This application claims priority to GB2112928.3, filed 10 September 2021.

Field of the Invention

The present invention relates to boxed loudspeakers for harsh environments and particularly, although not exclusively, to boxed loudspeakers for mounting to the exterior of vehicles and crafts, such as, for example, land vehicles.

Background

Boxed loudspeakers generally comprise an enclosure (or speaker enclosure) that houses a speaker unit, and boxed loudspeakers for harsh environments are known. Typically, the boxed loudspeaker is mounted externally to a vehicle or craft and therefore exposed to the uncontrolled external environment around the vehicle or craft. Here, the harsh environment may expose the boxed loudspeaker to a wide fluctuation of humidity, temperature, air pressure and other environmental conditions. The boxed loudspeaker for harsh environments is engineered to continue the desired operation, for example to emit an audible sound, across the expected environmental conditions.

The speaker unit of known boxed loudspeakers for harsh environments typically generate sound pressure through movement of a diaphragm attached to a motor system (e.g. an electromagnetic motor). The diaphragm is suspended at a rest position from which it has limited movement range in both directions. Sound is generated when the diaphragm oscillates around this rest position and excites the air around it. For reasons of efficiency, the diaphragm and components that move together with it are designed to be as lightweight as possible. Also, the surface area of the diaphragm is maximized so that it can move the most amount of air and be acoustically efficient. Here, the diaphragm is a dipole sound source, radiating sound to both directions of its movement axis. The acoustical radiation is in opposite phase for each of these directions. Therefore, to generate an audible sound, typically one side of the diaphragm is isolated to prevent the two opposite phases being allowed to sum, which would cause the radiation on these two sides to cancel each other, causing a compromised sound output, for instance a reduction in generated sound. The isolation of one side of the diaphragm can be achieved by mounting the diaphragm in an aperture in the speaker enclosure and forming the speaker enclosure surrounding the diaphragm to form a sealed enclosure. In this way, the opposite side radiation is not able to interfere with the radiation on the listener side in a destructive way.

Accordingly, known boxed loudspeakers for harsh environments have a diaphragm with a listener side open to the harsh environment and an opposed, isolated side typically housed in a sealed enclosure that is isolated from the harsh environment. Whilst the isolation prevents the sound cancellation, it is also known that if the static air pressure on the listener side of the diaphragm is not the same with the isolated side, a net force will be created on the diaphragm. This net force is known as static force F _n ,s. The static force Fn.son the diaphragm can cause it to shift away from its rest position, reducing acoustical performance and putting mechanical stress on components. Where the environmental conditions that the loudspeaker is installed change abruptly, there may occur large shifts of the pressure inside the sealed enclosure box. Most boxed loudspeakers are not designed to handle such a force and can undergo failures.

One practice to avoid failure due to changes in static force F _n , _s is to include venting routes through the speaker enclosure that creates the sealed volume (other than the venting routes) to the isolated side of the diaphragm. The venting routes act to equalize the pressure within the speaker enclosure with the outside environment. The venting routes are typically small so as not to provide a route for sound to radiate from the isolated side of the diaphragm that can compromise the generated sound. Whilst a small opening may provide pressure equalisation for gradual pressure changes, small openings can’t typically cope with sudden pressure changes. Also, the small openings can let dust and I or water or other foreign bodies into the isolated cavity.

An alternative practice to avoid failure due to changes in static force F _n , _s is to utilize a stiff suspension design for the diaphragm, which would reduce the displacement of the diaphragm under the static force Fn,s. However, the suspension stiffness affects the mobility and acoustical performance of the speaker unit. Moreover, the speaker unit and therefore also the boxed loudspeaker would typically become more bulky and heavier.

In addition, the speaker unit can be designed to accommodate the high potential static force F _n ,s. But again, this typically generates a greater weight and I or bulk and I or expense.

A particular example of a boxed loudspeaker for harsh environments is a boxed loudspeaker for mounting to the exterior of a land vehicle, for example a car. Such boxed loudspeakers can be referred to as Acoustic Vehicle Alerting Systems (AVAS). Here, one purpose of the boxed loudspeaker is to alert pedestrians to the presence of the vehicle. For instance, in electric drive vehicles, it can be a requirement for the vehicle to generate a warning sound when the vehicle is travelling at low speeds. Typically, the audible sound employs a frequency range of around 200Hz-2kHz. AVAS boxed loudspeakers are typically located at the front or rear of the vehicle so that they are positioned close to potential pedestrians in directions of travel of the vehicle. Typically, the front of the vehicle may have a highly enclosed engine compartment restricting location possibilities. Moreover, the aesthetics and aerodynamics of the vehicle can be affected by the mounting location of the AVAS boxed loudspeaker. Consequently, common locations for mounting the boxed loudspeaker are under the radiator and headlights or taillights, where holes can be opened up in an under-body panel or bumper for the listener side of the diaphragm to be exposed. The AVAS boxed loudspeakers are therefore often located close to the ground. Therefore, the boxed loudspeaker is exposed to a harsh environment, for instance, the boxed loudspeaker may be plunged into water if the vehicle drives through a body of water. The AVAS boxed loudspeaker may also be placed in proximity to the engine that may generate heat and further generate a harsh environment.

As explained, where the boxed loudspeaker for a harsh environment is designed with a enclosure (e.g. sealed or with substantially sealed with venting routes) to isolate one side of the diaphragm, the pressure inside the sealed enclosure can change due to a shift in the exterior temperature. A significant pressure change can be created when the AVAS boxed loudspeaker is plunged into water that has a temperature lower than the general temperature of the sealed enclosure. Here, rapid changes in temperature can cause the air in the speaker enclosure to expand and contract, creating a static net force Fn sthat pushes the diaphragm in and out of the speaker enclosure. Some AVAS boxed loudspeakers having a sealed enclosure surrounding the isolated side of the diaphragm have been known to fail when heated and then plunged into cold water, for instance by deformation of the diaphragm. It has also been found that the pressure drop can create a failure in the speaker enclosure, e.g. the plastic housing of the boxed loudspeaker. One example of a harsh environment that may generate a sudden change in temperature is submersion in cold water, which may occur due to the location of the AVAS boxed loudspeaker on the vehicle. Here, not only can the sudden change generate the destructive static force Fn.s, but any vents in the sealed enclosure to provide a pressure equalisation can allow egress of water that can also affect the speaker operation if the water is not drained. For instance, water weighing on the moving parts of the speaker unit (e.g. diaphragm) would reduce the amount of vibrations it can make. Water can also damage components that are not waterproof. Moreover when submerged in cold water, any holes or gaps in the sealed enclosure (e.g. venting routes) will be sucking water to the inside of the sealed enclosure. Therefore, pressure equalization through simple venting is not considered a fully effective solution for submersion in water or other fluids.

The present invention has been devised in light of the above considerations. In particular, it is an aim to provide a boxed loudspeaker for harsh environments that remains operational after being submerged in water generating a sudden temperature shock to the boxed loudspeaker.

Summary of the Invention

According to the present invention, the speaker enclosure of the boxed loudspeaker is an isolation enclosure arranged to provide a variable volume of trapped air. Suitably, the variable volume of the isolation enclosure is provided by dividing the isolation enclosure into a wet volume and a dry volume where the wet and dry volumes are connected by a chimney. An opening from the external environment is provided through the isolation enclosure to the wet volume. And the entrance to the chimney from the wet volume is configured to be arranged above the opening when installed. Advantageously, a fluid path is provided from the dry volume that surrounds the isolated side of the diaphragm to the external environment via the chimney and wet volume. When the boxed loudspeaker is not submerged in water, the fluid path provides pressure equalisation between the sides of the diaphragm to reduce static force F _n ,s. Here, the chimney provides a function of acting as an acoustic filter to reduce sound leakage from the isolated side of the diaphragm along the fluid path. The wet volume acts to trap air in the isolation enclosure when the boxed loudspeaker is submerged into water above the opening. If a temperature drop occurs in the volume of trapped air in the isolation enclosure when submerged, the pressure in the trapped air also drops, which acts to suck in water into the wet volume. By sucking in water, the volume of the trapped air is reduced. Thus, a variable volume of trapped air is provided. Due to the ideal gas law, by reducing the volume of the trapped air as the temperature drops, the corresponding pressure drop in the trapped air can be minimised and therefore the pressure differential between the two sides of the diaphragm can also be minimised. With a minimal pressure differential the static force F _n , _s is also minimised even when the loudspeaker is submerged.

According to the exemplary embodiments, there is therefore provided a boxed loudspeaker for harsh environments where the loudspeaker includes a sound transmitter, e.g. speaker unit, having a diaphragm arranged to vibrate to produce an audible sound. The diaphragm has a listener side exposed to an exterior environment of the boxed loudspeaker. The opposed side of the diaphragm to the listener side is enclosed within an isolation enclosure. The isolation enclosure is adapted to provide a variable volume of trapped air when the boxed loudspeaker is submerged in water.

In the exemplary embodiments, the variable volume of trapped air is provided by separating an internal surface of the isolation enclosure into a wet volume and a dry volume. The wet volume and dry volume are isolated from each other except for a fluid passage defined by a chimney. Suitably, the wet and dry volumes are separated by a divider, for instance the divider may be a wall or a plate extending across the isolation enclosure. For instance, the divider extends between opposed portions of the internal surfaces of the isolation enclosure. The divider therefore provides the isolation between the wet and dry volumes.

In the exemplary embodiments, the dry volume is the volume enclosed by a portion of the internal walls of the isolation enclosure that surround the isolated side of the speaker unit when bounded by the diaphragm and motor system, a dry side surface of a divider that separates the isolation enclosure into the isolated wet and dry volumes, and any walls of the chimney that protrude into the dry volume. Here, the boundary of the dry volume may form a dry enclosure and the dry enclosure and dry volume may be substantially the same.

In the exemplary embodiments, the wet volume is defined as a volume between upper and lower vertical extents. Here, the lower vertical extent is provided by a lower horizontal plane through an uppermost portion of the opening to the isolation enclosure. That is, a horizontal plane through a top of the opening. The upper vertical extent is provided by an upper horizontal plane through a lowermost portion of the entrance to the chimney from the wet volume. That is, a horizontal plane through a bottom of the entrance to the chimney from the wet volume. The wet volume is further bounded by sides of the isolation enclosure and I or sides of the chimney that intersect the upper and lower horizontal planes. Here, a wet enclosure may be formed by a portion of the internal walls of the isolation enclosure, when bounded by a wet-side surface of a divider that separates the isolation enclosure into the isolated wet and dry volumes, and any walls of the chimney that protrude into the wet volume. The wet enclosure is therefore typically larger than the wet volume. For instance, the wet enclosure typically may include upper and lower excess volumes. Here, the lower excess volume is the volume of the wet enclosure that may fill with water before the opening to the isolation enclosure is fully submerged. That is, the lower excess volume is the volume of the wet enclosure below the wet volume. And the upper excess volume is the volume of the wet enclosure that would not be filled with water until the chimney was also full with water. That is, the upper excess volume is the volume of the wet enclosure above the wet volume.

The wet volume provides a substantial volume relative to the size of the housing. Here, the wet volume is a substantial volume by being at least 25% or at least 30% or at least 35% or at least 40% or at least 45% of the total volume of the sealed enclosure formed by the housing. Alternatively, the wet volume may be a substantial volume by reference to the dry volume. For instance, the wet volume may be at least 50% or at least 60% or at least 70% or at least 80% or at least 95% of the volume of the dry volume.

Preferably however, the wet volume is substantially equal to a combined volume of the dry enclosure, the upper excess volume and the volume defined by the chimney. Here, the chimney volume is defined by the walls of the chimney between the entrance to wet and dry volumes respectively. That is, it has been found that to compensate for an air pressure drop in the sealed enclosure that is 0.5bar lower than the water pressure, the trapped volume needs to be reduced approximately in half. This means the wet enclosure needs to be sized such that water sucked into the wet enclosure is approximately half of the volume of the trapped air. Here, since, the trapped air is defined by the dry volume, the wet volume, the chimney volume and the upper excess volume, assuming the upper excess volume and the chimney volume are relatively small, to be sized to allow the volume of trapped air to be reduced by half without the water entering the chimney, the wet volume may suitably be sized to have a volume similar to the volume of the dry volume. The respective volumes can be adapted based on an intended pressure drop. Thus, it will be appreciated that the wet volume may suitably be sized relative to the dry volume, so that the dry volume is smaller than the wet volume. That is, the volume of the wet volume may be greater than the dry volume, for instance, the volume of the wet volume may be sized to be similar to the combined volume of the dry volume and the chimney volume, or the combined volume of the dry volume, chimney volume, and upper excess volume, or greater. However, in some embodiments it is envisaged that the wet volume may suitably be sized relative to the dry volume, so that the wet volume is smaller than the dry volume. In the exemplary embodiments where the wet volume is smaller than the dry volume, it is envisaged the minimum cross-sectional area of the chimney (Sp), where the cross-sectional area is relative to the direction of the acoustical radiation’s path along the chimney from the respective entrances to the wet and dry chambers, is less than about 5% or less than about 3% or less than about 1% of the total effective radiation surface area of the acoustical diaphragm (Sd). Thus, in a particularly suitable embodiment, Sp<Sd/100. Under the envisaged conditional parameters, it is considered the chimney will be too small to allow a significant volume velocity of air to pass, which in turn means the chimney section is an ineffective acoustical radiator. But the chimney still allow air flow at low speeds, which allows the air pressure between wet and dry volumes to be balanced when the boxed loudspeaker undergoes thermal-shock submersion, In the exemplary embodiments where the wet volume is smaller than the dry volume, it is further envisaged the tuning frequency of the chimney (fp, p is for port) is at least one octave lower than the first resonance frequency of the speaker unit in the closed enclosure (fc, c is for closed). Designing the tuning frequency of the chimney (fp) to be at least an octave away from the resonance frequency of the speaker in box (fc), means the speaker will be exciting the acoustical mass in the chimney very little due to the frequency mismatch. Here, the chimney will be acting simply as a filter that is not letting sound through in the usable frequency range of the speaker. Hence, it will not contribute acoustically to the radiation of the diaphragm at the listener’s side. Consequently, in a suitable embodiment fp < fc / 2. In some exemplary embodiments, the wet volume and cross-section and depth of the opening through the isolation enclosure are sized to act as an acoustical filter, where the opening and wet volume act as a mass-spring system. This acoustical filter can be tuned to filter the sound coming from the chimney if the chimney is not tuned to complement the audible sound from the listener side of the diaphragm. In another embodiment, the chimney can be designed to complement the audible sound from the listener side of the diaphragm. Here, suitably, the opening and wet volume is sized to have a natural frequency that is similar to or above the natural frequency of the chimney, and therefore not acting as a filter and allowing transmission of the sound from the chimney to the outside environment. The opening area may also be sized to be equal or larger than the chimney cross-sectional area. Here, the opening into the wet enclosure from the chimney (i.e. the entrance to the chimney from the wet volume) is discretely separated from the opening between the wet enclosure and the environment (i.e. the opening to the isolation chamber). That is, the wet enclosure separates the opening to the chimney from the opening to the environment. Here, the wet enclosure has an increased cross-sectional area to the opening to the chimney. Thus, when the boxed loudspeaker is designed to complement the acoustical output, the opening from the chimney to the wet volume opens into the larger wet chamber (in the cross-sectional area across the acoustical radiation’s path through the boxed speaker), on its way to the entrance to the environment.

In the exemplary embodiments where the chimney extends into the wet volume so that walls of the chimney add to the boundary of the wet volume, the wet volume can be maximised by minimising the cross sectional area of the chimney, without increasing the overall size of the isolation enclosure. Maximising the wet volume increases the pressure drop that the wet volume can compensate for without water entering the chimney through the opening to the chimney from the wet enclosure.

In the exemplary embodiments, the chimney provides a sealed passageway between an entrance to the dry volume and an entrance from the wet volume. The volume of air trapped in the chimney and the cross-sectional area of the entrances can be arranged to act as an acoustical filter. For instance, the mass of air trapped in the chimney can be configured to block sounds above a certain frequency. Here, the chimney characteristics and the dry volume and the wet volume can be arranged to tune the resonance frequency of the mass of air trapped in the chimney to act as an acoustic filter. For instance, a cross-sectional area of the chimney, the chimney length between the entrances to the respective wet and dry volumes, and the dry volume can be configured to tune the resonance frequency. In some exemplary embodiments, the dry volume defines a substantial box shaped enclosure. A height of the box shaped enclosure may be similar to the width. In some exemplary embodiments, the box shaped enclosure is a cylinder, where the width is a diameter. Here, the length of the chimney is arranged to be less than the height of the box shaped enclosure. The length of the chimney, is suitably also greater than 60% or greater than 70% or greater than 75% of the height of the box shaped enclosure. For instance, suitably, the length of the chimney is around 80% of the height of the box shaped enclosure defining the dry volume. Suitably, the cross section area of the chimney and the entrance from the wet enclosure to the chimney dos not restrict airflow. For instance, the entrance from the wet enclosure to the chimney is arranged spaced from a wall of the isolation enclosure so that a cross-sectional area between the opening and the isolation enclosure is around at least the cross-sectional area of the chimney and sufficiently large to allow the sound transmission without significant losses.

For the avoidance of doubt, the relative terms: above, below, upwards, downwards, vertical, horizontal and the like terminology; are provided in relation to the intended orientation of the boxed loudspeaker. For instance, the terms reference the orientation of the boxed loudspeaker when installed in a vehicle. It will be appreciated that the installation orientation of a boxed loudspeaker is generally readily discernible.

In some exemplary embodiments, the chimney extends in an upwards direction. Here, the wet volume is arranged to be substantially above the dry volume. However, in alternative exemplary embodiments, the chimney extends in a horizontal direction. Here, the wet volume is arranged to the side of the dry volume.

In some exemplary embodiments, the chimney is arranged to provide at least a portion of the isolation enclosure. That is, the chimney extends adjacent the isolation enclosure. However, in alternative exemplary embodiments, the chimney is arranged to extend spaced from the isolation enclosure. For instance, the chimney extends from a centre area of the isolation enclosure. Here, the chimney is surrounded by the wet and I or dry volume.

In the exemplary embodiments, the isolation enclosure is suitably a housing in which a sound transmitter having the diaphragm is housed. The isolation enclosure provides a hermetic enclosure to the isolated side of the diaphragm. The opening to the wet volume is suitably the only substantial opening through the isolation enclosure to the dry volume. That is, when the boxed loudspeaker is submerged, water is only able to egress into the dry volume along the fluid passage defined by the opening into the wet volume and the entrance in to the dry volume from the chimney. Here, the opening to the wet volume may comprise one or a plurality of apertures through the isolation enclosure. In embodiments where the opening is formed from a plurality of apertures, references to the size if the opening may suitably be references to a combined size of the apertures. Thus, suitably, for designs where the chimney and the openings are designed as acoustical resonators to complement the loudspeaker sound generation, the aperture(s) though the isolation enclosure may have a cross-sectional area or a combined cross-sectional area substantially similar to the cross-sectional area of the chimney.

In the exemplary embodiments, a sound transmitter comprising the diaphragm includes a motor system, for instance an electromagnetic motor, for driving the diaphragm to vibrate. Suitably, the diaphragm has a cone shape and is designed to be lightweight and with the surface area of the diaphragm maximised. The diaphragm may be formed from pressed paper or the like as is known in the art. As explained, the isolation enclosure is suitably a housing for the sound transmitter. Here, the diaphragm may be sealed within a diaphragm aperture in the isolation enclosure. For instance, the diaphragm may be suitably sealed to the isolation enclosure around a periphery of the diaphragm and diaphragm aperture.

It will be appreciated that according to further exemplary embodiments, there is provided a craft or vehicle having the exemplary boxed loudspeaker for harsh environments installed.

The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

Figure 1 shows a schematic representation of an acknowledged art boxed loudspeaker;

Figure 2 shows a schematic representation of a side view of a vehicle showing convenient location areas for an AVAS boxed loudspeaker;

Figure 3 shows a schematic representation of a front view of a vehicle showing convenient location areas for an AVAS boxed loudspeaker;

Figure 4 shows an underside view of a front of a vehicle showing convenient location areas for an AVAS boxed loudspeaker;

Figure 5 shows an underside view of a rear of a vehicle showing convenient location areas for an AVAS boxed loudspeaker

Figure 6. shows a schematic cross-sectional view of an exemplary boxed loudspeaker;

Figure 7 shows a schematic cross-sectional view of the exemplary boxed loudspeaker of Figure 6 when submerged in water;

Figure 8 shows a schematic cross-sectional view of the exemplary boxed loudspeaker of Figure 6 when submerged in water including a corresponding drop in temperature;

Figure 9 shows a graph of acoustical resonance frequencies of the chimney by function of dimensions of the loudspeaker of Figure 6;

Figure 10 shows a schematic cross-sectional side view of a further exemplary loudspeaker;

Figure 11 shows a schematic cross-sectional side view of a further exemplary loudspeaker; and

Figure 12 shows a schematic cross-sectional side view of a further exemplary loudspeaker.

Detailed Description of the Invention

Aspects and exemplary embodiments will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

Figure 1 shows an acknowledged art boxed loudspeaker as discussed in the background above. The boxed loudspeaker 10 comprises a housing 20 having a diaphragm aperture 22. Suitably, the housing 20 comprises a speaker box. The boxed loudspeaker 10 further includes an audible transmitter 30 (or speaker unit) arranged in the housing with respect to the diaphragm aperture. As is known in the art, the audible transmitter 30, is shown suitably as a dipole loudspeaker comprising a motor system 32 and a diaphragm 34. It will be appreciated that the audible transmitter comprises further components as is known, for instance suspension elements etc. Indeed, it is envisaged that the exemplary loudspeaker embodiments described herein will be suitable for use with known audible transmitters. That is, the known audible transmitters, will not have to be specifically adapted beyond general workshop variations. Whilst a detailed explanation of the audible transmitter (e.g. dipole loudspeaker) is not thought necessary, In brief, the diaphragm and driver may take various forms as is known in the art.

For instance, in exemplary audible transmitters, the diaphragm may be a single (monolithic) piece of material. The material forming the diaphragm is suitably lightweight, for instance, the material suitably has a density of 0.5 g/cm3 or less. In exemplary embodiments, the material is extruded polystyrene or extruded polypropylene or similar. In some examples, the diaphragm is covered by a skin, e.g. to protect the diaphragm. Suitably, the skin is formed from paper, carbon fiber, plastic foil, or the like. In some exemplary embodiments, the diaphragm includes several pieces of material attached together. For instance, the diaphragm includes several pieces of material attached to each other by glue. Here, the diaphragm may include a first cone and a second cone being first and second pieces attached together, wherein suitably the first and second cones are glued back to back to attach each cone to the other cone. Here, a front surface of one of the cones forms the first radiating (e.g. front) surface and a back surface of the other cone provides the second radiating (e.g. back) surface. In one exemplary embodiment, the first and second cones are formed from paper. The diaphragm may further include one or more cut-outs in one of the radiating surfaces (preferably the second radiating surface), wherein each cut-out is configured to have a respective rigid supporting element extend through it when the speaker unit is in use. This may allow the speaker unit to have a lower profile in the thickness direction of the diaphragm. Suitably, the diaphragm is watertight.

Further, in the exemplary embodiments the motor system suitably includes an electromagnetic drive unit that includes a magnet unit configured to produce a magnetic field, and a voice coil attached to the diaphragm. In use, the voice coil may be energized (have a current passed through it) to produce a magnetic field which interacts with the magnetic field produced by the magnet unit and which causes the voice coil (and therefore the diaphragm) to move relative to the magnet unit. The magnet unit may include a permanent magnet. The magnet unit may be configured to provide an air gap, and may be configured to provide a magnetic field in the air gap. The voice coil may be configured to sit in the air gap when the diaphragm is at rest. Such motor systems are well known. The magnet unit may be located in front of the second radiating surface of the diaphragm.

Referring back to Figure 1 there is shown an audible transmitter 30, such as a dipole loudspeaker mounted in the housing 20. The dipole loudspeaker 30 comprises a diaphragm 34 having a first radiating surface 35 and a second radiating surface 36. The first surface 36 is shown as a front surface, or a listener side, which faces outwards through the diaphragm aperture 22, and the second surface 36 is shown as the opposed rear surface, or isolated surface which faces into the housing 20. The dipole loudspeakers drive unit 32 is configured to drive the diaphragm 30 to produce audible sound as is known in the art. Consequently, in operation, the dipole speaker 30 is operational to drive the first radiating surface 35 to emit a sound (shown by ‘plus’ signs) and the second surface 36 to emit a response 180 degrees out of phase (shown by ‘minus’ signs). This out of phase response is an anti-phase sound and if allowed to interfere with the sound from the first surface would act to cancel out and compromise the sound. Consequently, the second surface of the diaphragm is isolated by being housed in the housing 20 that acts as a substantially sealed enclosure.

In the acknowledged art embodiment shown in Figure 1 , the housing provides an substantially sealed enclosure for the second or isolated surface of the diaphragm. Here, the diaphragm is installed in the housing so that sound does not escape between the diaphragm and periphery of the diaphragm aperture. The housing is a substantially sealed enclosure. Typically a box or a cylinder. The antiphase sound is therefore prevented from interacting with the sound from the first surface by the housing. As explained above, when adapted for harsh environments, it is known to provide venting channels (shown in Figure 1 pictorially as a capillary channel 24) for pressure equalisation between the interior volume of the isolated enclosure and the external environment to which the first surface is exposed.

A suitable example of a boxed loudspeaker for harsh environments is in an Acoustic Vehicle Alerting System (AVAS). As will be understood, in an AVAS the boxed loudspeaker 10 for harsh environments is mounted to a vehicle 40. Referring to Figure 2, the loudspeaker 10 is typically installed in a front 42 and I or rear 44 area of the vehicle so that it is in proximity to any pedestrians in a direction of travel of the vehicle. As will be appreciated, the boxed loudspeaker is located close to the bodywork of the vehicle so that the listener surface of the diaphragm is located in a position near to the pedestrian. As shown in Figures 3 and 4, common convenient free spots for locating the boxed loudspeaker 10 are under the radiator and headlights I taillights. Furthermore, referring to Figure 5, it is common to open holes in an under-body panel or bumper. Here, the boxed loudspeaker might be arranged so that the listener side of the diaphragm is arranged within the hole, or the hole might be an acoustic opening such as mesh or a perforated area (for instance as shown in Figure 5). In any event, the boxed loudspeaker is installed on the vehicle 40 so that the listener side of the boxed loudspeaker’s diaphragm is exposed to the external environment of the vehicle. It is also envisaged that the boxed loudspeaker could be manufactured with the housing integral to one or more parts of the vehicle, such as bodywork parts. Moreover, one or more boxed loudspeakers may be installed on a vehicle at spaced locations.

Figure 6 shows an exemplary embodiment of a boxed loudspeaker 10 for a harsh environment. In general, the boxed loudspeaker 10 is suitably a speaker unit for an AVAS, but as the problem and solution herein provided are not limited to AVAS, the further disclosure will be given to a general boxed loudspeaker suitable for use in a harsh environment. Thus, the boxed loudspeaker 10 is suitable or adapted for being mounted or assembled or firmed integrally to a vehicle, for instance a vehicle’s bodywork. Consequently, the boxed loudspeaker 10 has a defined installation orientation, defining a vertical and horizontal direction. References therefore to vertical and horizontal or the like relative orientation terms are to the boxed speaker’s intended installation orientation, for instance, when installed in a vehicle. Generally, the speaker 10 may be intended to be installed with a plane of the diaphragm being installed in either a vertical or horizontal plane. Or at least one wall or surface of the loudspeaker’s housing is intended to be installed in a horizontal or vertical plane. In these circumstances, as appropriate, references to vertical and horizontal or the like relative terms may be provided in relation to the features intended to be mounted in a horizontal or vertical plane. In particular, it is generally clear from a loudspeaker the intended mounting orientation. In the exemplary embodiment shown in Figure 6, the diaphragm is arranged in a generally horizontal plane. Moreover, the housing 20 has a generally orthogonal cross-section such that walls of the housing are generally arranged in vertical and horizontal planes. For instance, it is envisaged that the housing 20 may take the form having a generally box shape, wherein the housing has a square or rectangular cross-section in both horizontal and vertical planes, or a cylindrical shape, for instance, the housing has at least a circular or oval cross-section in one of the horizontal or vertical planes and typically in a plane orthogonal to an axis of the diaphragm. By way of further example, Figure 11 shows an exemplary embodiment wherein the diaphragm is arranged in vertical orientation. It will be appreciated however, that the exact shape of the housing may be influenced by the particular application, and in particular, any constraints on the shape of the housing dictated by the space in which the loudspeaker 10 is to be assembled or fitted.

The boxed loudspeaker 10 shown in the exemplary embodiment of figure 6, comprises a housing 20 defining an isolation enclosure that surrounds the rear, or isolated side 36 of the diaphragm 34. Assuming the housing 20 is designed with a generally circular cross-section in an axial direction of the diaphragm, the housing 20 therefore has a planar, circular top and bottom walls 23, 24 and a circumferential wall 25. The diaphragm aperture 22 is formed in the bottom wall 24, with the diaphragm 34 sealed therein. The listener side 35 of the diaphragm is therefore exposed to the external environment of the loudspeaker 10; that is externally to the housing 20. Internal surfaces of the housing walls are formed integrally or otherwise joined in an appropriate manner to create the isolation enclosure. The walls are shown as plates or consistent thickness walls, but it will be appreciated that other designs may be employed and other embellishments added, in particular, for instance mounting brackets or the like.

As shown in Figure 6, the isolation enclosure is provided with a variable volume created at the isolated side of the diaphragm. Suitably, the variable volume is provided by separating the housing into a dry volume 60 and a wet volume 70. Here, the separation is shown as suitably being provided by a divider 50. The divider 50 provides a separation of the isolation enclosure. For instance, the divider 50 provides a separation between opposed internal surfaces of the housing. The divider is shown as extending generally horizontally, for instance, parallel to the top and bottom walls 23, 24. However, again, the exact design of the divider can be varied to suit the application or installation. A chimney 80 connects the wet and dry volumes. The chimney is shown as extending from an aperture in the divider. Here, the aperture in the divider 50 creates an entrance 81 to the chimney 80 from the dry volume 60. The chimney is generally elongate, and as herein described is suitably sized so as to act as a frequency filter to limit sound leakage from the dry volume. At an opposed end to the opening to the dry volume, the chimney has an opening 82 from the wet volume 70. The chimney is suitably shown as a being formed from a chimney wall, for instance, the chimney wall may be a tube or the like. As will be appreciated, to isolate the wet and dry volumes, except via the chimney, the chimney is suitable a single piece with the divider or suitably joined.

An opening, shown as first and second opening apertures 84, 85 is formed through the housing 20 forming the isolation enclosure to provide a venting passage from the exterior to the wet volume. The first and second opening apertures provide the opening, and the opening is sized so as to allow water to move into and out of the wet volume when the housing is submerged in water. The opening, (that is the first and second opening apertures 84, 85) is arranged below the chimney’s entrance 82 that provides a fluid route from the wet volume to the chimney and therefore the dry volume. The wet volume 70 is therefore defined as being bounded by interior walls of the housing when intersected by upper and lower horizontal planes. The lower horizontal plane is defined through an uppermost extent of the opening (e.g. the uppermost extent of one of the opening apertures 84, 85) and generally indicated by level C in figure 6, which indicates a water level at which point the opening is completely submerged and therefore trapping a volume of air in the isolation enclosure. The upper horizontal plane is defined through a lowermost extent of the opening 82 to the chimney 80, which is generally indicated by level D in figure 6 that shows a maximum water level before the wet volume is filled and water would begin to enter the chimney. It will be appreciated that a volume defined between the interior walls of the housing and the divider minus the intrusion of any part of the chimney (that is a volume of the isolation enclosure not divided into the dry volume or the chimney) is larger than the wet volume. Thus a volume of a wet enclosure formed by the divider and chimney can be defined as the wet volume plus an upper excess volume and a lower excess volume. Here, the upper excess volume is the volume inside the isolation enclosure above the upper horizontal plane through a lowermost extend of the entrance 82 to the chimney. That is, the upper excess volume is suitably the volume of the isolation enclosure above level D as shown in Figure 6. The lower excess volume is the volume of water that ingresses into the wet enclosure before the opening is completely covered trapping air. For instance, as shown in Figure 6, the lower excess volume is the volume between the inner surfaces of the housing, above the divider but below the lower horizontal plane through the uppermost extent of the opening (that is, as shown in Figure 6, the volume of the wet enclosure between levels B and C).

The dry volume is the volume of the isolation enclosure bounded by the divider, the intrusion of the audio transmitter and any intrusion of the chimney. Here, the chimney 80 is shown as not intruding into the dry volume in Figure 6 (the chimney is formed above the divider in the wet enclosure). However, for instance, in Figure 12, an example is shown where the chimney 80 extends also into a dry enclosure 62, in which case a volume of the dry volume 60 is lower than a volume of the dry enclosure 62. But in Figure 6, the volume of the dry volume 60 is the same as the volume of the dry enclosure 62. The dry volume being the volume of air that is contained in the isolation enclosure to the dry side of the divider.

How the boxed loudspeaker 10 provides the variable volume of the isolation enclosure is explained with reference to Figure 7 and 8 when the boxed loudspeaker is plunged into (e.g. submerged) water. It will be appreciated though that, where the boxed loudspeaker is submerged up to a maximum level just beneath a lowermost extent of the opening (i.e. a lowermost extent of one of the opening apertures 84, 85), the opening act as vents, venting the isolation enclosure so that the pressure can be equalised between the exterior environment and the dry volume. Here, the opening (e.g. the opening apertures 84, 85) may be covered with filters to prevent debris of other particular contaminants entering the isolation enclosure. However, the tortuous path between the opening and the dry volume may also provide a prevention of contaminants entering the dry volume and interfering with the audio transmitter.

Additionally or alternatively, the entrances to the chimney may also be provided with filters. As the water level rises between level B and level C, which is between horizontal planes defining the lowermost extent of the opening and the uppermost extent of the opening, the opening begins to be covered with water and the lower excess volume of the wet enclosure becomes filled with water. At this point, some venting of the pressure is maintained through the opening, but as the water level nears level C, the opening is restricted (or completely covered) to prevent air from escaping through the opening (e.g. through any of the opening apertures 84, 85). Approximated at level C, the horizontal plane through the uppermost extent of the opening, the opening is sealed by the water and a volume of air is trapped in the isolation enclosure.

Figure 7 depicts the boxed loudspeaker being submerged in a water level that seals the opening and traps a volume of air in the isolation enclosure. Here, the volume of trapped air can be approximated as a combined volume of the volume of the dry volume, a volume of the chimney, the volume of the upper excess volume and the volume of the wet volume. Here, pressure in the dry volume cannot be equalised with the pressure on the listener side of the diaphragm (i.e. the water pressure) and the problem of static force Fn, _s arises. Referring to figure 8, if the water generates a temperature drop in the trapped volume of air, the pressure drops in the trapped air. Here, the arrangement of the divider and chimney provides a variable volume to compensate for the temperature drop without a corresponding pressure drop. For instance, as the pressure in the trapped air drops, it acts to suck in water reducing the volume of trapped air. That is, the volume of the trapped air is reduced by the volume of water sucked into the wet volume. Figure 8 shows a position where the volume of water sucked into the wet enclosure and caused by the temperature drop equals the volume of the wet volume. Here, the pressure differential between the isolated side of the diaphragm and the listener side of the diaphragm has been minimised, which in turn minimises the static force F _n ,s.

From Figure 8, where the temperature drop acts to suck in water up to level D, once the water level drops beneath level C, the water will begin to drain from the wet enclosure through the opening. It will be appreciated that when the water is fully drained from the wet enclosure, the loudspeaker 10 is fully functional because the water was prevented from entering the dry enclosure and therefore damaging the audio transmitter. Moreover, the static force Fn,s was minimised meaning damage to the diaphragm or housing caused by the pressure drop in the trapped volume of air is limited.

Of course, if a sudden pressure drop caused by plunging the loudspeaker in to cold water, for instance, if the temperature was elevated due to close proximity to a heat source such as the engine, or indeed, the water was particularly cold, a volume of water greater than the wet volume could be sucked into the wet enclosure through the opening, which would raise the water level inside the wet enclosure above the horizontal plane through a lowermost extent of the entrance to the chimney (i.e. water level D). At this point water would enter the chimney and therefore the dry volume. The wet enclosure, and importantly, the wet volume, therefore need to be sized suitably to accommodate a temperature drop caused by a predetermined pressure drop. Here, it has been found that to accommodate for a pressure drop of around 0.5 bar that might be seen in an AVAS example, the variable volume of the trapped air in the isolation enclosure needs to reduce by about half. Thus, the wet volume can suitably be sized to be at least 50% of the volume of the trapped air. That is, the volume of the wet volume can be sized to be at least the same as the volume of the dry volume, and preferably, at least the volume of the combined volume of the dry volume, the chimney volume and / or the upper excess volume.

As described above, the chimney not only enables a compact design of the separation of the wet and dry volumes within the isolation enclosure, bearing in mind the requirement for the opening from the wet enclosure to the dry enclosure (e.g. the opening to the chimney), to be above the opening through the isolation enclosure (e.g. opening apertures 84, 85), as well as the wet enclosure to have a volume of at least the dry enclosure, but also provides a sound resistance to sound from the isolated side of the diaphragm escaping the isolation enclosure. Here, the volume of air held within the chimney combined with the volume of the dry enclosure and the wet enclosure acts as a spring-mass system, which creates an acoustical filter. The filter blocks the transmission of sound above its resonance frequency. Therefore, the resonance frequency of the air in the chimney 80 needs to be lower than the operating frequency of the audible transmitter. For instance, for an AVAS boxed loudspeaker, the operating frequency of the audible transmitter may be in the frequency of 200Hz-2kHz. Here, the resonance frequency of the chimney can be tuned to by controlling the dimensions of the boxed loudspeaker, and in particular a volume (V0) of the isolation enclosure, a chimney cross-sectional area (A), and a chimney height (he). Here, the chimney height (he) is suitably the distance between the entrance to the dry volume and the entrance from the wet enclosure or chamber. For instance, the extents of the chimney length may be planes orthogonal to the general longitudinal extent of the chimney, taken from an end of the chimney. Considering a design of the housing wherein the housing has a cylindrical shape as described above, wherein the internal diameter (ID) of the housing (i.e. the isolation enclosure) is approximately equal to the height (h) of the dry enclosure. And, as an example, the chimney height (he) is approximately 20% lower than the height (h) of the dry enclosure. Figure 9 shows the resonance frequency by function of the dimensions for different ID and de values. Preferably, smaller chimney diameters (de) assist in maximising the volume of the wet cavity, which increase the temperature drop that the variable volume can accommodate.

In some embodiments as explained above, the chimney and dry volume are designed to act as a springmass system to create an acoustical filter. Here the opening may be sized (e.g. the combined size of opening apertures 84, 85) with the wet volume to also act as a spring-mass system to act as a further acoustical filter. However, in alternative embodiments, the chimney acoustics is designed to complement the sound from the listener side of the diaphragm. Here, the natural frequency of the ‘opening-wet enclosure’ mass-spring system needs to be similar to or higher than the natural frequency of the ‘chimney-dry enclosure’ mass-spring system. This is because the acoustical filter filters out the frequencies above its natural frequency. So assuming the speaker unit in the isolation enclosure, without the chimney taken into account, has 5 grams moving mass and a stiffness of 6000N/m is attached to it (2000 from diaphragm suspension, 4000 from the air suspension applying on the diaphragm from the dry volume), then the resonance frequency of this speaker in box will be f_b=1/2n ^(6000/0.005)=175Hz. At 175Hz the diaphragm velocity is more or less maximum (when neglecting resistance and losses). Below 175Hz, acoustical output will be reduced. So to support the output below 175Hz, the chimney can be tuned to 150Hz, and therefore function around the frequency around 130Hz-175Hz range, boosting the output. To make sure this 130Hz-175Hz can get out of the wet enclosure, the natural frequency of the openings-wet enclosure system should be 175Hz and above so as not to filter out what is coming from the chimney. The total surface area of the opening (e.g. the combined surface area of the opening apertures 84, 85) should be large enough to allow the sound inside the wet enclosure to easily pass through them. So, for both the chimney and the opening, the cross-section area should be sufficiently large so as to be able to let the intended amount of ‘air volume/sec’ without significant losses. For instance, suitably, the opening total area would be equal or larger than the chimney cross-section area.

Figures 10 to 12 show alternative exemplary embodiments showing additional or alternative features by way of example and that may be combined in singular or combination with the exemplary embodiments described herein. For instance, the exemplary boxed loudspeakers 10 shown in Figures 10 to 12 each include a housing 20, forming an isolation enclosure in which a sound transmitter 30 is arranged. The housing 20 is separated by a divider into a dry volume 60 and a wet volume 70. A chimney 80 provides a fluid path between the dry and wet volumes. The chimney has an entrance 81 to the dry volume and an entrance 82 that is in fluid communication with the wet volume. The entrance 82 is arranged above an opening 83 through the housing 20 to provide a vent to the exterior environment. Thus, the isolation enclosure is provided with a variable volume that can accommodated temperature drops with minimal pressure differential between the listener and isolated sides of the speaker’s diaphragm which can cause destructive static force F _ns .

In Figure 10, a capillary vent 90 is shown in the upper portion, and in particular the upper wall of the isolation enclosure. The capillary vent has a small size suitable such that the capillary vent 90 does not allow significant water ingress even when submerged, but does allow some air flow, in particular to assist in the draining of the wet enclosure. For instance, if the opening (e.g. the size of each opening aperture) is sized to be large, the water can drain freely from the wet enclosure, allowing air bubbles through the opening aperture to allow the draining. With smaller opening aperture sizes, the air bubbles do not freely pass through the opening apertures to allow the drainage, and here, capillary channels higher up the box, for instance in fluid communication with the upper excess volume, assists in the draining by allowing replacement air to flow in. For instance, these capillary holes may be similar to prior art drainage venting holes sized to allow air but not water passage when the boxed loudspeaker is submerged.

In Figure 11 , an alternative arrangement of the sound transmitter is shown. In this embodiment, a diaphragm 34 is arranged in a vertical or upwardly extending plane. For instance, the diaphragm 34 is arranged to seal in the diaphragm opening through the isolation enclosure wherein the diaphragm opening is in a vertical or side wall of the housing. As will be appreciated and as herein explained, the housing may have a box shape, or suitably a cylindrical shape, where the cylindrical shape here may have an axis parallel or coincident with an axis of the diaphragm.

In Figure 12, rather than dividing the isolation enclosure generally horizontally, the divider is arranged vertically such that the wet and dry volumes are in a side-by-side arrangement. Here, the chimney 80 is arranged horizontally, with the entrances 81 , 82 to the chimney from the respective dry and wet volumes generally vertical. The opening 83 is shown in a lower wall of the housing 20, but other locations include a lower portion of the side walls of the housing. However, the general function of the boxed loudspeaker 10 as herein described is maintained such that the variable volume of the trapped air when a water level rises above the uppermost portion of the opening 83 is provided.

According to the exemplary embodiments herein described, a boxed loudspeaker for harsh environments is disclosed wherein a variable volume of trapped air is provide to accommodate for rapid temperature drops when the speaker is plunged into a body of water or the like. For instance, the boxed loudspeaker is particularly suitable for an AVAS, where a boxed loudspeaker might need to be mounted low in a vehicle due to space or other design constraints or preferences, such that the boxed loudspeaker is susceptible to being plunged into road water. Such as large puddles or floodwater. Advantageously, the variable volume is provided by separating an isolation enclosure around an isolated side of a dipole diaphragm, into a wet and dry volume. Where the wet volume provides a volume between an opening for egress of fluid and an entrance to the dry volume. Other than the entrance to the dry volume, the wet and dry volumes are isolated to control sound leakage. Suitably, the wet and dry volumes have a similar volume. Advantageously, connecting the wet and dry volumes by a chimney can enable a compact design of the housing creating the wet and dry volumes. The chimney can also act as an acoustical filter to restrict sound leakage from the dry volume that surrounds the diaphragm.

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/- 10%.