mounted adjacent to this surface such that the primary sound- absorbing surface 18 is parallel to the perforated surface 46. The perforated surface 46 of the profile extends the volume through which soundwaves can propagate into an adjacent profile cavity 48 and thereby extends the sound- attenuating depth of the frame in a direction perpendicular to the primary sound- absorbing surface. Similarly, the profile of the panel optionally has a surface 46 which is at least partly perforated. The second sound-attenuating member 12 is mounted adjacent to this surface such that the secondary sound-absorbing surface 24 is parallel to the perforated surface 46. The perforated surface 46 of the profile extends the volume through which soundwaves can propagate into an adjacent profile cavity 48 and extends the sound-attenuating depth of the panel in a direction perpendicular to the secondary sound-absorbing surface 24.

The profile of the panel optionally has a surface 46 which is at least partly perforated where the third sound-attenuating member 40 is mounted such that the primary sound-absorbing surface is parallel to the perforated surface 46. The perforated surface 46 of the profile extends the volume through which soundwaves can propagate into an adjacent profile cavity 48 and extends the sound-attenuating depth of the panel in a direction perpendicular to the primary sound-absorbing surface.

In these embodiments, the profile of the panel and/or the frame may also be provided with perforations in other surfaces where the sound-attenuating members are mounted adjacent thereto to extend the volume through which soundwaves can propagate into an adjacent profile cavity, depending on the panel unit design.

A benefit of extending the sound-attenuating depth in this way is that the thickness of the sound-attenuating member in question can be made thinner: for example, if the profile cavity has a thickness of at least 10mm, the sound- attenuating member may have a thickness of 20mm, the thickness being in the direction perpendicular to the sound-absorbing surface which is approximately parallel to the perforated surface of the profile: in the absence of the air cavity provided in the profile, the thickness of the sound-attenuating member in question may need to be at least 30mm to provide sufficient sound attenuation, depending on the design of the panel unit.

In the fourth embodiment (figures 9 and 10), the fifth embodiment (figure 11 ), the sixth embodiment (figure 12) and the seventh embodiment (figure 13), the sound-attenuating members contain one or more optional ribs 30. Each rib preferably extends longitudinally along the length of the relevant sound-attenuating member. Each rib may have a profile which is curved (and is preferably arc- shaped or parabolic in shape), has an angled profile or is straight.

One longitudinally-extending edge of the rib preferably extends to a non- sound-absorbing surface or non-sound-porous edge of the sound-attenuating member and is therefore distal the air path: preferably, as shown in the relevant embodiments, this distal longitudinal edge of the rib extends to a non-sound- absorbing surface of its sound-attenuating member. This edge of the rib preferably touches this surface. This edge of the rib is preferably spaced from an adjacent edge of the sound-attenuating member (for example, a non-sound-porous edge at corner 42 and/or an edge which is porous to sound in one dimension (when viewed in cross-section) at corner 38).

Another longitudinally-extending edge of the rib extends to a sound- absorbing surface or sound-porous edge of the sound-attenuating member and is therefore proximate the air path: preferably, this proximal longitudinal edge extends to a sound-porous edge. This edge of the rib preferably touches this edge. The edge of the rib is preferably located at an edge which is porous to sound in two dimensions (when viewed in cross-section) at corner 34, 34’ although it may instead be located at an edge which is porous to sound in one dimension (when viewed in cross-section) at corner 38.

The ribs 30 of the fourth embodiment shown in figures 9 and 10 have a curved profile. Preferably the ribs do not make contact with one another.

The primary sound-absorbing surfaces 18, 20 of the first and second sound- attenuating members have a longitudinal edge of a respective rib at both

longitudinally-extending boundary edges of the primary sound-absorbing surface, one boundary edge being sound-porous from two directions (at corner 34) and one boundary edge being sound-porous from one direction (at corner 38).

The secondary sound-absorbing surface 22 of the first sound-attenuating member 10 has a longitudinal edge of a respective rib at both longitudinally- extending boundary edges of the secondary sound-absorbing surface, one boundary edge being sound-porous from two directions (at corner 34) and one boundary edge being sound-porous from one direction (at corner 38). The secondary sound-absorbing surface 24 of the second sound-attenuating member 12 has a longitudinal edge of a rib at one longitudinally-extending boundary edges of the secondary sound-absorbing surface, this boundary edge being sound- porous from two directions (at corner 34).

Thus, in this embodiment, only one longitudinally-extending boundary edge of the secondary sound-absorbing surface is met by a rib on the second sound- attenuating member 12 (on the panel), whereas both longitudinally-extending boundary edges of the secondary sound-absorbing surface 22 are met by a respective rib on the first sound-attenuating member 10 (on the frame). In the first sound-attenuating member, one of the ribs that meets the longitudinally- extending boundary edge of the primary sound-absorbing surface is also one of the ribs that meets the longitudinally-extending boundary edge of the secondary sound-absorbing surface, such that three ribs are provided in total.

Whilst each sound-attenuating member preferably comprises two or more ribs, it is possible for only one rib to be present (as in the fifth and sixth

embodiments) or for no ribs to be present. In the embodiment of figures 9 and 10, the first sound-attenuating member 10 contains three ribs and the second sound- attenuating member 12 contains two ribs. The number of ribs is not essential and is determined in part by the relative dimensions of the sound-attenuating member in question.

In embodiments where ribs are present, they are preferably configured such that the longitudinal edge of a rib, which edge is proximate the air path, is adjacent to or abuts a longitudinally-extending boundary edge (at corner 34) where the primary sound-absorbing surface meets the secondary sound-absorbing surface of the or each sound-attenuating member, as shown in figures 9, 10, 11 , 12 and 13. This boundary edge is porous to sound in two directions.

Then, the longitudinal edge of the rib that is distal the air path (and therefore distant from the externally-exposed surfaces of the sound-attenuating members) may be located adjacent to a surface of the sound-attenuating member that is remote from the air path, as shown in these figures. This remote surface is preferably non-sound-absorbing.

A plurality of ribs are present in the fourth embodiment of figures 9 and 10 and the distal longitudinal edges of the ribs are positioned close to one another along an internally-located, non-sound-absorbing surface of the sound-attenuating member, without making contact with one another.

The curved profiles of the ribs, when viewed in cross-section, preferably form an expansion pattern starting from their distal longitudinal edges, such that the spacing between neighbouring ribs gradually increases towards their longitudinal edges which are proximate the air path. Thus, in cross-section, in the embodiment shown in figures 9 and 10, tapering shapes are formed by the curved ribs with the resulting shapes tapering towards a face remote from the air path, and therefore tapering outwards towards faces which form part of the externally- exposed surface of each sound-attenuating member.

The ribs 30 of the fifth embodiment shown in figure 11 each have a curved profile. One curved rib is provided in each sound-attenuating member. The proximal longitudinal edge of each rib extends to a sound-porous boundary edge being sound-porous from two directions (at corner 34). The distal longitudinal edge of the rib in the first sound-attenuating member 10 extends to a non-sound- absorbing surface of its sound-attenuating member, spaced from adjacent corner 38. The distal longitudinal edge of the rib in the second sound-attenuating member 12 extends towards, but does not reach, a non-sound-absorbing surface of its sound-attenuating member. Rib 30 extends into the air cavity 32 in the first sound- attenuating member 10 but does not extend into the air cavity 32 in the second sound-attenuating member 12, although these arrangements are not limiting.

The ribs 30 of the sixth embodiment shown in figure 12 each have a curved profile. One curved rib is provided in each sound-attenuating member. The proximal longitudinal edge of each rib extends to a sound-porous boundary edge being sound-porous from two directions (at corner 34). The distal longitudinal edge of each rib extends to a non-sound-absorbing surface of its sound-attenuating member, spaced from adjacent corner 38.

The ribs 30 of the seventh embodiment shown in figure 13 each have a curved or a straight profile. The first sound-attenuating member 10 has no ribs.

The second sound-attenuating member 12 has one curved rib and one straight rib. The proximal longitudinal edge of the curved rib extends to a sound- porous boundary edge being sound-porous from two directions (at corner 34). The distal longitudinal edge of the curved rib extends to a non-sound-absorbing surface of its sound-attenuating member, spaced from adjacent corner 38. The

longitudinal edges of the straight rib extend between a non-sound-absorbing surface (at the rib’s distal longitudinal edge) of its sound-attenuating member and the primary sound-absorbing surface (at the rib’s proximal longitudinal edge) of its sound-attenuating member. This straight rib extends in a direction which is perpendicular (in cross-section) to the non-sound-absorbing surface and the sound-absorbing surface.

The third sound-attenuating member 40 has two curved ribs. The primary sound-absorbing surface has a longitudinal edge of a respective rib at both longitudinally-extending boundary edges of the primary sound-absorbing surface, one boundary edge being sound-porous from two directions (at corner 34) and one boundary edge being sound-porous from one direction (at corner 38). The secondary sound-absorbing surface of the third sound-attenuating member 40 has a longitudinal edge of a rib at a longitudinally-extending boundary edge which is sound-porous from two directions (at corner 34).

Each sound-attenuating member of the embodiments shown has a perforated housing 28 made from a sheet material. In these embodiments, the partially-perforated housing of each sound-attenuating member provides an externally-exposed surface which makes contact with incoming air and sound waves travelling in the air path.

In all but the seventh embodiment, the panel is moveable to an open positon such that it lies in a plane which is substantially parallel to the plane defined by the frame. However other means of arranging the panel with respect to the frame are envisaged, for example the panel may be tiltably mounted on the frame, such that it lies at an angle to the plane defined by the frame, as per the seventh

embodiment.

The openable panel unit is preferably mounted in a building fagade or another structure (eg a roof or an internal wall) that separates two air volumes.

Referring to figures 1 , 2, 5, 7 and 9, it can be seen that, in these

embodiments, when the panel unit is closed, the panel is able to lie flush with an adjacent fagade, panel unit or wall (for example). This is however not an essential feature as the panel may protrude forwards or backwards from the plane of the structure, depending on the design of the structure in question.

In the embodiments shown, the respective primary sound-absorbing surfaces of the sound-attenuating members have a greater surface area than the respective secondary sound-absorbing surfaces of the sound-attenuating members. This is not essential.

In the first to fourth embodiments, the primary sound-absorbing surface 18 of the first sound-attenuating member 10 presents a similar surface area to that of primary sound-absorbing surface 20 of the second sound-attenuating member 12. Also, the respective secondary sound-absorbing surfaces of the sound-attenuating members present a similar surface area to one another.

In the fifth to eighth embodiments, the primary sound-absorbing surfaces of the first sound-attenuating members 10 each have a greater surface area than the surface area of the respective primary sound-absorbing surfaces of the second sound-attenuating members 12.

The first sound-attenuating member 10 on the frame may therefore provide a greater surface area of an externally exposed surface than that of the second sound-attenuating member 12 on the panel.

Referring to figures 11 to 15, the embodiments shown have the following dimensions (when viewed in cross-section), with dimension‘a’ being the length of the primary sound-absorbing surface on the first sound-attenuating member 10, dimension‘b’ being the length of the secondary sound-absorbing surface on the first sound-attenuating member 10, dimension‘c’ being the length of the primary sound-absorbing surface on the second sound-attenuating member 12, and dimension‘d’ being the length of the secondary sound-absorbing surface on the second sound-attenuating member 12.

Dimensions‘c’’,‘d’’ refer to the lengths of the primary and secondary sound-absorbing surfaces, respectively, of the third sound-attenuating member 40 of the seventh embodiment (figure 13).

Dimensions‘e’, T,‘g’,‘h’, T,‘j’,‘k’ and T‘ are other lengths set out in figures

12 to 15: dimension‘g’ is the length of the sound-absorbing surface of the third sound-attenuating member 40 of figure 12. Figure 11

a = 130 mm; b = 50 mm; c = 100 mm; d = 50 mm

Figure 12

a = 80 mm; b = 35 mm; c = 90 mm; d = 35 mm; e = 130 mm; f = 70 mm; g = 60 mm; h = 50 mm

In another example, the dimensions of Figure 12 are instead:

a = 50 mm; b = 30 mm; c = 50 mm; d = 30 mm; e = 100 mm; f = 60 mm; g = 50 mm; h = 50 mm

Figure 13

a = 110 mm; b = 60 mm; c = 140 mm; d = 50 mm; c’ = 120 mm; d’ = 50 mm i = 280 mm; j = 80 mm

Figure 15

a = 76 mm; b = 51 mm; c = 52 mm; d = 41 mm; k = 64 mm; I = 30 mm

Sound-attenuating performance data has been calculated for the panel units of the fifth to seventh embodiments compared to an un-attenuated opening of identical dimensions. The results were all normalized to an un-attenuated opening that represents about 3% of a fagade area. In terms of sound reduction this is equivalent to R _w 15, where R _w is an abbreviation for weighted sound reduction, as defined by British Standard BS717-1 : 2013 relating to Acoustics: sound insulation in buildings and of building elements.

With the exception of the first sound-attenuating member 10 on frame 4 in the seventh embodiment (figure 13), the sound-attenuating members of the fifth to seventh were filled with a porous material being mineral wool at a density of 80 kg/m ³ mineral wool (except where the air cavity is provided in each of the sound- attenuating members of the fifth embodiment).

Also with the exception of the first sound-attenuating member 10 on frame 4 in the seventh embodiment (figure 13), the sound-attenuating members have a metal housing made of a 1 mm thick steel sheet with 10% of the surface area being perforated with apertures that are 2 mm in diameter.

The first sound-attenuating member 10 on frame 4 in the seventh

embodiment has a metal housing made of a 2 mm thick steel sheet with 0.5% of the surface area being perforated with apertures that are 0.7 mm in diameter (the sheet is micro-perforated). The sound-absorbing volume of this sound-attenuating member does not contain a porous material and no ribs are present, although the present invention encompasses the presence of a porous material and/or ribs in this first sound-attenuating member.

The ribs in the other sound-attenuating members of these embodiments are made of steel and are 2mm thick.

The panel units were modelled in 2D using finite element analysis software. The sound-attenuating members of the panel units were modelled using the material properties known to correlate to sound absorption. The appropriate specification of source and receiver room was provided either side of the panel unit in question, so that the difference, and thus the composite sound reduction, could be calculated.

In this regard, mineral wool can be modelled using various formulations, such as Johnson-Champoux-Allard or Delany-Bazley-Miki. The latter only requires knowledge of the flow resistivity. A value of 20 kPa.s/m ² was used in these tests.

The source and receiver spaces used to calculate the composite sound reduction were both free-field environments and grids to evaluate the data were described within 1500 mm of both sides of the opening of the panel unit. The ‘noise’ incident on the opening was modelled using eight monopole sources at various positions to simulate noise arriving from many different directions.

Figure 16 is a graph of frequency (Hz) against composite sound reduction (dB) showing the results of these tests.

The solid line represents the parallel-opening window of the fifth

embodiment (figure 11 ), with a sound reduction of R _w 24.

The dashed and dotted line represents the sliding window of the sixth embodiment (figure 12), with a sound reduction of R _w 25.

The dashed line represents the tilting (hinged) window of the seventh embodiment (figure 13), with a sound reduction of R _w 25.

It was found that the panel units of the fifth to seventh embodiments provide a 9 to10 dB(A) improvement over the baseline case of an open window. The y-axis is labelled‘composite sound reduction’ because the results are for panel units installed within a solid fagade, so the calculations are not based on the panel units in isolation.