GB2408405A - Sonic emitter - Google Patents

Abstract

A sonic emitter comprises a transducer 4, eg a micro-speaker, in a housing. The transducer has an emission surface facing a frontal resonant cavity 7, 8 and an opposed surface facing a rear resonant cavity 6. An acoustic conduit 9 links the transducer 4 to a sound outlet S2. The emitter may be incorporated in a pendant or other fashion-related article, or a mobile phone, digital camera, portable games console or hand-held computer. The conduit 9 may be flared and the cavities may contain acoustic damping material.

Description

SONIC EMITTER ARRANGEMENTS

The present invention relates to sonic emitter arrangements, and it relates more particularly, though not exclusively, to such arrangements including miniature loudspeakers, such as micro-speakers, and to their incorporation into portable electronic devices such as mobile phones, digital cameras, portable games consoles and hand-held computers, or into miniature loudspeaker enclosures, such as earphones and fashion-related items, such as pendants, intended to be worn by a listener. The invention also encompasses devices, such as portable electronic devices, and miniature loudspeaker enclosures incorporating such emitter arrangements.

Portable electronic devices, such as those mentioned above, are becoming increasingly popular. For example, it is now commonplace for mobile phones and digital cameras to incorporate music players using the MP3 format, and some of the individual technologies are converging to create hybrid devices such as mobile phones combined with digital cameras or gaming consoles.

Portable electronic devices in general are well-suited for use with personal earphones or headphones, because they are often used in public places.

Furthermore, it has not been possible (or worthwhile) hitherto for the housings of such devices to incorporate small loudspeakers which provide anything more than an extremely basic listening experience. There are several reasons for this.

Firstly, loudspeakers tend to add significantly to the physical size and cost of the end product. Secondly, the acoustical output quality of any loudspeaker is critically dependent on the way in which it is built and mounted.

Furthermore, because of the restricted space available in a hand-held portable electronic device, for example, the loudspeakers must be placed relatively close together; typically less than 10 cm apart, or even less than cm in a mobile phone. Thus, when stereophonic audio material is played, the 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". s

In order to address this emerging market for portable electronic devices with more sophisticated audio capabilities, loudspeaker manufacturers have recently developed extremely compact loudspeakers, known as "microspeakers", whose dimensions are similar to those of the driver units used in earphones. Such micro-speakers are typically less than 20 mm in diameter and less than 5 mm in thickness. Despite their small dimensions, however, such micro-speakers have significant power output capability, and can typically handle several hundred milliwatts.

Micro-speakers have been built into certain types of mobile phones by several manufacturers, but they are usually attached simply to the inner face of the phone's housing with their front surfaces exposed via a mesh or grille, or by several small holes in the housing. Although this is adequate for transmitting simple audio, such as ring-tones, to the listener, it is not adequate for delivering more sophisticated audio performance, such as 3D positional audio for games, or stereo expansion for ring-tone and music playback.

One feature of all mobile electronic devices is the very limited space which is available for the user interface, primarily the graphics display, keypad and other controller devices; the graphics display in particular taking priority when housing space is allocated during design. Accordingly, there is little or no space for loudspeakers, however small, to be mounted on the front panel, facing the listener.

Alternative options for incorporating micro-speakers into a mobile phone include (a) mounting micro-speakers to either side of the device, facing outwards to the listener's left and right sides, respectively; and (b) mounting the micro-speakers internally of the device body, and delivering the audio output via a conduit to respective output vents.

Of the alternative options, side mounting is the easier to implement, for example by mounting the micro-speakers in sealed pods on opposite sides of the device. However, although the pods are small, the overall additional bulk makes the phone body somewhat unsightly and the pods also detract from the smoothness of the body, making it less easy to move the phone in and out of a pocket.

Internal mounting is thus a favoured option although, as stated previously, the mounting arrangements for the micro-speakers are critical to the resultant performance, and a number of operational and designrelated criteria must be complied with, depending on the audio performance required.

If internally-mounted micro-speakers are to be used, it is axiomatic that a conduit of some sort must be used to convey the audio energy to the outside world. For micro-speakers mounted on an internal chassis or frame within an outer housing, suitable conduits can in principle be formed within the housing to which the front (sound-emitting) surfaces of the micro-speakers are exposed, and apertures can be cut into the housing at some small distance from the micro-speakers.

Problems arise with such arrangements however, since the conduits form resonant cavities, which create undesirable peaks and troughs in the emitted sound spectrum. Moreover, the housing itself is exposed directly to a considerable amount of sound energy present in the cavities, to which it is partly transparent, and hence the housing becomes an undesirable secondary emission source for both micro-speakers, further reducing the sound quality, and significantly impairing the effectiveness of 3D-positional audio.

One method of reducing the peaks and troughs caused by the resonant conduit would be to simply fill the cavities with sound damping material, but this inevitably results in the absorption of a considerable amount of the sound energy indiscriminately across the spectrum, and reduces the emitted volume very significantly.

It is an object of this invention to provide sonic emitter arrangements, comprising miniature transducers, such as micro-speakers, together with associated control means addressing one or more the above-mentioned problems to an extent rendering such arrangements capable of emitting sound of acceptable quality for use in one or more of the circumstances mentioned hereinbefore.

According to the invention from one aspect there is provided a sonic emitter arrangement comprising a sonic transducer encased within a housing dimensioned to be portable or wearable; said transducer having an emission surface and a further surface substantially opposed thereto, an acoustic conduit linking said emission surface to an outlet from said enclosure for sound produced by said transducer, and control means for influencing at least one parameter associated with sound emitted from said outlet, wherein the control means comprises first and second acoustic resonators respectively coupled to the emission surface and said other surface of the transducer.

Such arrangements are efficient, can be implemented at relatively low cost, and can be readily adapted to a wide range of different transducer types and sizes, though it is preferred that a miniature loudspeaker, such as a micro- speaker, is used.

It is further preferred that the said parameter influenced by the control means comprises the amplitude versus frequency characteristic of sound emitted from said outlet. By this means, unwanted peaks and troughs in the said characteristic can be reduced in amplitude and/or shifted in frequency to achieve a desired sound spectrum.

In further preferred forms of the invention, the arrangement comprises a further acoustic resonator linked to said acoustic conduit, in order to further influence said at least one parameter and/or a further parameter associated with sound emitted from said outlet. By this means, fine tuning and/or additional compensation for unwanted characteristics can be achieved.

In another preferred embodiment of the invention, said first acoustic resonator is connected by way of a passage to said outlet; said passage forming part of said acoustic conduit.

Further preferably, at least one dimension of the passage is flared so as to increase toward said outlet, such flaring preferably being linear or following an exponential or other curvilinear form.

It is preferred in some embodiments of the invention to provide a vent in the housing and to couple the second acoustic resonator to said vent by way of a tubular or similar duct. This permits further fine tuning of the acoustic emission spectrum to be achieved.

It is preferred that at least one of the acoustic resonators comprises a Helmholtz resonator. Further preferably, all of the resonators comprise Helmholtz resonators; such resonators being relatively simple to construct and exhibiting reliable performance over a wide range of operating conditions.

Any one or more of the acoustic resonators utilised in embodiments of the invention may conveniently be fabricated from superposed laminar components; such components being of metal, plastics or any other material which can be readily machined, moulded or otherwise formed to the required tolerances and which, when assembled, exhibits suitable acoustic performance.

Where such laminar components are used, edge-emission passages can conveniently be formed by opening an otherwise enclosed aperture, such as an aperture located substantially centrally within a plate, through to an edge of the plate by removal of the plate material, bearing in mind that other laminae will lie above and below the plate in question, thereby defining the passage.

If desired, any one or more of the resonators may be fabricated of, or may contain or have associated therewith, acoustic damping material of any convenient kind, such as glass fibre wool or tissue paper.

Arrangements according to any embodiment of the invention may be incorporated into electronic devices such as mobile telephones, digital cameras, mobile games consoles or portable sound and/or multimedia equipment.

In such circumstances, the said housing usually serves as the overall housing for the invention. Typically, such a device incorporates two or more such arrangements, preferably matched in performance, permitting the device to exhibit sophisticated audio performance characteristics, such as 3-D sound or enhanced stereophonic sound.

One or more arrangements according to any example of the invention may be incorporated into portable and/or wearable loudspeaker enclosures such as earphones and fashion-related items, such as pendants, intended to be worn by a listener.

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: Figures 1(a) and 1(b) show idealised representations of Helmholtz resonators, respectively unflanged and flanged; Figure 2 shows, in simplified and block diagrammatic form, a representation of a duplex resonator emitter arrangement corresponding to a first embodiment of theinvention Figure 3 shows, in exploded diagrammatic perspective, part of an arrangement in accordance with the first embodiment of the invention; Figure 4 shows, in plan view, an edge-emitter component that can advantageously be used with the embodiment of Figure 3; Figure 5 shows, in similar view to Figure 2, a representation of a triplex resonator emitter arrangement in accordance with another embodiment of the invention; Figure 6 shows, in exploded diagrammatic perspective, a practical implementation of the triplex arrangement shown in Figure 5; Figure 7 shows, in similar view to Figures 2 and 5, a representation of a triplex resonator emitter arrangement with rear speaker venting in accordance with a further embodiment of the invention; and s Figure 8 shows, for comparison, amplitude versus frequency characteristics for mobile phone devices with and without arrangements according to the invention.

In general, the inventor has determined that any enclosures and conduits associated with a micro-speaker, no matter how non-uniform, convoluted, and non-ideal, can be treated as acoustic resonators, particularly Helmholtz resonators. By "non-uniform", it is meant that any acoustic channels might vary grossly in dimension from one part to another. By "convoluted", it is meant that an enclosed space might not be a simple geometric structure, such as a sphere or cuboid, but might be an amorphous, multi-limbed, distributed volume. By "non-ideal", it is meant that a conduit might slowly emerge into an enclosed volume without a distinct, well-defined junction between the two. Despite these very significant departures from the ideal, the inventor has determined that it is possible to devise an arrangement comprising a network of two or more resonant cavities around a micro- speaker, and to tune and control their properties by adjusting their various parameters so as to provide acoustically acceptable output characteristics.

The properties of the Helmholtz resonator are described in the literature, for example by: Kinsler et al. Fundamentals of Acoustics (3rd edition), pp. 225-228; L E Kinsler, A R Frey, A B Coppens and J V Sanders John Wiley and Sons, New York (1984): ISBN 0-471-02933-5.

Diagrams of a theoretical Helmholtz resonator, without and with flanges, are shown in Figures 1(a) and 1(b) respectively. The resonator comprises a rigid walled cavity 1 of volume V, connected to the ambient by way of a neck 2 having length L, and cross sectional area S. The structure behaves as a resonant system when the dimensions of all of these parameters are significantly smaller than the wavelengths under consideration. For audio waves in the relevant range for the present invention (say 500 Hz to 6 kHz), the wavelengths lie between 680 mm and 57 mm. It is assumed that the neck constriction is sufficiently short that all fluid particles may be assumed to move in phase when actuated by a sound pressure wave.

Referring to the aforementioned publication of Kinsler et al., if \>>L, then the mass of air (or other fluid) in the neck moves as a unit, represented by an inertance. Similarly, if \>>V]/3, then the volume of air V represents a l 5 compliance element (with associated stiffness), and if V>>S'/2, the opening radiates sound as a simple source, corresponding to a resistance element.

This acoustical system is directly analogous to a mechanical oscillator, in which its inertance, compliance and resistance correspond to the mass, compliance and resistance of, for example, a damped, sprung piston, and also to the inductance, capacitance and resistance of a serial L-C-R resonant electrical circuit.

When sound energy is incident on to the entrance to the tube, the mass of air in the neck moves back and forth against the compliance of the internal fluid; resonance occurs when the reactance approaches zero. The structure is characterized by its resonant frequency, (OO (radians/sec), [or fO (Hz)] and the dimensionless "quality factor" or Q. as follows.

69, = C34 (1) Q= 27r4V(L'/19)3 (2) Here, the factor L' is used for the effective length of the neck 2, rather than the physical length L because of its radiation-mass loading.

According to Kinsler et al., at low frequencies a circular opening of radius a is loaded with a radiation mass equal to the fluid contained in a cylinder of area na2, and length 0.85a, if terminated in a wide flange 3 (Figure 1(b)), and 0.6a if not flanged (Figure 1(a)). From this, it can be shown that the relationship between L' and L is as follows.

L' = L + 1.7a (outer end flanged) (3) L' = L + 1.5a (outer end not flanged) (4) If the neck opening is a circular opening of radius a then, in the limit when L approaches 0, L' approaches a value of 1.7a (flanged) or 1.5a (not flanged).

In order to assess whether a tuned Helmholtz resonator might be usable in association with an efficient, miniature sonic emitter, and in order to determine whether the theoretical relationships might be applicable to the active, high power-density application of micro-speaker deployment, the arrangement shown in schematic form in Figure 2 was constructed in the manner shown in Figure 3.

Referring now to both Figures 2 and 3, the arrangement was constructed in laminar form and comprised a 16 mm diameter, 2 mm thick micro-speaker unit 4 (Foster type 2H54) mounted with epoxy sealant into an aluminium flange plate 5 of thickness 2 mm, measuring 28 x 28 mm and formed with a central aperture of diameter 16.2 mm. The plate 5 is shown to one side of the drawing for convenience but it will be appreciated that, in practice, it is orientated similarly to, and aligned with, all of the other components.

Bolted to the plate 5 are two piston-type variable-volume cavities, namely a rear enclosing cavity 6 and a frontal cavity 7; each cavity being variable in size from O to 10 ml. The cavities 6 and 7 were thus coupled respectively to the rear surface and to the front (sound-emitting) surface of the micro- speaker 4. In general, that surface of the micro-speaker 4 which is intended to be the sound-emitting surface is herein designated the "front" surface; the other (parallel) major surface of the micro-speaker correspondingly being designated as the "rear" surface. It will be appreciated that, in general, cavities of fixed volume are used; the cavities of variable volume being used by way of example to indicate that cavities with a range of dimensions can be used in arrangements according to the invention.

A 2 mm-thick plate 8, configured to define the cavity and neck of a Helmholtz resonator, is sited between the top of the loudspeaker mounting plate 5 and the base of the frontal cavity 7. The plate 8 is formed with a central 16 mm diameter aperture (the same as the underlying micro- speaker 4), and is coupled to the external environment via a conduit comprising a 10 mm-wide passage 9 formed between the central aperture and an edge of the plate. In 2 mm thick aluminium, the area at the neck opening is therefore 20 mm2, and the intrinsic central volume is 402 mm3 (0.4 ml). The length L is about 6 mm.

It will be appreciated that there is some uncertainty about the exact position of the dividing line between the central volume and the conduit, because one merges into the other.

The basic structure in question might be termed a "duplex" resonator emitter, as it comprises two resonant cavities, namely the rear enclosing cavity 6, wherein the compliance and inertance properties of the loudspeaker diaphragm correspond to the area and length parameters of the resonant cavity, and a frontal emission cavity (7 and 8/9), together with their associated parameters of volume, area and length.

Regarding the rear enclosing cavity 6, the following prevail: Area (equivalent) S1: determined by the micro-speaker properties.

Length (equivalent) L1: determined by the micro-speaker properties.

As regards the volume V1, it was found that reducing the volume below 1 ml impaired the low frequency (LF) response, whereas increasing V1 above 2 ml progressively improved LF performance. In general, a value of about 2 ml was found to represent a reasonable compromise between LF performance and space usage. A significant resonant peak, associated with the rear cavity 6, was observed at about 1 kHz.

Regarding cavity 7 in combination with emission cavity 8/9: Area S2: a value of 20 mm2 was used in the example described.

Length L2: a value of 6 mm was used in the example described.

Volume V2: a range of values between 0 ml and 10 ml was used.

This embodiment of the invention exhibited audio performance which, though adequate for some purposes, was compromised to an extent by the presence of a large resonant peak, associated with the emission cavity 8.

It was determined however that, although the existence of this resonant peak is unavoidable in such a structure, its effects could (if desired) be mitigated, at least to an extent, by designing and constructing the relevant components so as to reduce the Q factor of the resonance, and to move the spectral peak to as high a frequency as would be practically possible, ideally, above and beyond the spectral range of interest The Q factor can be defined as the quotient of the resonant frequency, cOO, and the width of the resonant peak at the half-power points (off and off) lying at the -3 dB points flanking the peak at coo. = A

{02-6) Kinsler et al. also note, in the publication referenced above, that the resonator acts as an amplifier, in effect, with Q representing the ratio of the pressure amplitudes inside and outside the cavity, and hence gain factor, G (dB), is given by: G = 20 log (6) Hence a typical Q value of 96, obtained experimentally, represents a gain factor of 40 dB.

By consideration of equation 2, the Q factor of the emitting cavity can be reduced either by reducing V, reducing L, or increasing S. or by any combination of or, preferably, all of these. In addition to the Q factor, another important feature of the resonant peak of a resonant system is its overall shape, and this is governed by the presence of any associated non-reactive impedances, such as resistive components, and their relative values. Their effect is to limit the range of the impedance of the system at resonance, such that it does not approach an infinitely large value or a zero value.

Accordingly, it is possible to have two resonant systems featuring an identical Q value, but which possess differing spectral profiles. One might display a resonant curve in the frequency domain featuring concaveupwards flanks to the resonant peak, for example, whereas the other might exhibit flanks having a concave-downwards profile. In the present invention, resistive elements are l 0 introduced, as described later, in order to refine and match the shape of the resonant profile of a compensating cavity to that of an emitting cavity. The equations described herein can be extended to include such resistive elements and model resonant system behaviour in even more detail.

The inventor has determined two useful general emitter design principles from equations (1) and (2). Firstly, it is noted that C is a constant (343 ms').

then if it is required to minimise L, then, say, let L be equal to 1; it can be shown that: to =: (C and Lconstant) (7) Hence, if it is desired to change Q but keep me at a fixed value, then the SN ratio must be kept constant.

The second principle, for minimising Q. can be derived by substituting (7) into equation (2), which yields: s LAO) (C and L constant) (8) Hence, for any particular chosen resonant frequency, we, the larger the area S. then the smaller the Q value will be. In accordance with these determinations and utilising these newly derived principles, a new resonant emitter was designed, and a practical embodiment thereof is depicted at 10 in Figure 4; this plate 10 being intended to replace the plate 8 shown in Figure 3.

to In accordance with the aforementioned principles, volume V2 was reduced by removing the entire frontal emission cavity variable-volume unit 7, such that only the intrinsic volume present in the edge emitter plate 10 was present; the upper surface of the plate 10 being covered with a simple flat blanking plate (not shown). This residual intrinsic volume was further reduced by using a smaller diameter bore of 5 mm diameter, in order to just encompass the central emitting surface of the micro-speaker 4 rather than its entire upper surface.

Area S2 was increased by extending the width of the aperture to 12 mm, and using 2 mm thick plate to provide an area of 24 mm2. In this configuration, the intrinsic volume is linked to the aperture area, because they are both dependent upon the thickness of the plate.

In order to provide a gradual transition from the 5 mm wide central aperture to the 12 mm wide emitting aperture in plate 10, and avoid any impedance discontinuities, a linearly "flared" profile 20 was adopted, as shown in Figure 4; and is termed hereinafter for convenience a "5-12 flared emitter", referring to the 5 mm central aperture diameter and the 12 mm length of the emitting edge. Flaring characteristics other than linear (for example exponential or other curvilinear characteristics) can be used if preferred.

These expedients reduced the Q factor and increased the resonant frequency, as intended. Consequently, the high-frequency response was sustained to 10 kHz and beyond and, except for a still-present resonant peak in the 5 kHz region (and the rear-cavity related 1 kHz peak), the response was fairly flat, and was substantially free (within the frequency range of interest) from HE roll-off, sharp peaks and sharp troughs.

It is found in practice that the flared emitter 10/20 transmits significantly more signal than the emitter 8/9 in important regions of the sound spectrum, and provides more acceptable acoustic performance.

However, it is a further objective of the invention to produce a still higher- quality sonic emitter by further reducing or even eliminating the still- present resonant peaks present in the emitted spectrum. An obvious approach is to insert damping material into the frontal cavity. However, as already noted, this expedient significantly reduces the emitted volume level across the spectrum, and the micro-speaker cannot be simply driven harder to compensate for the reduction in sound volume if it is already operating near its maximum output capability.

A further embodiment of the invention therefore utilises a third resonant cavity, linked to the emitter cavity 10/20, tuned to the unwanted resonant frequency and having an appropriate Q factor. The third cavity absorbs acoustic energy specifically at the relevant resonant frequency and to the desired spectral profile, without reducing the sound output across the spectrum, as would happen if damping material were simply introduced into the cavity.

Accordingly, a three-cavity structure of this kind, shown schematically in Figure 5, was fabricated using laminar components as shown in Figure 6, in order to eliminate the still-present spectral peak at about 5 kHz. s

As shown in Figures 5 and 6, the lower two cavities are as before; a 2 mm thick, 5-12 flared emitter 10/20 on top of the plate 5 carrying the microspeaker 4, which was backed by a 2 ml rear cavity 6. The third (upper) resonant cavity 11 was linked to the emitter cavity 10/20 via a small aperture 12, having associated length L3 and area S3, directly overhead the emitting cavity volume, i.e. on the central axis 13 of the assembly. The compensating cavity 11 comprises an 8 mm diameter aperture 14 in a 2 mm thick aluminium plate 15, forming the volume element, coupled to the resonant emitter via a 1.5 mm hole 12 in a 2 mm thick aluminium plate 16; the cavity 11 being closed by an upper blanking plate 17. The parameters used were thus: V3=0. 1 ml; S3= 1.8 mm2; L3=2mm.

The foregoing arrangement provides useful improvements in sound quality.

Any remaining unwanted artefacts can if desired be mitigated by the incorporation of a resistive acoustic element, for example a single layer of very thin tissue paper, at the entrance to the compensating cavity 11. This partly de-couples the compensating cavity and affords control of the shape of the resonant peak, and produces an acoustically acceptable result. An alternative damping procedure is to introduce some acoustic damping material, such as cotton or glass fibre wool, directly into the compensating cavity 11. This works equally well, and is preferred for a manufacturing process.

The parameters of the triple resonator system are tabulated below.

Cavity Length L Area S Volume V (mm) (mm2) (ml) Rear Enclosing Cavity, 6 N/A (micro- N/A (micro- 2 speaker) speaker) Frontal Emission Cavity, 10 10 24 0.26 Frontal Compensating 2 1.8 0.1 damped Cavity, 11

_

In addition to the tuning of the frontal emitter cavity 10/20 for minimalartefacts, and the incorporation of a (possibly damped) compensating cavity 11 to enhance the smoothness of the spectral response, the rear enclosing cavity 6 can also be tuned advantageously to enhance the frontal emitter output Firstly, by the incorporation of cotton or fibre wool damping material into the rear cavity 6, the spectral peak at about 1 kHz associated with that cavity can be substantially eliminated. By choosing different materials having differing densities and structure, the degree of damping can be varied to suit different circumstances.

Moreover, if a vent tube or conduit having particular, calculated dimensions is introduced to the rear cavity, a small but significant increase in mid-frequency response can be achieved under certain circumstances.

As shown in Figure 7, therefore, the triplex arrangement of Figure 5 is further modified by the connection to the rear cavity 6 of a breather tube 18. The physical construction of the Figure 7 arrangement is identical to that shown in Figure 5; except that the rear cavity 6 provides 2 ml rear volume and opens into an aluminium tube, of length 15 mm and internal diameter 3.2 mm, which l O constitutes the vent tube 18. With the tube 18 open, there is a significant audible improvement. This embodiment of the invention thus incorporates damped compensating and rear cavities, together with a tuned rear cavity.

Figure 8 provides an indication of the benefits that can be gained from the invention. It shows for comparison, amplitude versus frequency performance curves derived under identical circumstances from an existing mobile phone of good quality and from a mobile phone enclosure containing an arrangement according to the embodiment of the invention described with reference to Figures 5 and 6. Both devices utilised identical microspeakers.

It will be seen that the invention provides a substantially smoother characteristic than the prior arrangement.

Laminar constructions such as those shown in Figures 3, 4 and 6 hereof and fabrication methods therefor are considered to be inventive in their own right and are accordingly further described and claimed in a co- pending patent application (applicant's reference SPQ003) filed contemporaneously herewith and by the same applicant.

In general, it is noted in relation to the invention that: 1. The parameters of the tuned sonic emitter can be selected and adapted so as to enable its integration into a wide range of differing device body sizes and shapes.

2. Although aluminium was used for device fabrication in the above examples, it is equally effective to use other materials, such as plastics materials, although it is much preferred to use a rigid plastic material rather than a soft plastic (such as polystyrene) in order to minimise secondary emission.

3. To further reduce secondary sonic emission, it is preferred to acoustically isolate, in so far as it is possible, the sonic emitter from the housing of the host device. This can be done by minimising the physical contact between the two.

4. The structure can be extended to include additional resonator elements.

For example, a plurality of emitting cavities, rather than a single emission cavity, may be provided if desired, in order to provide acoustic filtering to boost a certain region of the spectrum.

Furthermore, and with particular regard to the reproduction of 3D-audio using micro-speakers, an intrinsic and critical element of all loudspeaker-based 3D audio is the provision of transaural crosstalk cancellation, as is described in GB 2,340,005. This relates to the natural acoustic crosstalk which occurs when an individual is listening to a pair of conventional stereo loudspeakers.

When the left-channel sound signal is emitted by the left-hand loudspeaker, it travels not only to the left ear, but also, a little later in time, it crosses to the right ear (and vice versa). The brain recognises the high degree of correlation between the two signals, the primary signal and the transaural crosstalk signal, and attributes their source - correctly - to the left-hand loudspeaker.

The presence of the transaural crosstalk signal inhibits 3D audio effects, and so it must be cancelled by generating a cancellation signal which is equal in magnitude, and opposite in polarity, from the opposite loudspeaker, as is described in GB 2,340,005.

In order to achieve adequate transaural crosstalk cancellation, the cancellation signal must match the crosstalk signal in magnitude and phase within fairly precise limits. This means that the relative time of arrival of the signals at the listener's ears must be synchronized very carefully. Anything which interferes with the integrity of the left and right-channel signals will degrade the crosstalk cancellation and hence impair the effectiveness of the perceived 3D-audio. During playback on portable devices, where the loudspeakers might only be 40 mm or so apart, the timing must synchronise to within a few microseconds for optimum effect. Bearing in mind these constraints, and other considerations, the following rules should preferably be followed for rendering 3D-audio via micro-speakers.

The rear of each loudspeaker should be adequately enclosed.

The emission sources should be small, and spaced as far apart from each other on the device as possible, and on the same horizontal axis.

The loudspeakers and their emitted acoustic energy should be acoustically well isolated from the host device casing, insofar as is possible.

The frequency response of the system should be smooth, and free from significant troughs and peaks.

The system should possess good left-right matching in all respects.

It will be appreciated that the invention may be implemented in other forms and utilised in other applications than those specifically described herein, and that accordingly the examples used herein are not intended to limit the scope of the claims hereof.

Claims (19)

1. Claims: 1. A sonic emitter arrangement comprising a sonic transducer

   encased within a housing dimensioned to be portable or wearable; said transducer having an emission surface and a further surface substantially opposed thereto, an acoustic conduit linking said emission surface to an outlet from said enclosure for sound produced by said transducer, and control means for influencing at least one parameter associated with sound emitted from said outlet, wherein the control means comprises first and second acoustic resonators respectively coupled to the emission surface and said other surface of the transducer.
2. 2. An arrangement according to claim 1 wherein said transducer comprises a micro-speaker or an alternative miniature loudspeaker.
3. 3. An arrangement according to claim 1 or claim 2 wherein the said parameter influenced by the control means comprises the amplitude versus frequency characteristic of sound emitted from said outlet.
4. 4. An arrangement according to any preceding claim comprising a further acoustic resonator linked to said acoustic conduit, in order to further influence said at least one parameter and/or a further parameter associated with sound emitted from said outlet.
5. 5. An arrangement according to any preceding claim wherein said conduit includes a passage linking said first acoustic resonator to said outlet.
6. 6. An arrangement according to claim 5 wherein said passage linking said first acoustic resonator to said outlet is flared to increase in width as it approaches said outlet.
7. 7. An arrangement according to claim 6 wherein said flare is substantially linear.
8. 8. An arrangement according to claim 6 wherein said flare is substantially exponential or is otherwise curvilinear.
9. 9. An arrangement according to any preceding claim wherein at least one of said acoustic resonators comprises a Helmholtz resonator.
10. 10. An arrangement according to any preceding claim wherein at least one of the acoustic resonators is fabricated from superposed laminar components.
11. 11. An arrangement according to claim 10 comprising a laminar component in the form of a plate having an aperture therethrough and wherein an edge-emission conduit is formed in said plate by a passage linking said aperture to an edge of the plate.
12. 12. An arrangement according to any preceding claim wherein at least one of said acoustic resonators is fabricated of, or contains or has associated therewith, acoustic damping material.
13. 13. An arrangement according to any of claims 1 to 11 wherein at least one of said acoustic resonators is fabricated from, or incorporates, a sound absorbing medium.
14. 14. An arrangement according to any preceding claim wherein said second acoustic resonator is connected by way of a tube or conduit to a vent formed in the said housing.
15. 15. A sonic emitter arrangement substantially as herein described with l O reference to and/or as shown in the accompanying drawings.
16. 16. An electronic device incorporating at least one arrangement according to any preceding claim.
17. 17. A device according to claim 16 wherein said housing serves as the overall housing for the electronic device.
18. 18. An earphone including an arrangement according to any of claims 1 to 15.
19. 19. A pendant or other fashion-related article intended to be worn by a user and including an arrangement according to any of claims 1 to 15.