GB2428531A - Loudspeaker mounting and enclosure arrangements - Google Patents

Abstract

A loudspeaker mounting and enclosure arrangement wherein first <B>71</B> and second <B>72</B> microspeakers are mounted in a common casing <B>73</B>; each microspeaker having a first sound emissive surface disposed to deliver sound outwardly from the casing and a second sound emissive surface disposed to deliver sound inwardly of the casing; the arrangement comprising a respective enclosed cavity <B>74</B>, <B>75</B> associated with each said second sound-emissive surface; and wherein acoustic emission apertures <B>77</B>, <B>78</B> through which the sound emerges from the casing are disposed with a minimum separation distance <B>B</B> of at least 25mm. The cavities may share a common rigid, partition or they may be formed entirely separately without any common walls or borders. The arrangement may be incorporated in personal electronic devices such as cellular phone handsets or personal stereo players, thus enabling stereophonic, expanded stereo and/or 3d audio sound.

Description

LOUDSPEAKER MOUNTING AND ENCLOSURE ARRANGEMENTS

This invention relates to loudspeaker mounting and enclosure arrangements, and it relates especially, though not exclusively, to such arrangements fbr the incorporation of miniature loudspeakers into portable, personal electronic devices such as cellular phone handsets, personal stereo players and the like (hereinafter referred to collectively as "PEDs") with sufficient efficiency to permit useful rendering of stereophonic sound, expanded stereo andlor 3D-audio. The invention also encompasses PEDs incorporating such loudspeaker mounting and enclosure arrangements.

PEDs are becoming increasingly popular and it is now commonplace for cellphones to incorporate music players using the MP3 format. Moreover, some of the individual technologies are converging to create hybrid devices such as cell-phones combined with gaming consoles.

Hitherto, it has not been considered worthwhile to incorporate small loudspeakers into PEDs, other than to provide a very basic, monophonic listening experience. It has especially not been considered worth adding stereophonic pairs of loudspeakers to such devices, because the limited space available in a hand-held PED requires the loudspeakers to be placed relatively close together; typically less than 10 cm apart, or even less than 5 cm apart in a cell-phone. In such circumstances any stereo effect is lost, because the left and right channels are being reproduced from virtually the same point in space, whereas stereo is intended for playback on more widely spaced loudspeakers, typically about 2 metres apart, in order to create a spatial "sound image".

Moreover, by means of special signal-processing algorithms, it is now possible to create expanded stereo and/or three-dimensional sound-fields from a pair of loudspeakers. This generally comprises a binaural processing stage, which creates virtual 3D-audio suitable for playback via headphones, followed by a transaural crosstalk cancellation stage, which enables 3D-audio playback via loudspeakers as described, for example, in W094/22278 Al, or which can be configured to provide expanded stereo, as described in W098/20707 Al. For example. EP 0975201 describes a transaural crosstalk cancellation algorithm which is well- suited to personal portable devices, and capable of optimisation. Another cancellation algorithm is disclosed in WO/9730566, which also shows the accompanying loudspeaker enclosure configuration, in which a pair of loudspeakers 10 and 12 are mounted closely together in one small enclosure 14, as shown in Figure 1 hereof In principle, the use of 31)-audio algorithms should enable PEDs to create immersive 3D- audio for the listener, which is especially valuable for movie playback with 5-channel surround sound, interactive computer games and multi- channel music playback, such as the Super-Audio CD (SACD) standard. Accordingly, a limited number of cell-phone handsets and game consoles which incorporate this type of technology are now becoming commercially available. 1-lowever, their 3D-audio effectiveness is generally very poor, despite the best efforts of some of the world's major manufacturers.

In order to address this emerging market for sophisticated PEDs, loudspeaker manufacturers have developed very small, compact loudspeakers, hereinafter referred to as "micro-speakers", similar in size to the driver units used fbr "in-ear" earphones. A section diagram of a microspeaker 20 is shown in Figure 2, wherein a current-carrying coil 22 is freely suspended between the poles of an aimular, concentric magnet 24, 26 by a plastic diaphragm 28, to which it is attached. By applying a current to the coil 22 by way of connectors 21 and 23, the resultant electromagnetic field interacts with the static magnetic field and causes a displacement force, moving the diaphragm away from its resting position, against the restoring force of the diaphragm suspension at the outer edges. When the current is stopped, the restoring force returns the diaphragm to its rest position again.

The components of the microspeaker 20 are disposed within a housing 25 and covered by a protective screen 27. The housing 25 is provided with an annular rear vent 29 through which rearwards-directed sound energy can escape.

Although such microspeakers are typically 16 mm (or less) in diameter, and only 3 mm or 4 mm in thickness, they can exhibit considerable power handling capability; being typically rated at several hundred milliwatts.

Difficulties have been experienced, however, even by experienced audio technologists, in devising loudspeaker mounting and enclosure arrangements which enable significant advantage to be taken of the capabilities of such microspeakers and the technology which exists for using them to generate high quality stereophonic sound, expanded stereo and 3D- S audio in PEDs.

The invention aims to alleviate at least some of the aforementioned difficulties, thereby to facilitate the production of PEI)s capable of delivering stereophonic sound, expanded stereo and/or 3D-audio of useracceptable quality, and therefore the invention encompasses certain new and useful loudspeaker mounting and enclosure arrangements, and also PEDs embodying the same.

According to the invention from one aspect there is provided a loudspeaker mounting and enclosure arrangement, wherein first and second mierospeakers are mounted in a common 1.5 casing; each microspeaker having a first sound-emissive surface disposed to deliver sound outwardly from the casing and a second sound- emissive surface disposed to deliver sound inwardly of the casing; the arrangement comprising a respective enclosed cavity associated with each said second sound-emissive surface; and wherein acoustic emission apertures through which the sound emerges from the casing are disposed with a minimum separation distance of at least 25 mm.

By this means, certain undesired secondary emissions of sound originating from the second surfaces of the microspeakers; (a) from the casing and (b) from one microspeaker through the other, are significantly reduced whilst, at the same time, the differences between the left and right head- related transfer functions (HRTFs) are maximised.

It is preferred that the minimum separation distance between the acoustic emission apertures is at least 30 mm, and that the apertures be disposed on the same horizontal axis.

It is further preferred that the emission apertures comprise elongate slots of width less than mm, and ideally less than 2 mm. In one embodiment, the slots are substantially rectangular.

In one preferred embodiment, the two cavities share a common, rigid and at least substantially fluid-tight partition; although the two cavities may he formed entirely separately; i.e. without any common walls or borders, if desired.

The invention also encompasses a PED incorporating any such arrangement as aforesaid and, in a further preferred embodiment of the invention, the PED incorporates also processing means conditioned to implement crosstalk cancellation algorithms for 3D-audio and/or stereo expansion. It is further preferred that acoustic ducting means are provided to convey audio signals from the microspeakers to the emission apertures of the casing; the ducting means having associated therewith means for controlling the acoustic frequency transmission characteristics within hounds prescribed for 3D-audio.

In some preferred embodiments, acoustic ducts in the form of specially configured conduits are used to couple sound from the (or each) speaker to the respective emission apertures. The conduits arc configured such as to control the amplitude versus frequency characteristics of emitted sound to an extent that renders such arrangements capable of emitting sound of acceptable quality for the reproduction of music and/or for 3-D positional audio imaging. In particular, the cross-sectional area of each conduit is increased by flaring along at least a part of its path from the transducer surface to the emission aperture. ftc sound emission characteristics can be further enhanced by linking tuned resonator sinks, such as Helmholtz resonators or quarter-wave resonant tubes, to the conduits.

In order that the invention may be clearly understood and readily carried into effect, certain embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1, which has already been referred to, shows, in schematic crosssectional view, certain components of a prior art microspeaker arrangement used in a PED; Figure 2, again already referred to, is a cross-sectional diagram showing the typical structure of a microspeaker; Figures 3a and 3h show, in schematic cross-sectional view, respective operational states of a microspeaker; Figure 4 shows, in similar view to Figure 1, a prior art arrangement incorporating microspeakers in the handset of a cellular phone; Figures 5a, 5b and Sc are diagrams illustrative of transaural crosstalk; Figures 6a, 6b and 6c arc diagrams, similar to those of Figure 5, indicating the principles of crosstalk cancellation; Figure 7 is illustrative of crosstalk cancellation required for deriving 3D-audio from microspeakers; Figure 8 shows secondary emission pathways in a cellular phone handset; Figure 9 shows undesired acoustic coupling resulting from the use of microspeakers with a shared rear cavity; Figure 10 shows, in cross-sectional view, part of an arrangement in accordance with one example of the invention; and Figure 1 1 shows, in similar view to Figure 10, part of an arrangement in accordance with another example of the invention.

Referring now to Figure 3a, when the diaphragm of the microspeaker 20 is driven forward (i.e. in the upward direction in the Figure), a region 30 of compression is established in front of the diaphragm 28 (not shown separately in Figures 3a and 3b), and a rarefaction region 32 is formed behind it. These pressure perturbations, equal in magnitude, but opposite in polarity, propagate as sound waves outwards and away from their origin (the diaphragm 28 of the microspeaker, as shown in Figure 2). Similarly, referring to Figure 3b, when the diaphragm is driven backward (i.e. in the downward direction in the Figure), a rarefaction wave 34 propagates outwards and away from the front of the diaphragm, and a compression wave 36 from the rear.

Microspeaker pairs of this type are now being built into cellular phone handsets by various manufacturers. In general, it is common to find, as sho in Figure 4, that the left and right speakers; 41, 43 respectively, arc attached to the inner face 42 of a cellular phone 44, with their front surfaces exposed to the outside world via acoustic emission apertures in the form of meshes or grilles 45, 46, or one or more small outlet apertures in the cellular phone body.

Many of these handsets also incorporate 3D-audio algorithms in compliance with the 2004 NTT I)oCoMo network specifications. The speaker configuration of Figure 1 has also been used in a portable games console by a major manufacturer, which also features 3D-audio algorithms.

Referring again to Figure 4, typically the microspeakers 41 and 43 are 16 mm in diameter, and Is the emission apertures 45, 46 are often in the form of a "pepper pot" matrix of 1 mm diameter holes arranged within respective discoidal areas juxtaposed directly over the active surface of the microspeakers (Figure 4, dimension D). A cellular phone handset is usually about 48 mm in width (Figure 4, dimension A), and this constrains the microspeakers to be mounted relatively closely together. Allowing for mounting arrangements around the margin of the microspeaker of about 2 mm, then the maximum inter-speaker separation (centre-to-centre, dimension C in Figure 4) would be 28 mm. Consequently, the closest distance between the outlet apertures (Figure 4, dimension B) is only 12 mm, and the acoustic emission occurs over the entire 16mm width of the microspeaker (Figure 4, dimension D).

The present invention provides, inter alia, a means of generating effective 3D-audio or stereo expansion for a listener from closely spaced microspeakers in a PED. In particular, the invention relates to principles governing the physical positioning and mounting arrangements of a microspeaker pair, suitable for use in conjunction with 3D-audio algorithms, including transaural crosstalk cancellation means.

Transaural crosstalk cancellation is an intrinsic and critical element of all speaker-based 3D- audio, as is described fully in EP 0975201. This relates to the natural acoustic crosstalk that occurs when an individual is listening to a pair of conventional stereo loudspeakers; as shown in the schematic plan view of Figure 5. When the right-channel signal 50 is emitted by the right- hand loudspeaker 51, it moves outwards and travels not only to the right ear 52 of a listener 53 but also crosses (as indicated at 54) to the left ear 55, where it is received a little later in time. The brain recognises the high degree of correlation between the two signals; i.e. the primary signal 50 and the crosstalk signal 54, and correctly attributes their source to the right-hand loudspeaker 51. A directly analogous situation also exists of course with the left- hand loudspeaker 56, which is the source of primary and crosstalk signals (not shown) to the left and right cars 55, 52 respectively.

The presence of a transaural crosstalk signal such as 54 inhibits 3Daudio effects, and so it must be cancelled, as indicated schematically in Figure 6, by generating a signal 57 which is equal in magnitude, and opposite in polarity, from the opposite loudspeaker 56, as described in EP 0975201. This can be achieved with a signal-processing scheme such as that shown schematically in Figure 7 and it will he appreciated that, in general, 3D audio algorithms create cancellation signals which are, by definition, at least substantially out of phase with the signals it is desired to cancel. Indeed, the principal signals fed to the left and right speakers for 31) audio are themselves substantially out of phase.

In order to achieve adequate crosstalk cancellation, the cancellation signal must match the crosstalk signal in magnitude and phase within fairly precise limits; namely about 3 dB of amplitude and 20 of phase for 9 dB cancellation. This means that the relative time of arrival of the signals at the listener's ears must he synchronised very carefully. Anything which interferes with the integrity of the left and rightchannel signals will degrade the crosstalk cancellation and hence the effectiveness of the perceived 3D-audio. During playback on portable devices, where the loudspeakers might only be a few centimetres apart, the timing must synchronise to within a few microseconds for optimum effect.

Accordingly, in order to achieve precise transmission of the acoustic signals to the listener, sufficient for effective transaural crosstalk cancellation to be achieved, the inventors have established the following set of principles.

I. The sound for each channel must be emitted from a single "pointsource", or as close to this as possible. If the emitting area source is relatively large, then the wave-front origin will he distributed over the emitting area, and this disperses the transmitted signal, making it less coherent.

2. Following on from the above there must be minimal secondary emission, that is, undesired acoustic leakage from locations other than the respective primary sound- emission apertures for the two microspeakers. Figure 8 indicates three significant sources of secondary emissions; namely, emissions from parts of the case other than the desired emission apertures; alternate speaker emissions, wherein sound generated by one speaker finds its way to, and is emitted through, the other speaker; and leakage emissions.

3. The left- and right-channel sources must be placed as far apart as is practical in order IS to maximise the differences between the left and right head-related transfer functions (HRTFs).

Clearly also, the left- and right-channels should be well matched in all respects, including loudspeaker matching with respect to both phase and amplitude, and channel volume levels.

Also, ideally, the frequency response of the speakers should be fairly smooth - without significant notches or peaks - because 3D-audio relies on spectral shaping, and any spectral features might degrade the 31) effects.

Techniques capable (inter alia) of providing suitably smooth response characteristics are disclosed in GB 2408404-A, WO 2005/051037 and GB 2408405-A; and described in UK patent application No. GB 0510438.5, all assigned to the present applicant. These documents describe, inter alia, sonic emitter arrangements, comprising one or more miniature transducers (such as microspeakers) disposed within a common housing. Specially configured conduits couple sound from the (or each) speaker to emission apertures. The conduits are configured such as to control the amplitude versus frequency characteristics of emitted sound to an extent that renders such arrangements capable of emitting sound of acceptable quality for the reproduction of music and/or for 3-D positional audio imaging. In particular, the cross- sectional area of each conduit is increased by flaring along at least a part of its path from the transducer surface to the emission aperture. The sound emission characteristics can be further enhanced by the use of tuned resonator sinks, such as Helmholtz resonators or quarter- wave resonant tubes.

Tn any event, in order to achieve effective transaural crosstalk cancellation, and hence effective 3D-audio or expanded stereo, the foregoing principles must he satisfied by deploying a microspeaker pair differently from prior-art configurations.

In particular, the inventors have discovered that there are two critical criteria related to microspeaker deployment which must be satisfied simultaneously.

1. The volume of air behind the rear of each microspeaker must he enclosed in a rigid, substantially fluid-tight cavity, and the rear volumes of each microspeaker should be isolated from each other; for example, by means of a rigid partition, or by the use of two separate enclosure cavities or by other means.

2. The left-channel and right-channel acoustic emission apertures must be spaced as far apart from each other on the device as possible, with a minimum separation distance of 25 mm. Preferably, the separation should be more than 30 mm, with the apertures disposed on the same horizontal axis.

Only when these two protocols are implemented together is it possible to achieve effective 3D-audio.

It is also preferable for the left-channel and right-channel acoustic emission apertures to be small (for example rectangular or otherwise elongate slots of width less than 5 mm; ideally less than 2 mm).

In various prior-art devices, as depicted in Figure 8, microspeakers such as 61 and 62 share a common rear cavity volume 63, which appears to act as an acoustic "short circuit" between the two microspeakers, allowing the pressure waves from the rear of one speaker to transmit, via the common rear volume, out of the casing through the other microspeaker diaphragm.

This secondary emission signal, in turn, interferes with the primary signals, partially nullifying the crosstalk cancellation mechanism and inhibiting the 3D-audio.

Figure 9 illustrates the nature of the acoustic coupling mechanism that exists when both microspeakers 61, 62 share a common rear volume 63. When, for example, the diaphragm of the speaker 61 is driven forwards and the speaker 62 is not driven at all, a compression region 64 is formed in front of the diaphragm of speaker 61, which compression propagates forwards and outwards (upwards in the Figure). At the same time, a reduced pressure is created in the common, shared rear volume 63, and this causes the diaphragm of the speaker 62 to move backwards (downwards in the Figure). This creates a rarefaction region 65 in front of the diaphragm of the speaker 62, which propagates forwards and outwards (upwards in the Figure). Consequently, when the speaker 61 is driven, there is a primary, positive signal emitted therefrom. However, there is also a secondary, negative signal emitted from the is speaker 62; this signal being reduced somewhat in volume because of resistive losses in the diaphragms. Thus, instead of the emission of a single, well- deiThed signal, two interfering signals are emitted from differing spatial locations, resulting in an ill-defined, low coherence signal. The process also occurs, of course, in mirror image when the speaker 62 is driven, creating secondary emission from the speaker 61.

In conventional stereophonic signals, where the left and right channel signals are largely in- phase, the effect of the acoustic coupling between the speakers is less noticeable, but nevertheless, it is still undesirable. The stereophonic effect is impaired, although it is already poor because of the relatively close speaker spacing. As previously mentioned, crosstalk cancellation involves the use of signals which are substantially out of phase and thus 3-D audio and expanded stereo are more susceptible to the results of acoustic coupling.

All prior-art 3D-audio mobile devices feature a common rear microspeaker cavity. Often, but not always, this is the handset or console casing itself. in one known instance, there is formed a specific rear volume plastic moulding, but it is common to both microspeakers.

The present invention eliminates the above problem by the inclusion of a partition, between the rear volume regions of the two microspeakers, which is at least substantially fluid-tight; as depicted in Figure 10. In this configuration, there is substantially no acoustic couple between the rear volumes of the microspeakers and so the movements of one diaphragm do not significantly influence the other. It will be appreciated that each loudspeaker diaphragm has two sound-cmissive surfaces; the first or frontal surface being disposed to deliver the wanted sound outwardly from the casing and the second or rear-facing sound-emissive surface being disposed to deliver a phase-inverted, and in principle unwanted, version of the sound inwardly of the casing.

Figure 10 thus shows a first embodiment of an arrangement in accordance with the present invention, in which left-channel and right-channel microspeakers 71 and 72 are deployed in an enclosure 73, such that a respective rear cavity 74, 75 is provided for each microspeaker in communication with the second surface of its diaphragm. The cavities are scaled from the outer environment, and separated from each other by a partition 76 which, in this example, is rigid and fluid-tight so as to isolate the speakers 71 and 72 from each other.

Preferably, the volume of the rear cavity 74 or 75 behind each speaker should be as large as possible, so as to provide a high compliance diaphragm loading, which promotes a good low- frequency response. Ideally, a rear cavity volume of 2 ml or more is preferred, but in practice there is often insufficient space available, and volumes of I ml or less are typically employed.

In addition to the partitioned rear volumes, the minimum spacing between the emission areas (i.e. that between their closest points; dimension B in Figure 10) must be adequately large. In practice, it has been found that this dimension must be no less than 25 mm, and preferably greater than 30 mm. It is further advantageous, but not so critical, that the width of each emission aperture 77, 78 (dimension D in Figure 10) is small; ideally less than 5 mm, and preferably 2 mm or less. One method of implementing this is to form the emission apertures as elongate, vertical slits (when operationally disposed, facing a listener), say 1 mm wide and 10 mm in height.

- II -

An alternative embodiment of the invention is shown in Figure 11, in which the central partition 76 of Figure 10 has been replaced by the use of respective individual cavity enclosures 86a and 86b around the rear of each microspeaker 81, 82, so as to create mutual isolation. This serves the same purpose as a single partition, and is more practical in use for some applications. The same dimensional properties apply to this configuration as to that of Figure 10. - 12-

Claims (16)

1. What we claim is: I. A loudspeaker mounting and enclosure arrangement,

   wherein first and second microspeakers are mounted in a common casing; each microspeaker having a first sound- emissive surface disposed to deliver sound outwardly from the casing and a second sound- emissive surface disposed to deliver sound inwardly of the casing; the arrangement comprising a respective enclosed cavity associated with each said second sound-emissive surface; and wherein acoustic emission apertures through which the sound emerges from the casing are disposed with a minimum separation distance of at least 25 mm.
2. 2. An arrangement according to claim 1, wherein the minimum separation distance between the acoustic emission apertures is at least 30 mm.
3. 3. An arrangement according to claim I or claim 2, wherein the acoustic emission apertures are mounted with a common horizontal axis.
4. 4. An arrangement according to any preceding claim, wherein the emission apertures comprise elongate slots of width less than 5 mm.
5. 5. An arrangement according to claim 4, wherein the emission apertures comprise elongate slots of width less than 2 mm.
6. 6. An arrangement according to claim 4 or claim 5, wherein the slots are substantially rectangular.
7. 7. An arrangement according to any preceding claim, wherein the two cavities share a common, rigid partition.
8. 8. An arrangement according to any of claims I to 6, wherein the two cavities are formed entirely separately and without any common walls or borders.
9. 9. A loudspeaker mounting and enclosure arrangement substantially as herein described with reference to, and/or as shown in, Figures 10 or 11 of the accompanying drawings.
10. 10. A portable, personal electronic device such as a cellular phone handset, personal stereo player or the like (hereinafter "PED") incorporating an arrangement according to any preceding claim.
11. 11. A PED according to claim 10, further comprising processor means conditioned to implement transaural crosstalk cancellation algorithms for 3D-audio and/or stereo expansion.
12. 12. A PEE) according to claim 11, further comprising respective acoustic conduits con ligured and disposed to convey audio signals from the microspeakers to the emission apertures of the casing; the ducting means having associated therewith means fbr controlling the acoustic frequency transmission characteristics within bounds prescribed for 3D-audio.
13. 13. A PED according to claim 12, wherein the cross-sectional area of each conduit is increased by flaring along at least a part of its path from the microspeaker to the emission aperture.
14. 14. A PED according to claim 13 further comprising a respective tuned resonator sink associated with each said conduit.
15. 15. A PED according to claim 14, wherein said tuned resonator sinks comprise Helmholtz resonators.
16. 16. A PEI) according to claim 14 or claim 15, wherein said tuned resonator sinks comprise quarter-wave resonant tubes.