LOUDSPEAKER

Abstract: A loudspeaker has a bending wave panel (11) and an exciter (13) mounted on the panel to excite bendingg wave modes in the panel. A rear box (15) defines a cavity (17) in cooperation with the panel (11). A resonance of cavity and panel at a coupled mode resonance frequency may be controlled by providing a duct (19) coupled to the cavity to selectively reduce sound pressure at the coupled mode resonance frequency. Damping (21) may be provided in the duct (19).

Description: TITLE: LOUDSPEAKER DESCRIPTION This invention relates to loudspeakers and more particularly to loudspeakers incorporating resonant panel acoustic radiators. In some applications it would be preferable to mount loudspeakers in a shallow enclosure. This is particularly true of distributed mode loudspeakers intended for wall mounting. If such loudspeakers have an open back, then the adjacent wall will affect the sound output in an uncontrollable manner since the environment will not be constant from one loudspeaker location to the other. The provision of a shallow enclosure can alleviate this problem. However, there are some disadvantages with shallow enclosures. In particular, strong coupling occurs between the panel and enclosure. The air in the shallow enclosure acts as a spring and the panel can oscillate on the spring moving as a rigid body backwards and forwards at a frequency that is often inconveniently in the audio range. This resonance can give rise to uneven frequency response in the output of the loudspeaker when it is used. The response can give rise to a large audible peak in the frequency response . For low quality applications, electrical circuits can adequately equalise the peak. However, such equalisation may not be desirable in high quality products - for example ambient temperature may affect the electrical circuits differently from the acoustical properties. Damping is another way to reduce such coupled mode peaks. This is again more suited to low quality applications. Moreover, damping increases the width of the peaks and so a system can sound worse with damping than without . It would also be advantageous if classical pistonic loudspeakers could be mounted in a shallow cavity. It is normal to mount such loudspeakers in enclosures, since if there is no enclosure behind such a loudspeaker, the sound output from the rear of the loudspeaker is in antiphase with the sound output from the front and tends to cause cancellation at low frequencies. However, in such pistonic speakers the whole body mode resonance is almost always arranged to be at the low frequency end of the response of the loudspeaker. In this way the increase in sound output caused by the resonance can compensate for a falling off of sound at low frequency, and extend the bass response of the loudspeaker. This does mean that the box cannot be too shallow, and significant volumes are required behind the loudspeaker. To make the box shallow and so move this resonance from its useful low frequency to higher in the audio range where it impedes an even frequency response would fly in the face of conventional teaching. According to the invention there is provided a loudspeaker comprising a panel member for emitting sound, an exciter for exciting the panel member to emit sound, a box behind the panel member defining in co-operation with the panel member a shallow cavity, causing a coupled resonant mode of the panel member and cavity at a coupled mode frequency, characterised by a duct acoustically coupled to the cavity for selectively reducing sound pressure in the cavity at the coupled mode frequency. The duct may act as a pressure relief means that allows pressure waves at the selected frequency to be absorbed. Although it might be thought that simply adding an acoustic absorber to the cavity would work, the use only of an absorber would add absorption over a wide frequency range. Sufficient volume damping for control of the resonance will unnecessarily attenuate acoustic power below and above the resonance due to a lack of selective absorption. Also, a shallow box has insufficient depth for a suitably large thickness of absorbent . By shallow cavity is meant a cavity which has ceased to function as a volume behind the panel but in which the finite thickness gives rise to effects such as the coupled whole body mode within the acoustic range. In general, the cavity will need to be less than half the smaller dimension of the panel in its plane before such effects become significant, preferably less than a quarter or further preferably less than 10% of the smaller dimension. The shallower the cavity, the thinner the loudspeaker can be and this is generally desirable. Although the sound is selectively absorbed at the coupled mode frequency it is not required that the absorption is exactly tuned to a specific frequency. For example, the whole body mode resonance can be quite broad and the absorption can advantageously be suitably broad as well. In embodiments, the solution proposed may allow several advantages over the alternative approaches. Firstly, the cost of providing the duct can be absorbed in the cost of providing the initial tooling to form the cavity. The approach may also be tolerant of temperature changes, since these are likely to affect the air in the duct and the cavity in parallel . The approach is also permanent . The coupled mode can be the whole body mode, since this is in general the dominant mode in a shallow cavity and hence the mode that requires reduction. One end of the duct may be open to the cavity. The other end may be closed. Slots may be provided in the duct to adjust the properties of the cavity. The duct can be tuned as a quarter wavelength duct to a frequency within 10% of the coupled mode frequency. Although a starting point for the length of the duct is a quarter wavelength of the sound wave at the required frequency, the exact length depends on end corrections, and whether the duct is bent. Such corrections are well known in the art. Accordingly, by "quarter wavelength duct" is meant an appropriately tuned duct, not a duct of exactly that wavelength. As mentioned above, the coupled mode absorption may be quite broad over the frequency range so it is not necessary for the length to be precisely a quarter of the wavelength of the frequency of the coupled mode . A more accurate tuning than the 10% margin indicated above would be preferable; accordingly it is preferred that the duct is tuned to a frequency within 5% of the coupled mode frequency. Although pure acoustic resistances are unavailable, a quarter wavelength converts a real impedance to an imaginary, and vice versa. Accordingly, a quarter wavelength duct can act as an effective acoustic absorber for sound at the frequency for which its length is a quarter wavelength. The ducts need not be straight. Slight curvatures barely affect them, and sharp bends add acoustic mass, causing them to tune low. This effect can be countered by altering the length of the duct appropriately, as is known . The ducts can be located either inside or outside the main cavity, as required. Experiments have shown that the improvements are greater when sound-absorbing material is provided in the duct. The sound absorbing material may be acoustic fibre and/or foam plastic. In an alternative arrangement the duct may be both coupled to the cavity and separated from the cavity by a membrane, so that the duct in effect forms an auxiliary cavity. The coupled system of the duct and membrane may be arranged to resonate at approximately the coupled mode/whole body mode frequency in the cavity. The cavity can be in a different plane to the main cavity. Again, absorbing material in the duct may be useful. The membrane itself may be absorbing, by providing damping in the membrane, and/or by providing a damped suspension of the membrane . Alternatively, the duct may be an aperture connecting the cavity with the ambient. Whereas with a quarter wavelength closed duct the length determines the tuning of the duct, in this arrangement the width and area of the duct provide some tuning. That is because the area can be too small for lower frequencies to readily pass through, whereas at higher frequencies the area has a low sound radiation efficiency. In embodiments a strip may be provided around the edge of the panel to support the panel on the box, wherein the strip is omitted over part of the edge to so that the panel, box and strip define the duct; in this case the duct may be open to the air away from the cavity. This arrangement can be simpler than the solution of providing separate ducts. The strip may be a resilient strip to resiliently support the panel. In this way, the panel can be freely mounted, i.e. not clamped at the edges. In embodiments the duct can be a hole provided in the front of the panel, or alternatively in the box. Such holes are preferably in the central area of panel or of the rear of the box. Such ducts are less frequency selective than a quarter wave plate, but this can be sufficient for some applications. In practice a plurality of ducts as described above may be provided for reducing sound pressure at the coupled mode frequency. It is also possible to provide a plurality of ducts to absorb sound at a plurality of frequencies. A particularly useful approach using a plurality of ducts will now be described. In this approach, a plurality of quarter-wave ducts tuned to different frequencies are provided acoustically connected to the cavity. This changes the resonant modes in the cavity, and in particular the ducts may be selected to reduce the fundamental frequency of the cavity and also increase the density of coupled resonant modes in frequency. The panel may be a distributed mode panel which operates by having a variety of resonant modes distributed in frequency. The increase in the number of modes for the coupled enclosure system can significantly improve the properties of a distributed mode loudspeaker having such multiple selective frequency control . The quarter-wave ducts may be provided along one side of the cavity, in which case that side of the cavity may be considered to act as a sound absorber. Although the invention, in all its forms, is particularly suitable for resonant bending wave loudspeakers the technique can also be used for removing unwanted resonance or resonances in any panel coupled to a shallow cavity. This may allow a shallow box pistonic loudspeaker to be manufactured. Specific embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings in which Figure 1 shows a schematic drawing of a first embodiment of the invention, Figure 2 shows a schematic drawing of a second embodiment of the invention, Figure 3 shows experimental results of the arrangement of Figure 2 both with and without the ducts, Figure 4 shows a third embodiment of the invention having a membrane, Figures 5 and 6 show a fourth embodiment of the invention using slots, Figures 7 to 9 show details of the acoustic response calculated as the slot length is varied in the arrangement of Figure 5 , and Figure 10 shows a fifth embodiment of the invention having a plurality of ducts of different lengths. In Figure 1, a distributed mode panel 11 is produced using the teaching of WO97/09842. The panel is of a preferred distributed mode aspect ratio, i.e. 1:882 or 1:0.707. A transducer 13 is mounted on the rear of the panel at a preferred transducer location. A shallow box 15 defines a shallow cavity 17 in co-operation with the panel 11. At one end of the cavity, a duct 19 is provided. The duct is one quarter of the wavelength of the coupled whole body mode of the panel 11 on the spring constituted by the air in the cavity 17 and any resilience in the panel 11 supports. Sound absorbing material 21 is provided in the duct 19. Referring to Figure 2, the second embodiment is shown viewed from behind through the rear of the cavity 17 that is 260mm by 210mm. The panel 11 forms the upper surface of the cavity and the transducer is shown at 13. Four ducts 19 are provided, one on each side wall of the cavity. The ducts are each 120mm long; accordingly, they are all intended to tune the same resonance, here the whole body mode resonance. Figure 3 shows the results achieved. The original frequency response is shown with a dashed and dotted line and the response with the ducts with a dashed line. The results show a significant reduction in the size of the resonance at around 740Hz due to the ducts. A reduction of 4dB is acoustically highly significant. With damping, the reduction in the resonance is further improved . Figure 4 shows similar arrangement to the embodiment of Figure 1 except that a membrane 23 is provided to divide the duct 19 from the cavity 17. The duct 19 is tuned to resonate at the whole body mode frequency. The duct 19 may be in a different plane to the main cavity. Damping 21 is provided in the duct: the damping may be acoustic fibre or foam as conventionally used for acoustic damping. The membrane may have a density chosen to act in cooperation with a resilient support of the membrane to vibrate at a resonant frequency to determine the selected resonant frequency of the duct. The resilient support may be provided in the duct, and may comprise foam, the air in the cavity or a combination of the two. Figures 5 and 6 show a fourth embodiment of the invention. The panel 11 is supported on the box 15 by a resilient strip 29 around the panel edge. The panel is supported 5mm from a rim 31 on the box. The box 15 and panel 11 are separated around the circumference of the panel by a 2mm gap therebetween. The strip has gaps in its length defining slots 33 along one side of the panel. The slots function as ducts but act in a different way to a quarter wavelength duct - they act as vents releasing sound pressure particularly at the whole body mode frequency. Damping 21 is provided in the cavity, fixed to the panel 11. Calculations have been carried out by finite element analysis to show the effect of varying the length of the slots and these results are presented in Figures 7 to 9. Each graph relates to a different slot length, as indicated in the key in mm. The slot 144.44mm long provides a particularly smooth response. As can be seen the strong peak around 760Hz can be cancelled out by using slots of various lengths. Moreover, the length of the slot can be fine-tuned to give a desirable acoustic response and quality to the loudspeaker produced. There is some difference in effect between a single contiguous slot and a plurality of slots making up the same total effect (see Figure 9) but the difference does not appear highly significant. Figure 10 shows a schematic diagram of a cavity with six quarter wavelength ducts 19 of different lengths, from 80 to 180mm in 20mm steps. These provide multiple frequency selective acoustic termination. The ducts 19 function to make the boundary acoustically effectively invisible over a range of frequencies. Calculations have shown that the number of useful resonant modes of the coupled system can significantly increase; the modes qualitatively differ from the modes without the ducts. Table 1 lists the resonant modes with the quarter wave ducts in the first three columns and without in the final three. It can clearly be seen that the number of modes is much higher with the ducts than without. With Ducts Without 388.12 1309 2223 9 1320.3 2238 8 479.63 1420.1 2355 7 1492.5 2334 1 568.32 1512.1 2411 3 1516.6 2479 9 646.05 1679.6 2556 8 660 16 1632 2599 2 749.62 1801.4 2624 6 746 26 1980.5 2640 7 837.16 1931.6 2686 6 1992.7 2744 1 962.92 2002.1 2816 2 996 35 1141.6 2081.8 2844 9 1236.2 2190.9 2937 4 2116.4 TABLE 1 Moreover, the cavity appears less stiff by controlling sound pressure at the whole body mode frequency so that the lowest coupled mode is now at a beneficially lower frequency. In the example the lowest mode is at 388Hz rather than 660Hz without changing the volume of air or the external dimensions of the box. The modal density of the system is also increased by about 50% in the range up to 3KHz range shown in the table.

Claims: CLAIMS 1. A loudspeaker comprising a panel member for emitting sound, an exciter for exciting the panel member to emit sound, a box behind the panel member defining in cooperation with the panel member a shallow cavity, causing a coupled resonant mode of the panel member and cavity at a coupled mode frequency, characterised by a duct acoustically coupled to the cavity for selectively reducing sound pressure in the cavity at the coupled mode frequency. 2. A loudspeaker according to claim 1 wherein the coupled mode is the whole body mode in which the panel member oscillates on its suspension. 3. A loudspeaker according to claim 1 or 2 wherein sound absorbing material is provided in the duct. 4. A loudspeaker according to any preceding claim wherein the duct is tuned as a quarter wavelength duct to a frequency within 10% of the coupled mode frequency. 5. A loudspeaker according to claim 4 wherein the duct is tuned to a frequency within 5% of the coupled mode frequency. 6. A loudspeaker according to claim 1, 2 or 3 wherein the duct is coupled to and separated from the cavity by a membrane wherein the coupled system comprising the membrane and the duct is tuned to a frequency within 5% of the coupled mode frequency in the cavity. 7. A loudspeaker according to any of claims 1 to 3 wherein the duct is an aperture provided at the edge of the cavity. 8. A loudspeaker according to claim 7 wherein a strip is provided around the edge of the panel to support the panel on the box, wherein the strip is omitted over part of the edge to so that the panel, box and strip define the duct. 9. A loudspeaker according to claim 8 wherein the strip is resilient. 10. A loudspeaker according to any of claims 1 to 3 wherein the duct is an aperture provided in the cavity in the centre of the panel member or in the rear of the box facing the centre of the panel member. 11. A loudspeaker according to any preceding claim wherein a plurality of ducts are provided for absorbing sound at the coupled mode frequency. 12. A loudspeaker according to any preceding claim wherein a plurality of ducts are provided for absorbing sound at a plurality of frequencies. 13. A loudspeaker according to any preceding claim wherein the panel is a distributed mode panel.

Inventors: BANK GRAHAM, HARRIS NEIL, BANK, GRAHAM, HARRIS, NEIL
Applicants: BANK GRAHAM, HARRIS NEIL, NEW TRANSDUCERS LTD, BANK, GRAHAM, HARRIS, NEIL, NEW TRANSDUCERS LIMITED
Application year: 2000
Main classification: H04R1
IPC codes: H04R1, H04R7