Field of the First Aspect of the Invention

A first aspect of the present invention relates to a device for aiding a person's hearing, and in particular for helping a person to hear sound from one direction while suppressing sounds that come from other directions.

Background of the First Aspect of the Invention

Human ears enable a person to hear sounds from all directions, but there are some circumstances in which it would be desirable to be able to hear sounds from one direction more clearly, while suppressing sounds from other directions. This would apply for example when having a conversation in a place where there is background noise from machinery or from other people talking. By way of example US 1 761 666 describes a form of ear trumpet with a generally rectangular aperture facing forward, and defining curved channels that lead to a discharge port which connects through a tubular extension to a nipple that can be positioned within the orifice of the wearer's ear. US 4 997 056 describes an acoustic device which includes parabolic reflectors that fit behind the ears, and which therefore focus sound into the ear. Both these devices would be very prominent when worn, and are not practical for everyday use.

Summary of the First Aspect of the Invention

Accordingly a first aspect of the present invention provides a device for aiding a person's hearing, the device comprising a tubular portion and a shell portion; the tubular portion defining a cylindrical bore and adapted to communicate at one end to a user's ear canal, and at the other end being connected to the shell portion; the shell portion defining a tapering channel that tapers down to a narrow end that communicates with the bore of the tubular portion, the tapering channel defining a longitudinal axis, and the shell portion also defining an outer shell which surrounds the longitudinal axis on at least two sides, but is open on at least one side; and the device comprising a reflector surface at the narrow end of the tapering channel to reflect sound into the bore of the tubular portion.

In use sound waves that enter the device along or substantially parallel to the longitudinal axis tend to reflect from the outer shell to travel down the tapering channel, and are then reflected by the reflector surface into the bore of the tubular portion, and so are transmitted into the user's ear canal. In contrast, sound waves from other directions that reach the outer wall of the shell portion will tend to be diffracted; if as a consequence they then hit an opposing part of the outer wall, they will tend to be reflected away by virtue of the divergent taper of the outer shell, while if as a consequence of the diffraction they would hit a side which is open, again they will not be transmitted into the tapering channel.

The reflector surface may be a 45° plane reflector, or may be a slightly curved or parabolic reflector.

The tubular portion may extend along a longitudinal axis which is orthogonal to the longitudinal axis of the tapering channel, but alternatively the angle between these longitudinal axes may differ from 90°, for example being between 70° and 120°.

The tapering channel may have a conical shape, or may have a number of flat surfaces with linear tapers, for example being pyramidal, or for example a six-faced or eight-faced pyramid. The outer shell may have a divergent tapering shape, and in this case the divergent taper of the outer shell may be continuous with the taper of the tapering channel. The outer shell may therefore be generally of frustro-conical shape, but with at least one side being open. Alternatively the outer shell may have a taper which is different to that of the tapering channel, or indeed the outer shell may not be tapered, for example being cylindrical (although with at least one side being open). The opening may have an arc length, relative to the longitudinal axis as centre, of at least 100° in plan, and may be more than 130°. Indeed the opening may correspond to an arc length of as much as 180°; in the case where the outer shell is cylindrical, the opening may correspond to half the cylinder.

The device may be of plastic, for example moulded hard plastic, or may be of metal such as aluminium or titanium. The device may be sufficiently large to be prominent in use, for example the shell portion being of width up to 50 mm and of length up to 75 mm; or alternatively may be sufficiently small to be much less noticeable and more convenient, for example having a shell portion of width no more than 20 mm, and preferably no more than 10 mm, and of length no more than 30 mm and preferably no more than 15 mm. For example a small device may have a shell portion of width 7 mm and of length 12 mm. In the case of the larger devices, a user may wear two such devices, one in each ear, linked by an adjustable headband, as is common with headphones. The smaller devices can be worn individually, one in each ear, being secured merely by engagement with the opening of the ear canal, in an analogous way to an ear plug or earphone. Typically the tubular portion, at the end that is to communicate with the user's ear canal, if it is to be inserted into the ear canal, has an external diameter substantially that of an earphone, and is provided with a resilient suspension cap, as commonly provided with earphones. The suspension cap is of a compressible material such as foam, memory foam, silicone or rubber; such a material has a significantly lower density and a much lower modulus of elasticity than the material of the tubular portion and shell portion. In some cases the suspension cap may be custom-moulded to an individual user’s ears, and may be made from silicone or acrylic.

In use, the device would be arranged to have the wide end of the shell portion, which is open, facing towards the direction from which the user wishes to hear sounds. It will be appreciated that although the device may not cut out unwanted low-frequency sounds, it can nevertheless considerably enhance the intelligibility of what the user hears, as the higher frequencies are where most of the intelligible information is found. So, for example, in a busy restaurant where many people are talking, the user will find that the higher frequencies emanating from around him are filtered out, while the higher frequencies from in front will be heard clearly.

It will also be appreciated that the device as described above is an entirely passive device. In a further modification the device may incorporate a miniature electronic microphone in the vicinity of the reflector surface, connected to a miniature driver which provides sound to the user's ear canal. This combination of electronic components might be provided within the tubular portion. Alternatively, in a second aspect of the invention, the miniature electronic microphone may be provided near the narrow end of the tapering channel; and the driver may be provided within the tubular portion. In this alternative the narrow end of the tapering channel would not have to communicate with the bore of the tubular portion, and the reflector surface would be omitted.

Field of the Second Aspect of the Invention

A second aspect of this invention relates to an acoustic device such as loudspeaker, or a housing for a loudspeaker; it also relates to a sound-suppressing duct for such a device.

Background of the Second Aspect of the Invention

A loudspeaker usually incorporates a loudspeaker driver, which oscillates in order to produce sound, and a loudspeaker enclosure or housing, to which the loudspeaker driver is mounted. The shape, material and construction of the loudspeaker enclosure, along with the way in which the loudspeaker driver is mounted to the loudspeaker enclosure, have a strong influence on the quality of sound output by the loudspeaker.

A particular problem is that as the driver oscillates forwards and backwards, it creates sound waves in the air behind the driver as well as in the air outside the loudspeaker. The sound waves behind the driver may be contained within the enclosure, if the enclosure is substantially rigid and has no apertures or ports through which the sound waves can emerge. However, with such an enclosed space behind the driver, the pressure fluctuations in the air behind the driver can impede the movement of the driver, and so distort the sound; this problem can be minimised by having a sufficiently large enclosed space. As an alternative, if the space behind the driver is provided with an aperture or port through which the sound waves can emerge, this avoids the problems that arise from pressure fluctuations, but on the other hand there may be interference between sound waves produced by the front of the driver and those produced by the back of the driver and which emerge through the port. This issue is particularly of concern with loudspeakers for producing low frequencies, because of the size of the driver; and such a port may be referred to as a "bass-reflex port". A number of different designs of loudspeaker port have therefore been developed, for example as described in US 4 650 031 (Yamamoto/Bose Corp) and US 6 275 597 (Roozen et al/Philips Corp.). An alternative approach is that described in WO 2014/147378 in which such a loudspeaker housing is provided with at least one sound suppressing duct that incorporates a vortex chamber to absorb sound waves propagating through the duct. The document describes a number of different designs of vortex chamber. An improved design would be desirable.

Summary of the Second Aspect of the Invention

According to a second aspect of the present invention there is provided an acoustic device in the form of an enclosure with a plurality of sound-suppressing ducts through the walls, wherein the enclosure comprises a cavity enclosed by walls defined by a stack of framing sheets, and end plates of a rigid material, the enclosure also comprising multiple bolts extending between the end plates to compress the stack of framing sheets; wherein each framing sheet defines a central aperture surrounded by a peripheral strip, the central apertures of the stacked sheets defining the cavity; and where each peripheral strip defines a plurality of circular apertures and a plurality of slots that extend between the circular apertures and either the inside or the outside of the peripheral strip; each slot being aligned tangentially to the circular aperture such that any air flowing into the circular aperture from the slot would tend to follow a circular path around the wall of the aperture; wherein at least some adjacent framing sheets in the stack have the circular apertures of the adjacent framing sheets aligned to form vortex chambers, at least some aligned circular apertures in adjacent framing sheets having slots that extend to opposite edges of the peripheral strip from diametrically opposite positions within the vortex chamber such that for those slots the circular paths have the same rotational sense.

The peripheral strips may also define locations for the bolts, and these may be in the form of notches or slots extending from the inside or the outside edge of the peripheral strip. For example these may be rounded V-shaped notches extending from the inside edge of the peripheral strip. Alternatively bolts may be located substantially at the centre of aligned circular apertures, and so at the centre of vortex chambers, the diameter of the bolt being significantly less than the diameter of the vortex chamber.

The framing sheets may be arranged such that the circular apertures of all the framing sheets are aligned to form vortex chambers. It will be appreciated that for each vortex chamber the slots that communicate with the opposite edges of the peripheral strip are defined by different framing sheets, and therefore are not in the same plane. Preferably the thickness of the framing sheets is less than 5 mm, for example between 1 mm and 3 mm, for example 2 mm, and it has surprisingly been found that under these circumstances satisfactory sound suppression is achieved, despite the axial separation of the planes of the slots that communicate with the opposite edges.

The stack of sheets may also comprise spacer sheets which define the said central aperture surrounded by the peripheral strip, but in which the peripheral strip defines no circular apertures or slots; such a spacer sheet would provide an end face to the vortex chambers defined by the adjacent framing sheets in the stack. For example one such spacer sheet may be provided for every n of the framing sheets, such that the axial length of the vortex chambers is equal to the thickness of n framing sheets. The number n may for example be between 4 and 40.

The enclosure may be of any desired shape, for example a rectangular shape, or of a cylindrical shape; and the walls may define any desired shape of the cavity, for example a rectangular cavity or a circular cavity. The central apertures may be of uniform size and shape, or alternatively the central apertures may be of varying size or shape, for example such that the resulting cavity has a tapered frustro-conical form. Since each circular aperture has a slot communicating with only one edge of the peripheral wall, the framing sheet may be stamped out of a sheet material, while remaining as a single piece, and the remainder of the sheet material also remaining as a single piece. A benefit of having notches or slots to locate the bolts is that these notches can also be stamped out at the same time as stamping out the walls, with the remainder of the sheet material remaining as a single piece.

The sheet material may for example be plastic, paper or cardboard. Such materials can be cheap and easily stamped out, so this provides a cost-effective way of making such an acoustic device. It may also be possible to utilise other sheet materials such as MDF, or a thin sheet of aluminium that can be stamped out. The use of paper or cardboard provides a comparatively lightweight structure.

One end plate would typically define an aperture or port, while the other end plate would typically not define any apertures, apart from any apertures required for the bolts. The acoustic device may be used as a housing for a loudspeaker, if a loudspeaker driver is mounted in the aperture or port of the end plate. Alternatively the acoustic device may be used as a sound-suppressing module, to suppress sound emerging from a duct or outlet port, by mounting the acoustic device so the sound that would otherwise emerge from the duct or outlet port enters the aperture or port of the end plate of the device.

Considering the acoustic device as a housing in which a loudspeaker driver is mounted at one end plate, in operation of the loudspeaker sound waves will be generated by the front and rear surfaces of the diaphragm (which may for example be a cone). Sound waves from the rear surface propagate through the cavity and then enter the slots that lead from the inner edge of the framing sheets into the vortex chambers. It has been found that to achieve good sound suppression the total area of all the entrances to the slots, which is the open area on the inner wall, is advantageously at least 75% and more preferably at least 95% of the area of the moving diaphragm as viewed along the axis of movement of the diaphragm, i.e. measured in a plane orthogonal to the axis of movement; indeed the open area of the inner wall may be more than 100% of the corresponding area of the moving diaphragm.

Alternatively the acoustic device may be used as a sound-suppressing module, to suppress sound emerging from a duct or outlet port, by mounting the acoustic device so the sound that would otherwise emerge from the duct or outlet port enters the aperture or port of the end plate of the device

For example a conventional loudspeaker cabinet may have an outlet port for sound waves produced from the rear of the loudspeaker driver, to minimise pressure fluctuations behind the loudspeaker driver; and the acoustic device may be mounted either inside or outside the loudspeaker cabinet with the outlet port of the cabinet communicating with the aperture or port of the end plate the device, so that sound waves would have to pass through the wall of the device, and hence would have to pass through the tangential slots and vortex chambers. This would effectively suppress sound transmission.

The compressive force applied by the bolts increases the rigidity or stiffness of the walls. The compressive force must be applied such that the walls are all under substantially uniform compression and so are uniformly rigid. So for example the spacing between the compressing members (such as the bolts) should be sufficiently low, and the rigidiity of the end plates sufficiently high, that all portions of the sheets in the stack remain under sufficient compression. The end plates may for example be of a metal such as steel, brass, zinc or aluminium, and of thickness at least 2.5 mm thick, and in some cases 5 or 10 mm thick or up to 25 mm or more thick. The dimensions partly depend upon the size of the acoustic device, so a larger diameter stack will typically require a greater thickness of the end plates, but also on the desired appearance.

It will be appreciated that loudspeakers are primarily intended for generating audible sound, that is to say sound within the range of frequencies that is audible to a person with normal hearing, which may be taken as about 20 Hz up to about 18 kHz. Nevertheless under some circumstances loudspeakers may be required to generate infra-sound, for example to generate 15 Hz or 10 Hz; and may be required to produce ultrasound frequencies, for example 20 kHz or more. The loudspeakers of the invention can be expected to provide satisfactory performance both in the audible range, and at frequencies above and below the audible range.

Brief Description of the Drawings

The first and second aspects of the invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a perspective view of a device in accordance with an exemplary embodiment of the first aspect of the invention;

Figure 2 shows a sectional view on the longitudinal axes of the tubular portion and of the tapering channel, in the device of figure 1 ;

Figure 3 shows a sectional view on the line 3-3 of figure 2;

Figure 4 shows a plan view in the direction of arrow 4 of figure 2;

Figure 5 shows a perspective view of an alternative device in accordance with an exemplary embodiment of the first aspect of the invention;

Figure 6 shows a sectional view on the longitudinal axes of the tubular portion and of the tapering channel, of the device of figure 5;

Figure 7 shows graphically how the sound angle varies with the axial length of the semi- cylindrical wall, in the device of figure 5;

Figure 8 shows a perspective view of an over-ear device in accordance with an exemplary embodiment of the first aspect of the invention;

Figure 9 shows a plan view in the direction of arrow 9 of figure 8;

Figures 10a and 10b show sectional views corresponding to figure 6, of modifications to the tubular portion of the device of Figure 5;

Figure 11 shows a front view of a loudspeaker housing in accordance with an exemplary embodiment of the second aspect of the invention, along with a loudspeaker driver;

Figure 12 shows a view in the direction of arrow 2 of figure 11 ;

Figure 13a and Figure 13b show plan views of framing sheets used to form the loudspeaker housing of figure 1 1 ;

Figure 14 shows a plan view of two adjacent framing sheets of figures 13a and 13b superimposed;

Figure 15 shows a perspective view of part of the stack forming the loudspeaker housing of figure 11 ; and

Figure 16a and Figure 16b show plan views of framing sheets used to form an alternative loudspeaker housing in accordance with an exemplary embodiment of the second aspect of the invention.

Detailed Description

First Aspect of the Invention

Referring to figure 1 , a device 10 for aiding a person's hearing consists of a tubular portion 12 and a shell portion 14. At one end the tubular portion 12 is provided with a resilient bud 18 (indicated in broken lines in figure 1 ) which in this example is of memory foam, so that end can be inserted into a user's ear canal. The tubular portion 12 and the shell portion 14 are integral with each other. Referring also to figure 2, the shell portion 14 defines a conically-tapered wall 20 which has a longitudinal axis 21. The tubular portion 12 has a bore 15 with a longitudinal axis 16. The longitudinal axis 16 of the tubular portion 12 is orthogonal to the longitudinal axis 21 of the conically-tapered wall 20.

Referring also to figures 3 and 4, the wall 20 defines a conically-tapering channel 22 which tapers down towards the bore 15, with which it communicates, and there is a 45° plane reflector surface 24 at the narrow end of the tapering channel 22. In this example part 20a of the wall 20 projects above the tubular portion 14, so that part of the longitudinal axis 21 is completely surrounded. A remaining part 20b of the wall 20 is arcuate in plan, so that there is an opening 26 at one side of the wall 20.

It will be appreciated that the device 10 may be modified in various ways. For example the part 20a of the wall 20 may not extend above the top of the tubular portion 12. Preferably the opening 26 has an axial length at least half that of the wall 20, and the axial length of the opening 26 may be three quarters that of the wall 20. The shape of the opening 26 may differ from that illustrated here, for example the part 20b of the wall 20 might be U-shaped in plan, rather than part-circular.

In use of the device 10, the end of the tubular portion 12 would be inserted into the user's ear canal, cushioned by the bud 18, and the device 10 oriented so that the longitudinal axis 21 associated with the shell portion 14 points towards the direction from which the user wishes to hear sound. Sounds that come from that direction into the open end of the shell portion 14 are reflected by the conical surface of the channel 22 to hit the reflector surface 24, and hence are transmitted along the bore 15 to the user's ear.

It will be appreciated that the opening 26 is adjacent to the user's head.

Sounds that approach the device 10 from directions that are not parallel to the longitudinal axis 21 are shielded by the shell portion 14, and any sounds that diffract around the outer edge of the wall 20 pass through the opening 26 and therefore hit the user's head, where they tend to be diffusely scattered or absorbed. Hence the device 10 suppresses sound from directions other than those at least approximately parallel to the longitudinal axis 21. Referring now to figure 5, an alternative device 30 for aiding a person's hearing consists of a tubular portion 32 and a shell portion 34. At one end the tubular portion 32 would be provided with a resilient bud 18 (not shown in figure 5 or figure 6) which as described above may be of memory foam, so that that end can be inserted into a user's ear canal. The tubular portion 32 and the shell portion 34 are integral with each other.

Referring also to figure 6, the shell portion 34 defines a conically-tapered wall 36 which has a longitudinal axis 37. The tubular portion 32 has a bore 38 with a longitudinal axis 39. The longitudinal axis 39 of the tubular portion 32 is orthogonal to the longitudinal axis 37 of the conically-tapered wall 36.

The conically-tapered wall 36 thus defines a conically-tapering channel 40 which tapers down towards the bore 38, with which it communicates, and there is a 45° plane reflector surface 42 at the narrow end of the tapering channel 40. At its wider end the conically-tapered wall 36 is integral with a curved wall 44 which is also centred on the longitudinal axis 37, the radius of curvature of the curved wall 44 being the radius of the wider end of the conically-tapered wall 36. The curved wall 44 extends along an arc of 180°, so it is only a half cylinder, and may be referred to as a semi-cylindrical wall, and the conically-tapered wall 36 is similarly cut away; consequently there is an opening 46 extending from the open end of the semi-cylindrical curved wall 44 to a lip 48 just above the join between the shell portion 34 and the tubular portion 32.

The shell portion 34 in this example is formed of plastic, and the conically-tapered wall 36 is the inner surface of the shell portion 34, while the outer surface in this example is of uniform cylindrical shape: the wall thickness increases in a direction parallel to the axis 37 to create the taper of the inner surface of the wall 36.

In use of the device 30, the end of the tubular portion 32 would be inserted into the user's ear canal, cushioned by the bud 18, and the device 30 oriented so that the longitudinal axis 37 associated with the shell portion 34 points towards the direction from which the user wishes to hear sound. Sounds that come from that direction into the open end of the shell portion 34 are reflected by the inner surfaces of the shell portion 34 to hit the reflector surface 42, and hence are transmitted along the bore 38 to the user's ear.

Sounds that approach the device 30 from directions that are not parallel to the longitudinal axis 37 are shielded by the shell portion 34, and any sounds that diffract around the outer edge of the wall 44 pass through the opening 46 and therefore hit the user's head, where they tend to be diffusely scattered or absorbed. Hence the device 30 suppresses sound from directions other than those at least approximately parallel to the longitudinal axis 37.

It will be appreciated that the device 30 may be modified in various ways. The shape of the opening 46 may differ from that illustrated here, for example the wall 44 might extend along an arc greater than or less than 180°, so that it is not semi-cylindrical. Furthermore, it will be appreciated that it is the shape of the inner wall of the shell portion 34 which is particularly significant, whereas the external shape is of much less significance. For example if the device 30 is made of a material such as metal, then the wall may be of a more uniform thickness, so that the external shape would match the internal shape more closely.

Furthermore the axial length of the curved wall 44 may differ from that shown here, as the figures are not intended to be exactly to scale. It has been found that with the device 30 arranged with the longitudinal axes 37 and 39 horizontal, the angle in a horizontal plane over which sound can be heard, relative to the longitudinal axis 37, depends on the axial length of the curved wall 44. As illustrated in figure 7, to which reference is now made, for example with a device 30 in which the widest diameter of the conical wall 36 is 10 mm, increasing the axial length, L, of the curved wall 44 from 0 mm to 10 mm decreases the angle, Q, relative to the longitudinal axis 37 from about 45° to about 22.5°, while increasing the axial length from 10 mm to 30 mm decreases the angle relative to the longitudinal axis 37 from about 22.5° to about 10°.

It will also be appreciated that a device of the invention such as the device 10 or the device 30 described above may be held adjacent to the user's ear canal for example by means of a head band, rather than being inserted into the user's ear canal. In this case the tubular portion 12 or 32 would not require the bud 18, and may be shorter than shown above.

So, referring now to figures 8 and 9, there is shown a headphone device 50. This consists of a headband 52 which links two sound-insulating ear covers 54 equivalent to an ear muff, ear pad or ear protector. Each ear cover 54 would itself substantially prevent any sound from reaching the ear. At the centre of each ear cover 54 is mounted a device corresponding to the device 30 (but with a slightly shorter tubular portion 32 and without a bud 18). The shell portion 34 in each case protrudes outside the ear cover 54, with its longitudinal axis 37 aligned horizontally and with its open end facing forwards. The tubular portion 32 in each case extends through the ear cover 54 to terminate immediately adjacent to the user's ear canal.

The device 50 thus combines two devices equivalent to the devices 30 into a single device that can be conveniently worn, and does not require the insertion of any parts of the device into the user's ear canal. This may be found to be more comfortable by some users.

As shown in figure 10a, to which reference is now shown, an alternative device corresponding to the device 30 but which is intended to be mounted at the centre of an ear cover 54 as shown in figures 8 and 9, has a tubular portion 32' which differs from the tubular portion 32 of the device 30 not only in being somewhat shorter, and without a bud 18, but also in that the bore 38' has an end portion 56 of gradually increasing diameter. In this example the end portion 56 increases in diameter linearly, so having a conical flare. As shown in figure 10b, another alternative device corresponding to the device of figure 10a differs in that the bore 38" has an end portion 56' which increases in diameter non-linearly, having a trumpet-like flare. The devices of figures 10a and 10b improve the transmission of sound from the bore 38' or 38" towards the ear canal, because they provide a less abrupt transition at the end of the tubular portion 32' or 32".

Mention has been made above of forming the device of various types of plastic, or of metal. It will be appreciated that a device of the invention may be made of a very wide range of different materials, and the most suitable material will depend upon the size of the device, the desired appearance, and the use to which it is to be put, The device may be made by conventional manufacturing techniques such as casting or injection moulding, or by machining (for example computer-controlled machining), or 3D printing. Some suitable materials would include resins, silicone, or card (for example moulded paper pulp); another suitable material would be wood, or wood-based materials such as medium density fibre board (MDF), or metals such as brass, titanium or steel. Brass can additionally be plated with metal such as chromium and gold, to alter its appearance, while titanium can be anodised to provide a range of different colours.

Second Aspect of the Invention

Referring to figure 1 1 , a loudspeaker housing 10 is of generally cylindrical shape, and comprises a stack of framing sheets 12a, 12b (not shown in figure 1 1 ). The front face (as shown) includes an annular aluminium plate 14 of thickness 5 mm, which forms an end plate of the stack. There are a number of bolts 16 (represented schematically) that hold the stack together, and the heads of the bolts 16 may locate in recesses in the aluminium plate 14. The annular aluminium plate 14 supports a loudspeaker driver 20 which includes a cone which is caused to oscillate electromagnetically; the loudspeaker driver 20 is conventional.

Referring now to figure 12, this shows a side view of the loudspeaker housing 10. The stack consists of two types of framing sheets, 12a and 12b, which in this example are arranged in groups of four, except at the ends where there are groups of two framing sheets 12b side- by-side; the remainder of the stack consists of groups of four framing sheets 12a alternating with groups of four framing sheets 12b. At the other end of the stack is a circular aluminium end plate 24 of the same external diameter as the annular plate 14 and of the same thickness, but without any aperture. In this example the bolts 16 engage with threaded holes in the circular end plate 24, and compress the walls of the housing 10. In this example each framing sheet 12a or 12b is of card, and of thickness 2 mm.

Referring now to figure 13a and figure 13b, these show plan views of the framing sheets 12a and 12b. Each framing sheet 12a or 12b is of annular form, defining a central aperture 26 surrounded by a peripheral strip 28. Each peripheral strip 28 defines nine circular apertures 30. Within each framing sheet 12a each circular aperture 30 communicates with the outside of the peripheral strip 28 through a slot 32 which extends tangentially from the circular aperture 30, broadening out or flaring slightly at the outside of the peripheral strip 28; within the framing sheet 12b each circular aperture 30 communicates with the inside of the peripheral strip 28 through a slot 33 which extends tangentially from the circular aperture 30, broadening out or flaring slightly at the inside of the peripheral strip 28.

Referring now to figure 14, this shows a plan view of a framing sheet 12a with a framing sheet 12b underneath it; this shows how the framing sheets are arranged when in the stack. When the framing sheets 12a and 12b are assembled into the stack, all the circular apertures 30 are aligned with each other, forming vortex chambers. Any one vortex chamber is hence defined by circular apertures 30 in at least one framing sheet 12a adjacent to at least one framing sheet 12b. In figure 4 the slots 33 are shown in broken lines. The position at which a slot 32 communicates with a vortex chamber defined by a circular aperture 30 is substantially diametrically opposite the position at which a slot 33 communicates with the same vortex chamber, although in a slightly different plane. Referring again to figures 13a and 13b, each framing sheet 12a and 12b also defines four equally-spaced V-shaped notches 34 around the inside edge of the peripheral strip 28, and one such notch 34 has a step 35 at one side. During assembly, the steps 35 provide an alignment feature to ensure that all the framing sheets 12a and 12b are correctly aligned; and bolts 16 locate at the base of the notches 34. It will be appreciated that each shape of framing sheet 12a and 12b can readily be cut out or stamped out from sheet material, and both the framing sheet 12 and the cut away sheet material are single pieces after being cut out.

Referring also to the perspective view of figure 15, it will thus be appreciated that there are multiple channels communicating between the cavity defined by the central apertures 26 and the outside wall of the stack, each channel including a vortex chamber defined by a number of circular apertures 30 defined by adjacent framing plates 12a or 12b, the vortex chamber communicating with the wall of the chamber through the slots 33, and the vortex chamber communicating with the outside surface of the stack through the slots 32. If air were to flow into the slots 33 from within the housing it would tend to flow around the wall of the vortex chamber, because the slot 33 introduces the air next to the wall and in a tangential direction, so the air flow would tend to be in an anticlockwise direction around the vortex chamber in this example. The alignment of the diametrically-opposite slots 32 is such as to similarly cause an anticlockwise air flow around the vortex chamber if air were to flow into those slots 32 from outside the housing. Consequently if air were to flow into the slots 33 from within the housing the slots 32 would inhibit the flow from leaving the vortex chamber, so the air would tend to form a vortex within the vortex chamber, flowing round and round. Equally, if air were to flow into the slots 32 from outside the housing, the slots 33 would inhibit the flow from leaving the vortex chamber, so again the air would tend to form a vortex within the vortex chamber. Air flow in either direction would therefore be inhibited.

As mentioned above, in use the driver 20 is mounted within the loudspeaker housing 10, locating within the cavity defined by the central apertures 26. The driver 20 is driven to oscillate and to generate sound waves that propagate away from the front surface of the driver 20. Sound waves are also created within the housing 10. It has been found that the sound waves from within the housing 10 are inhibited from reaching the outside of the housing by the multiple vortex chambers.

The dimensions of the framing sheets 12a and 12b must obviously be suited to the dimensions of the driver 20. In one example the peripheral wall may have an inner radius of about 80 mm and an outer radius of about 120 mm; each circular aperture 30 may have a diameter of about 27 mm, while each slot 32 and 33 may have a width of 3.6 mm. Typically the diameter of the circular apertures 30 should be between about 3 times and 10 times the width of the slots 32 and 33, for example between 5 times and 8 times. Clearly the diameter of the circular apertures must be less than the radial width of the peripheral strip 28, while the inside diameter of the peripheral strip 28 must suit the diameter of the driver 20. It has been found that it is desirable to match the total cross-sectional area of all the channels defined by the slots 33 to the projected area of the driver 20, that is to say the area over which the driver 20 generates waves in a plane immediately adjacent to its front surface. This therefore provides an indication of how many framing sheets 12a and 12b are required in the stack.

It will be appreciated that a larger driver 20 will require more slots 33, and therefore may require more of the framing sheets 12b. However, the design of the framing sheets may be modified to provide a larger number of the circular apertures 30 and consequently a larger number of slots 33 within any one framing sheet.

Referring now to figures 16a and 16b, these show plan views of an alternative design of framing sheets 42a and 42b that may be assembled into a loudspeaker housing in an equivalent way to those described above. As described above, each framing sheet 42a or 42b may be of 2 mm-thick card. Each framing sheet 42a or 42b is of annular form, defining a central aperture 46 surrounded by a peripheral strip 48, and the peripheral strip 48 defines thirty circular apertures 50. Within each framing sheet 42a each circular aperture 50 communicates with the outside of the peripheral strip 48 through a slot 52 which extends tangentially from the circular aperture 50, broadening out or flaring slightly at the outside of the peripheral strip 48; within the framing sheet 42b each circular aperture 50 communicates with the inside of the peripheral strip 48 through a slot 53 which extends tangentially from the circular aperture 50, broadening out or flaring slightly at the inside of the peripheral strip 58.

When the framing sheets 42a and 42b are assembled into the stack, all the circular apertures 50 are aligned with each other, forming vortex chambers. Any one vortex chamber is hence defined by circular apertures 50 in at least one framing sheet 42a adjacent to at least one framing sheet 42b. The position at which a slot 52 communicates with a vortex chamber defined by a circular aperture 50 in a framing sheet 42a is substantially diametrically opposite the position at which a slot 53 in an adjacent sheet 42b communicates with the same vortex chamber, although in a slightly different plane. Framing sheets 42a and framing sheets 42b may be arranged alternately within the stack, or may be arranged in groups of identical framing sheets 42a or 42b, the groups being arranged alternately within the stack, each group consisting of the same number of framing sheets 42a or 42b, for example two, three or four. It will be appreciated that the more identical framing sheets 42a or 42b form a group, the greater the axial length of the group of slots 52 or the group of slots 53, and so the greater the mean axial distance between one group of slots 52 and the next group of slots 53. To ensure satisfactory vortex formation the mean axial distance between a slot or group of slots 52 and the next slot or group of slots 53 is preferably no more than 10 mm, and may be no more than 6 mm.

Each framing sheet 42a also defines J-shaped slots 54 and 55 that communicate with the outer edge of the peripheral strip 48, diametrically opposite each other (at the top and bottom of the figure as shown), and each framing sheet 42b also defines J-shaped slots 56 and 57 that communicate with the inner edge of the peripheral strip 48, at corresponding positions to the J-shaped slots 54 and 55. These provide alignment features when forming the stack. The stack is held together by bolts 16 extending between rigid end plates 14 and 24 (not shown, but equivalent to those shown in figure 1 ). One such bolt 16 extends through the curved parts of the J-shaped slots 54 and 56, another such bolt 16 extends through the curved parts of the J-shaped slots 55 and 57, and two other bolts 16 extend through the centres of vortex chambers defined by apertures 50a that are at positions 90° around the peripheral strip 48 from the J-shaped slots 54-57.

By way of example the peripheral strip 48 may have an external radius of 105 mm and an internal radius of about 80 mm. Each circular aperture 50 may be of diameter about 13 mm, and each slot 52 and 53 may be of width about 3.6 mm. In that case the diameter of the circular aperture 50 is about 3.6 times greater than the width of the slot 52 or 53 that communicates with it. As compared to the loudspeaker housing 10 of figure 1 1 , a loudspeaker housing formed from the framing sheets 42a and 42b provides more than three times as many slots 53, so to match the area of the slots 53 to the projected area of the driver 20 would require about a third as many framing sheets 42a and 42b than would be required in the stack formed from the framing sheets 12a and 12b. In a modification the dimensions may be the same apart from each circular aperture being of diameter 14.5 mm; this would provide a slightly larger ratio between the diameter of the circular aperture 50 and the width of the slot 52 or 53, of about 4.0. In a further modification, if the diameter of the circular aperture 50 was 15 mm but the width of the slots 52 and 53 was reduced to 3.0 mm that would increase the ratio to 5; this may improve the effectiveness of the vortex chambers in suppressing sound transmission, but would increase the number of sheets required in the stack in order to match the projected area of the driver 20.

In the example given above in which the circular apertures 50 are of diameter 13 mm, the bolts 16 may for example be M6 or M8 bolts, that is to say having a threaded shaft of external diameter 6 mm or 8 mm respectively. Consequently the bolts 16 that extend through the circular apertures 50a obstruct only the centres of those apertures 50a, so the bolts 16 would not prevent formation of a vortex flow around the vortex chamber.

As commented above, the dimensions of the framing sheets 12a and 12b or 42a and 42b must obviously be suited to the dimensions of the driver 20. In the case of a larger driver, it may be appropriate to provide a peripheral wall of greater radial width, with vortex chambers of greater diameter. So for example with a base driver of external diameter 533 mm (21 inches), the radial width of the peripheral wall might be 125 mm, with the vortex chambers formed by the circular apertures 30 or 50 being of diameter 90 mm or 100 mm, and with slots 32 and 33 or 52 and 53 being of width between say 5 mm and 25 mm, for example 10 or 15 mm. As another example, with a miniature driver 20 of diameter 25 mm, the peripheral wall might be of width only 10 mm, the vortex chambers being of diameter say 5 or 6 mm and the slots being of width say 1 mm or 1.3 mm.

Other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features that are already known and which may be used instead of, or in addition to, features described herein. Features that are described in the context of separate embodiments may be provided in combination in a single embodiment. Conversely, features that are described in the context of a single embodiment may also be provided separately or in any suitable sub combination.

It should be noted that the term“comprising” does not exclude other elements or steps, the term “a” or“an” does not exclude a plurality, a single feature may fulfil the functions of several features recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims. It should also be noted that the Figures are not necessarily to scale; emphasis instead generally being placed upon illustrating the principles of the present invention.