GB2245450A

GB2245450A - Speaker for use in a sound output system

Info

Publication number: GB2245450A
Application number: GB9013640A
Authority: GB
Inventors: Mervyn Stanley Curtis; Hirokazu Negishi
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1990-06-19
Filing date: 1990-06-19
Publication date: 1992-01-02
Also published as: WO1991020162A1; GB9013640D0

Abstract

A loudspeaker in which out-of-phase sound is attenuated comprises a drive unit (10) in or on a housing (12). A sound attenuating path from the concealed face of the drive unit (10) to the walls of the housing (12) is by a first region (42) air-tightly separated from a surrounding second region (44) by a sack-shaped partition (26) of rubber or other suitable soft material. The region (44) may be at atmospheric or reduced pressure or it may be constituted by a skinned layer of plastics foam.

Description

SPEAKER FOR USE IN A SOUND OUTPUT SYSTEM FIELD OF THE INVENTION The present invention relates to a speaker for use in a sound output system and to a sound output system comrprising a pair of such speakers to reproduce stereophonic sound.

BACKGROUND TO THE INVENTION A loudspeaker for use in a sound output system normally comprises a drive unit fed with the signal to be reproduced and a housing in which the drive unit is mounted. Acoustic energy is radiated both from the front of the drive unit which is a source of direct sound to the listener and from the rear of the drive unit which is a source of indirect sound and there is a phase difference between the direct and the indirect sound. A function of the housing is to contain the air which is present at the rear of the drive unit and reduce the proportion of the indirect or out-of-phase sound which reaches the listener and which tends to distort the direct sound which reaches him.

Loudspeaker designers have usually constructed the housing out of rigid materials because they are concerned with the structural integrity of the housing, but rigid materials usually have poor acoustic performance with low attenuation of sound vibrations. As a result, out-of-phase vibrations are liable to appear at the exterior of the enclosure with colorations resulting from the superposition of the panel vibrations on the out-of-phase sound.

Our EP-A-0330319 discloses a dual chamber loudspeaker enclosure having inner and outer walls of metal, glass or plastics, and with the outer chamber evacuated to reduce sound propagation by the structure of the enclosure.

SUMMARY OF THE INVENTION It is an object of the invention to provide a speaker for use in a sound output system comprising a drive unit mounted to a housing, in which a significant proportion of the unwanted sound from the concealed side of the drive unit is damped or attenuated so that it is not transmitted from the housing.

This problem is solved according to the invention by providing a speaker for use in a sound output system which has a drive unit for producing sound in response to an input signal and a housing to which the drive unit is mounted. A partition is provided that divides the interior of the housing into a first region that receives out-of-phase sound from a concealed side of the drive unit and a second region between the first region and the housing. The presence of the partition which is arranged to damp unwanted sound in transmission gives rise to a poor sound transmission path between the concealed face of the drive unit and the housing and if the partition is made of a soft material which inherently damps sound the partition also provides a poor path for forward reflection of unwanted sound.

The division of the housing into inner and outer regions as aforesaid, with the two regions air-tightly divided from one another, brings about a significant attenuation of the sound passing to the second region and onward to the walls of the housing even where the second region is at ambient or somewhat reduced pressure less than a vacuum. This attenuation is increased if the partition is made from a material that attenuates transmitted sound and preferably also minimises reflected sound. A variety of compliant materials have the appropriate properties including natural and synthetic rubbers, vinyl plastics materials, polyethylene and polypropylene. The material may be selected as to composition, thickness and structure to bring about the sound attenuation and low level of sound reflection required.

The invention also provides a loudspeaker enclosure comprising an inner wall, outer walls spaced from the inner wall and defining a volume space, one of said outer walls being formed with means defining an opening for receiving a loudspeaker drive unit, and the inner wall being of a sound-absorbent material.

DESCRIPTION OF PREFERRED FEATURES The speaker may comprise a single drive unit mounted in the housing or it may comprise more than one drive unit mounted in the housing with individual partitions for each drive unit so that the housing is divided into a first region associated with each individual drive unit and a common second region. In another form of the invention in which a plurality of drive units are mounted in the housing, there may be provided a single partition which divides the interior of the housing into a first region that is common to the drive units and a single second region that surrounds the drive units.A third form of the invention provides a housing that has at least first and second drive units of which at least the first drive unit is in a first compartment of the housing separated from a second compartment of the housing by means of a wall that forms part of the housing. In this case, first and second partitions for the individual drive units are provided that divide the interior of the housing into first regions for the respective drive units and second regions between each first region and the housing. The invention also includes speakers in which there are more than two drive units in each housing.

Preferably the or each partition is of an irregular shape to minimise standing waves in the first region which could affect the forwardly transmitted sound.

It is also desirable that the or each partition should have surface irregularities to promote multiple internal reflection of out-of-phase mid-frequency sound entering the first region from the rear face of the drive unit.

Desirably the partition is of a plastics or rubber material in sack or bladder shape that is flexible or compliant at ambient temperatures and is "lossy".

Thereby the volume of air is enclosed behind the drive unit which forms a poor transmission path of the out-of-phase vibration to the walls of the housing.

The volume of enclosed air in the internal space between the cone of the loudspeaker drive unit 10 and the partition 26 will depend upon the acoustic performance required for the drive unit. A high frequency "tweeter" may require only a small space, whereas a base speaker may need about 10 litres. For purity of tone and most reproducible results in mass production, the space 42 is preferably empty, but if required it can be partly or completely filled with a damping material such as loose cellulosic fibre wadding, lambswool, plastics foam or glass fibre matting.

Because of the inherent damping properties of the material, there is little forward reflection of sound from the partition through the rear of the cone of the drive unit and towards the listener. Suitable materials include plastics or rubber materials having a glass transition temperature (Tg) of less than OOC 0 and preferably less than -50 C. Suitable plastics or rubber materials include natural and synthetic rubbers which typically have Tg of -65 to -950C, polyethylene (Tg about -210C), polypropylene (Tg about -350C) and polyvinylidine chloride (Tg about -170C). The plastics or rubber material may be provided in the form of a sheet or layer of thickness from 2 to 10 mm, typically about 5 mm to assist in providing the appropriate sound attenuation.In most materials other than natural and synthetic rubber, a thickness greater than 2 mm will cause the partition to be stiff enough to maintain its shape and the partition then becomes an efficient sound reflector and is not so "lossy". Relatively stiff plastics materials other than rubber may be used to make the partition if they are of a foamed or cellular structure. However, such cellular materials have to have sufficient mass to give the required "lossy" function since the air cells or pockets tend to reduce the mass of the material and hence its "lossy" character.

In this form of the invention, the partition is provided in a sack or bladder-shaped moulding formed at its mouth with a flange or other formation to assist its attachment to the front face of the housing. The moulding of the partition to the required shape may be by any conventional technique for rubber or plastics and the material being moulded may have conventional fillers and processing aids, provided that these do not interfere with the required sound attenuating properties. Such mouldings have the advantage that they are easy and inexpensive to make.

The front face of the partition may be smooth, but if it is desired to promote the internal reflection of sound, it may be formed with protuberances or depressions over the whole of its surface or over regions of its surface, these protuberances or depressions being typically of height from 2 to 8 mm.

The partition means may be air-tightly attached to and may extend rearwardly from a front panel of the housing on which the drive unit is mounted, and a cage may also be provided for limiting the deformation of the partition means so that it does not touch the walls of the housing. In another form, the partition means may be provided with a number of attachment points which are spaced apart from the front panel and spaced apart from one another, and straps or other suitable fasteners or connectors may extend between these attachment points and the walls of the housing so as to define a predetermined concave general shape for the partition whilst at the same time preventing the partition from touching the walls of the housing and transmitting sound thereto.

The or each second region of the housing may contain air at ambient pressure or it may contain air at a reduced pressure or partial vacuum (e.g. 0.1 atmosphere or less). In this latter form of the invention, the partial vacuum assists in maintaining the shape of the partition means as if it were an inflated balloon, the partition means being of material that by its nature or thickness is relatively difficult to stretch at the reduced pressures used.

Another merit of reduced pressure is that it attenuates transmission of any movement of the partition towards and away from the walls of the housing, as described in our EP-A-0330319. In a further form of the invention, the second region is filled with a cellular material at least partly and preferably wholly, and a skin on the cellular material may then define the partition. Suitable cellular materials are plastics foams, for example polyether foams and polyurethane foams. The foams may be moulded in situ in the housing after the housing has been constructed. Desirably, the foam is flexible at room temperature and has a glass transition temperature of below OOC as aforesaid.

The forward going sound from the drive unit may reach the listener either directly or via a sound mirror attached to or forming part of the housing generally as disclosed in our Patent Specification No.

GB-A-2213677.

The speakers as aforesaid will normally be comprised within a sound output system in which there is a pair of right and left speakers for providing stereophonic sound, and such systems are also within the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Various forms of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional view of a first form of loudspeaker unit according to the invention; Figure 2 is a diagrammatic sectional view of a second form of loudspeaker unit fitted with an audio mirror:: Figure 3 is a diagrammatic sectional view of a third form of loudspeaker unit in which, in use, a partial vacuum is to be maintained within the housing; Figures 4A and 4B show the path of mid-frequency sound from the rear face of the drive unit reflected respectively by a partition having a smooth inner surface and by a partition having an irregular inner surface; Figures 5A and 5B are fragmentary sections of the smooth inner surfaced partition and the irregular inner surfaced partition of Figures 4A and 4B, respectively; Figure 6 is a diagrammatic perspective view of a partition and cage assembly which forms part of a loudspeaker unit according to a further form of the invention; Figure 7 diagrammatically shows another partition and cage assembly fitted to a housing;; Figure 8 shows a housing containing a partition according to a further form of the invention; Figures 9A, 9B and 9C are diagrammatic sectional views of a housing showing successive stages in the manufacture of a loudspeaker in which there is a region of air-tightly skinned cellular material between the drive unit and the outer walls of the housing; and Figure lOA-lOC show forms of the loudspeaker unit having more than one device unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In Figure 1 there is shown a loudspeaker unit comprising a drive unit 10 and a housing 12 which may be generally rectangular and may comprise end walls 14, side walls 16, a rear wall or base 18 and a front wall 20. A first portion 22 of the front wall 20 projects into the area defined by walls 14, 16, and is formed with a second region 24 defining a lip around the walls 14,16 as shown. The first portion 22 is formed with a central circular or elliptical aperture to receive a correspondingly moulded partition 26 of rubber or other suitable material of thickness about 5 mm, the partition 26 being concave or sack-shaped when viewed from the front and being formed at its mouth with a flange or lip 28 which overlies and rests on portions 22,24 of the front wall 20.The forward face of the flange or lip 28 is covered by a support plate 30 on which the drive unit 10 is mounted, the plate 30 having a suitably shaped central aperture to receive a frame 32 of the drive unit 10, the frame being received in a rebate 34 with an intervening gasket 36. The drive unit 10 is held to the plate 30 by fixing bolts 38 as shown. The flange 28 of the partition is sandwiched between the plate 30 and the wall 20, and the assembly is held together by clamping bolts 40. The sandwiched flange or lip 28 serves to decouple the drive unit 10 from the plate 30 and other parts of the housing 12.

With this arrangement the interior of the housing 12 is divided by the partition 26 into a first space or chamber 42 which is bounded by the support plate 30, the inner face of the partition 26 and by the drive unit 10 so that out-of-phase sound from the loudspeaker drive unit 10 first enters the space 42.

A second or outer space or chamber within the housing 12 is defined by the inwardly projecting portion 22 of the front wall 20, by the walls 14,16,18 and by the rear or convex face of the partition 26. The outer surface of the partition 26 is clear of the walls 14,16,18, and sound transmitted through the partition 26 therefore has to travel through the outer region 44 before it reaches the walls 14,16,18 of the housing 12. Because the partition 26 is made of a sound-attenuating or "lossy" material, only a proportion of the sound from the rear face of the drive unit passes from the inner region 42 into the outer region 44, and that sound is further attenuated within the region 44 before it reaches the walls 14,16,18.The partition 26 is preferably of a flexible natural or synthetic rubber 5 mm thick which provides sufficient mass to absorb sound rather than transmit it in the manner of a balloon or drumskin and because of its flexibility and composition is a poor reflector of sound. Furthermore, the partition 26 has a generally asymmetrical inner surface which discourages the formation of regular reflections or standing waves and this further reduces forward-going sound.

The air within the outer region 44 is at ambient pressure as also is the air within the inner region 42. There is normally a sufficient slow leakage path through the cone of the loudspeaker drive unit 10 to enable the pressure to be equalised as between the inner region 42 and the atmosphere, but to ensure that there is no pressure differential that could damage the loudspeaker cone, the support plate 30 may be further provided with one or more air leakage holes 46.

The loudspeaker shown in Figure 2 is generally similar to that shown in Figure 1, except that sound from the drive unit 10 is reflected by a reflector 50 supported in a generally overlying relationship to the drive unit 10 on a support 52. The reflector is generally as disclosed in Patent Specification No. GB-A-2213677 and is generally conical, with its axis offset slightly from the axis of the drive unit 10, in the case of a circular drive unit, or in the case of an elliptical drive unit from a line joining the foci.

The form of the loudspeaker shown in Figure 3 is similar to that in Figure 1, except that there is a partial vacuum (e.g. an internal pressure of 10 m bar to about l/lOth atmosphere) in the space 44 which assists in maintaining the shape, position and volume of the partition 26 and also contributes somewhat to the attenuation of sound. The partition 26 provides good damping and sound insulation in its own right.

Although it has its own major damped resonance which rises in frequency as the degree of vacuum is increased, the resonant frequency can be adjusted to a band by selection of material, mass (wall-thickness) and the degree of vacuum. At frequencies above its resonance the partition offers good damping and sound isolation. Air may leak into the space 44 over an extended period of use of the loudspeaker and to enable the vacuum to be restored, an outlet passage 60 is provided in the wall 18 which may be closed off by a valve 62. When required, a vacuum pump 64 may be connected via valve 62 to the outlet passage 60 for restoration of the vacuum in the region 44.

The partition 26 as shown in Figure 4A may have a smooth inner surface but is overall of irregular shape, as mentioned above, to minimise standing waves.

In that case, as shown in Figure 4A, the sound from the drive unit 10 (diagrammatically represented as a sound source 70) is reflected only a limited number of times at the inner face of the partition 26 before it is transmitted forwardly out of the space defined within the partition 26. On the other hand, as shown in Figure 4B, if the inner surface of the partition 26a is formed with a pattern of projections and depressions 72 defining an irregular inner surface, the number of reflections of the sound, and hence the attenuaton of the sound before it passes out of the space defined by the partition 26a, is increased.

These projections and depressions may occupy a part only but preferably occupy the whole of the available front surface of partition 26. The projections 72 will typically be of height 2 to 8 mm, and they are most effective in producing further attenuation of sound of a mid-range of frequencies, typically 500-15000 Hz.

The corresponding profiles of the partitions 26,26a are shown on an enlarged scale in Figures 5A and 5B.

In the absence of a vacuum in the outer region 44, the partition 26 which is not itself shape-retaining may require an external support to maintain its shape, position and volume, particularly where it and the drive unit 10 are to be used in the forwardly-facing attitude of Figures 1 and 3, rather than in the upwardly-facing attitude of Figure 2. In that case, the partition 26 may fit within an openwork cage 80 (Figure 6), the cage 80 and the partition 26 being supported on a common mounting plate 82 that fits onto support plate 30 of housing 12. In an alternative, the cage 80 and the partition 26 may be individually mounted on the support plate 30 as shown in Figure 7.

In Figure 8 there is shown a support plate 30 to which a partition 26b is attached by bolts or screws. In order to maintain the shape and position of the partition 26b, it is formed on its outer surface at spaced-apart intervals with a multiplicity of integral or attached locating stakes or fingers 85 which extend between the surface of the partition 26b and the walls of the housing 12 to which they may be attached or within which they may float.

In Figures 9A-9C, the space 44 is defined by a body of cellular foamed material 90. The foamed material 90 may be formed in situ in a partly pre-assembled housing 12. For this purpose, the walls 14, 16, 18 of the housing are arranged so that the open part of the housing faces upwardly and sections 92 of a sectional mould 94 are positioned within the interior space of the housing 12 so as to occupy a space corresponding to the intended inner region 42. A composition 96 to form a flexible plastics foam is poured or otherwise introduced into the interior space of the housing 12 and allowed to rise and cure in situ, an air-tight skin 92a forming at the same time as a result of the inherent properties of the foamable material.The sections 92 of the mould are then removed as shown in Figure 9B, after which a support plate 30a having a drive unit 10 mounted thereto and acoustically decoupled therefrom by an intervening flexible support is attached to the housing 12 as shown in Figure 9C.

As mentioned above, each loudspeaker assembly may contain a single drive unit 10, or it may contain more than one drive unit as indicated in Figures 10A to lOC. In Figure IOA there are provided first and second drive units 91,93 in a housing 12 devoid of internal walls. Partitions 95,95a are associated with the respective drive units 91,93 to define respective inner chambers 42a,42b and a common outer chamber 44a.

In the alternative arrangement shown in Figure lOB, the drive units 91,93 have a single partition 97 associated therewith to define an inner region 42c and an outer region 42b. The arrangement shown in Figure 10C is similar to that in Figure 10A, except that the housing 12 is divided into separate compartments by an internal wall 99, so that there are separate outer regions 44c,44d in each of the compartments.

Various modifications may, of course, be made to the embodiments of the invention described above without departing from the invention, the scope of which is defined by the appended claims. For example, the shape of the housing 12 need not be rectangular but could be any other convenient shape, e.g. circular or elliptical, depending on the styling desired for the speakers. The housing 12 need not itself form the outer case of the loudspeaker unit but could itself be housed in a decorative outer case e.g. of wood or plastics.

Claims

CLAIMS:

1. A speaker for use in a sound output system comprising a drive unit for producing sound in response to an input signal, a housing to which the drive unit is mounted, and partition means air-tightly dividing the interior of the housing into a first region that receives sound from a concealed side of the drive unit and a second region between the first region and the housing, said partition means being of a non-rigid material that damps sound transmitted into the second region.

2. A speaker according to claim 1, wherein a plurality of drive units are mounted in the housing and a plurality of partition means for respective drive units divide the interior of the housing into first regions for each drive unit and a common second region.

3. A speaker according to claim 1, wherein a plurality of drive units are mounted in the housing and a single partition means for the drive units divides the interior of the housing into a common first region for the drive units and a common second region.

4. A speaker according to claim 1, wherein the housing has at least first and second drive units of which at least the first drive unit is in a first compartment separated from a second compartment for the second drive unit by a wall forming part of the housing, first and second partition means for respective drive units dividing the interior of the housing into first regions for respective drive units and second region between the respective first regions and the housing.

5. A speaker according to any preceding claim, wherein the or each partition is of irregular shape to minimise standing waves in the first region.

6. A speaker according to any preceding claim, wherein the or each partition has surface irregularities to promote internal reflection of mid-frequency sound in the first region.

7. A speaker according to any preceding claim, wherein the partition is of a plastics or rubber material that is flexible at ambient temperatures.

8. A speaker according to any preceding claim, wherein the plastics or rubber material has a glass transition temperature of less than 0 C.

9. A speaker according to claim 8, wherein the plastics or rubber material has a glass transition temperature of less than -500C.

10. A speaker according to claim 8, wherein the plastics or rubber material is selected from natural and synthetic rubbers, polyethylene, polypropylene and polyvinylidine chloride.

11. A speaker according to any preceding claim, wherein the plastics or rubber material is of a sheet or layer of thickness 1 to 10 mm.

12. A speaker according to claim 11, wherein the plastics or rubber material is of thickness about 5 mm.

13. A speaker according to any preceding claim, wherein the partition is a generally sack-shaped moulding.

14. A speaker according to any preceding claim, wherein the partition is formed with protuberances or depressions extending from 2 to 8 mm from a datum surface of the sheet or layer.

15. A speaker according to any preceding claim, wherein the partition means is air-tightly attached to and extends rearwardly from a panel of the housing on which the drive unit is mounted.

16. A speaker according to claim 15, wherein the partition is supported within a cage for maintaining the partition means at a clearance from the housing.

17. A speaker according to any of claims 1 to 15, wherein the partition means has a multiplicity of attachment points at locations spaced apart from one another and spaced apart from the front panel of the housing for attachment of the partition means to the housing so as to maintain the partition means at a spacing from the housing.

18. A speaker according to any preceding claim, wherein the second region is at ambient pressure.

19. A speaker according to any of claims 1 to 17, wherein the second region is at reduced pressure or vacuum.

20. A speaker according to any preceding claim, wherein the second region is at least partly filled with a cellular material.

21. A speaker according to claim 20, wherein the second region is wholly filled with a cellular material, and a skin of the cellular material defines the partition.

22. A speaker according to claim 20 or 21, wherein the cellular material is a plastics foam.

23. A speaker according to claim 22, wherein the plastics foam is formed in situ in the housing.

24. A speaker according to any preceding claim, wherein an unobstructed cavity is defined between the partition and the drive unit.

25. A speaker according to any of claims 1 to 23, wherein a cavity is defined between the partition and the drive unit and said cavity is at least partly filled with a sound-damping material.

26. A speaker according to any preceding claim, further comprising an audio mirror for controlling the directivity of sounds which are output from the speaker.

27. A sound output system comprising right and left speakers, each speaker being as claimed in any of claims 1 to 26.

28. A speaker for use in a sound output system comprising a drive unit for producing sound in response to an input signal, a housing to which the drive unit is mounted, and partition means air-tightly dividing the interior of the housing into a first region that receives sound from a concealed side of the drive unit and a second region between the first region and the housing, said partition means being arranged to damp reflected sound from the first region transmitted through the drive unit to the listener.

29. A speaker for a sound output system having a transducer and a housing in or on which the transducer is mounted, sound from a reverse face of the transducer passing via a path including an inner and an outer chamber divided by a sound attenuating material before it reaches walls of the housing.

30. A loudspeaker enclosure comprising an inner wall, outer walls spaced from the inner wall and a defining a volume space, one of said outer walls being formed with means defining an opening for receiving a loudspeaker drive unit, and the inner wall being of a sound-absorbent material.