CN1135058C - Acoustic device based on bending wave principle and driving method thereof - Google Patents

Abstract

An acoustic device has an outer shell(1)supporting bending waves and enclosing a volume(2). The bending waves couple to the enclosed volume to provide coupled resonant modes. A transducer(3, 5)is coupled to the shell to excite the coupled resonant modes. The coupling of the enclosed volume can improve the distribution of resonant modes.

Description

Acoustic device based on bending wave principle and driving method thereof

field of invention

The present invention relates to an acoustic device and its driving method in the form of using an element supporting the action of bending waves on its entire surface, and the bending waves are coupled to the surroundings. Such a device can be used as a speaker or a microphone.

background of the invention

International patent application WO97/09842 and related applications describe loudspeakers and other acoustic devices having an acoustic element and a transducer coupled to the acoustic element. In these devices, various parameters of the element can be adjusted so that the resonant bending wave modes of the element are evenly distributed in frequency. Resonant bending wave modes can also be distributed over the surface of the element. This case also reveals the preferred location for mounting transducers on components. A typical preferred installation location is at a near center location, but not at the center. However, depending on the shape of the element, other preferred positions are also possible.

However, it is not always easy to provide sufficient modal density, especially at lower frequencies. Therefore, it would be advantageous if improved modal density or other enhancements could be provided, especially for lower or intermediate frequency responses.

Overview of the invention

According to a first aspect of the present invention there is provided an acoustic device comprising a substantially continuous enclosure supporting bending waves, the enclosure being curved to at least partially enclose an air volume such that bending waves couple into the volume to provide coupled resonant modes, and A transducer coupled to the housing for coupling the electrical signal at the transducer to the coupling mode and thus to the ambient sound.

In the device according to the invention, there are additional modes besides the resonant bending wave mode available in conventional distributed mode devices. Calculations will be provided later, showing the increased number of modes for air coupling to the enclosure. Therefore, according to the apparatus of the present invention, the number of modes existing in a predetermined frequency range can be increased.

Preferably, the coupled resonant modes are evenly distributed in frequency within a predetermined frequency range, advantageously about 1 to 2 or 3 octaves above the fundamental resonant frequency. In this range, the resonant bending wave modes are sparsest, and the distribution of bending and in-plane modes in this range yields the greatest benefit.

The housing can be completely closed to completely enclose the volume. Alternatively, apertures or vents may be provided in the housing. The orifices or vents can be designed to provide specific resonance effects and specifically enhance or control the limit output in the lower acoustic frequency range.

While the enclosure need not completely surround the enclosed volume, the enclosure must be practically continuous so that it can exhibit effective acoustics. In other words, the housing must not have too many holes or windows. Highly perforated elements are not suitable as acoustic radiators, since the radiation from the front of the element will destructively interfere with the radiation emitted from the rear, which is in phase opposition to the radiation from the front. Also, the shell should be continuous enough for the coupling of the shell to the enclosed volume to be significant.

Tests on boards have shown very low coupling of the perforated element to ambient air. Thus, the housing may delimit no more than 20%, preferably no more than 10%, more preferably no more than 5% of the surface area of its total surface area.

Air within the volume may also exhibit cavity resonances.

According to the acoustic device of the present invention, resonant bending wave modes are supported at the surface of the three-dimensional housing and coupled to a volume at least partially enclosed by the housing. In contrast, conventional distributed mode loudspeakers have resonant bending wave modes distributed over a single panel.

In the aforementioned WO97/09842 it is proposed to mount a distributed mode panel to the front of a frame. In such prior art devices, any resonant bending wave modes are effectively limited by the panel area, not by the frame. Therefore, these devices do not provide the improved modal density and thus the improved acoustic performance provided by the device according to the invention.

Another previous publication, WO98/31188, describes a flat plate mounted on a tray. The tray is highly perforated, has more apertured areas than solid areas, and is therefore not actually continuous. The tray will thus not be effectively coupled to the surroundings, nor to the air within the tray.

The shell may be of constant thickness. Alternatively the thickness of the shell can be changed slowly or continuously or more rapidly.

Ribs or other extensions may be provided on the housing.

The housing can be a single unitary housing. Alternatively the housing may comprise a combination of elements, mechanically coupled to create the desired acoustic radiator structure. In conjunctions between elements, the boundary conditions may advantageously be specified.

The housing may be in the form of a polyhedron having a plurality of individual faces or a portion thereof.

Each end face may have a natural resonance frequency, and the natural resonance frequencies may be chosen to have different values. This increases the modal density of the overall enclosure. Furthermore, the different natural resonance frequencies can be chosen such that the modes resonating at different end faces are interleaved in frequency. This approach may be particularly useful for increasing the modal density for the lower 10-20 resonant modes.

The individual end faces or individual plate elements need not have uniform mechanical properties, and they may vary in stiffness, isotropy of stiffness, damping, or thickness.

The audio device can be a loudspeaker, and the transducer can be an exciter.

The acoustic device may comprise front and rear end faces in the form of plates, together with at least one other plate, providing a path for resonant modes from the front to the rear and thus coupling the front and rear plates. The front and rear panels may actually be planar. The front and rear boards can be driven by separate discrete drivers, or a single driver can be coupled to the front and rear boards.

In embodiments, a driver's voice coil may be coupled to one of the plates, and the driver's magnet assembly coupled to the other board. Due to the weight of the magnet assembly, its coupling to the board will cause high frequency attenuation. This enhances the bass response of the audio device. Many actuators are available. The actuators may be driven in phase, out of phase or in any suitable phase relationship to each other.

In a conventional flat panel speaker, any in-plane compression waves produce little or no audible output. This is because the in-plane compression and expansion of the flat plates are not coupled to the surrounding air. In contrast, in devices according to the invention, the shell is curved so that in-plane compression and expansion cause local or general contraction and expansion of the shell, which acts as a mechanism for coupling compressional waves to the enclosed volume and to the surroundings. Therefore, in-plane compression waves can beneficially contribute to coupled modes. In fact, in some embodiments, the resonant mode can couple bending waves with the planar mode and the enclosed volume. Modal density can thus be improved.

According to a second aspect of the present invention there is provided an acoustic device comprising a substantially continuous enclosure enclosing a volume, supporting a plurality of resonant modes, coupling the enclosure and the enclosed volume, the resonant modes spanning the enclosure, and a transducer coupled to The housing, which couples the electrical signal to the resonant mode at the transducer, and thus to the surrounding sound.

In a device according to the second aspect of the invention, the resonant modes span the surface, from front to back, from side to side, and from top to bottom. This allows good mode coupling to the enclosed volume over the entire surface. The modes need not necessarily cover the entire surface; the enclosure may for example have apertures or regions that are not resonant.

According to another aspect of the present invention there is provided a method of driving an acoustic device, comprising: providing a substantially continuous housing, the housing being curved to at least partially enclose an air volume, providing two transducers, the two transducers being coupled to the housing; supporting bending waves such that bending waves couple into the volume to provide coupled resonant modes for driving the two transducers in phase with a common electrical signal such that the transducers drive the coupled mode of the housing and volume into a monopolar configuration; and Radiates sound energy into the surrounding air from coupled modes.

Brief description of the drawings

Certain embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1a shows a section through a loudspeaker according to a first embodiment of the invention, the loudspeaker having an ellipsoidal enclosure;

Figure 1b shows a section through another loudspeaker according to a first embodiment of the invention, which loudspeaker has an ellipsoidal housing;

Figure 2 shows a section through a loudspeaker according to a second embodiment of the invention;

Figure 3 shows a section through a loudspeaker according to a third embodiment of the invention, the loudspeaker having an aperture;

Figure 4 shows a modified profile through an orifice;

Figure 5 shows a view of a loudspeaker according to a fourth embodiment of the invention in the form of a vented box;

Figure 6 shows a view of a loudspeaker according to a fifth embodiment of the present invention, the loudspeaker having a closed box;

Figure 7 shows various excitation techniques that can be used with the loudspeaker shown in Figure 6;

Figure 8 shows the stimulus response in an air-free vented box;

Figure 9 shows the excitation response of the vented box shown in Figure 8, including the effects of air coupling;

Figure 10a shows the velocity response of the exciter as a function of frequency in the case of Figure 8 without air;

Figure 10b shows the velocity response at the exciter as a function of frequency in the box in Figure 9, including air effects;

Figure 11 shows the pressure inside the tank of Figure 9b;

Figure 12 shows the mode in a loudspeaker with an enclosed enclosure, with two exciters driven in antiphase;

Figure 13 shows the mode in the loudspeaker of Figure 12, with two exciters driven in phase;

Figure 14 shows the velocity response of the devices in Figures 12 and 13;

Figure 15 shows the velocity response of a six-sided closed box in which the bending stiffness of the front face does not match that of the rear face;

FIG. 16 shows a view of a loudspeaker in the form of a frusto-conical body according to the invention;

Figure 17 shows a view of a loudspeaker in the form of a tetrahedron according to the present invention;

Figure 18 shows a view of a loudspeaker in the form of a dodecahedron according to the present invention;

Figure 19 shows a view of a loudspeaker in the form of a cylinder according to the invention;

Figure 20 shows a view of a loudspeaker in the form of a cone segment according to the present invention;

Figure 21 shows the root mean square (rms) central difference of mode frequencies as a function of the device front facet aspect ratio of Figure 5;

Figure 22 shows the velocity distributions for three exciter locations for the device of Figure 5 having a front face with an aspect ratio of 2:1;

FIG. 23 shows a plot of actuator position efficacy as a function of actuator position on the device for the model used in FIG. 22 .

Detailed description

Please refer to Fig. 1, a closed ellipsoid shell 1 encloses a volume 2, and transducers 3, 5 are fixed inside the shell, and are installed at opposite positions on the short axis of the ellipsoid. The enclosure 1 supports resonant modes formed from resonant bending wave components coupled into the enclosed volume.

The transducer couples the electrical signal to coupled resonant modes of the housing and volume. In this embodiment, the transducer is an exciter which, in use, can be driven to excite the coupling mode to produce an acoustic output. The transducer may be of the conventional type in which a voice coil moves relative to a grounded magnet assembly when current is passed through the voice coil. The transducer may be inertial, in which case the magnet assembly is free and the force of the voice coil acts against the inertia of the magnet assembly. Alternatively, a grounded transducer can be used, in which case the magnet assembly supports it. In this example, commercially available exciters, commonly used to drive distributed mode plates, were used in an inertial configuration.

The transducer is driven with a known polarity whereby a desired polarity state can be produced in the emitted sound. For a unipolar source, the transducers can be driven in-phase, whereas for a dipolar source, the transducers can be driven in-phase. Alternatively, the actuators may be driven at any suitable phase relationship.

The invention allows loudspeakers to be used without baffles. In conventional pistonic or distributed mode loudspeakers using a single diaphragm or plate, the sound radiated from the rear is in phase opposition to the sound radiated from the front. Therefore, to avoid interference effects, it is necessary to enclose the diaphragm with a box, or provide a baffle around the speaker, so as to prevent the sound radiated from the rear from reaching the front. By driving the loudspeaker according to the invention as a monopole with the two transducers operating in phase, the need for this baffle can be avoided.

The coupling mode can consist of two types of vibration coupling of the enclosure to the enclosed volume. One of these two types is a bending wave that bends the shell away from the local plane of the shell. Another pattern is expansion or contraction in the plane of the shell.

A perfectly flat plate will not provide this coupled expansion-contraction mode with resonant bending wave modes. While a flat plate can have an oscillatory mode of expansion and contraction, this only moves the plate in its own plane and does not affect the motion of surrounding air molecules. Therefore, in these modes of the flat panel, there is little or no acoustic effect. In contrast, if the plate itself is bent back enough, or even forms a completely closed body, in-plane compression wave modes cause a general expansion and contraction of the body, which can couple to the air and thus have an acoustic effect.

The actual mode of housing vibration need not be a pure bending wave mode, or a pure compression wave mode. Instead, the modes may interfere with each other and couple to provide coupled modes. However, such modes may still retain predominant bending wave properties. These waves at the enclosure are then coupled into the contained volume to generate coupled resonant modes.

The transducer is to be mounted on the stub axis and is not necessary. It may be convenient to mount it off-axis as shown in Figure 2, or indeed in any suitable location. The transducers are preferably mounted in a location selected for an optimal or desired response. Using a regular geometry, such as an ellipsoid, makes this easier. Alternatively methods such as finite element analysis can be used to find out the proper transducer location. In general, an approach similar to the use of distributed mode loudspeakers may be appropriate; in particular asymmetrical transducer positions may prove appropriate. Examples of this situation will be discussed later with reference to FIGS. 21 to 23 .

For some applications a single transducer may be sufficient; others may require several transducers spread out over the housing. The placement of the transducers may affect the directionality of the housing's coupling to the environment.

Providing an enclosed volume allows the use of orifices to control resonance within the volume. Figure 3 shows an orifice 7 in the form of a simple hole at one end of the ellipsoid. Alternatively, a conduit opening 9 may be provided as shown in FIG. 4 .

The apertures may result in an effect similar to that which such apertures would produce in conventional piston type speakers or tubes. The orifices may have an asymmetric profile.

As indicated above, the volume need not be entirely enclosed by the shell. Instead, all that is required is for the enclosure to bend back enough on itself that the resonant modes of the enclosure couple to the air in the enclosed volume to create the acoustic effect. FIG. 5 shows an open box comprising a large front face 11 surrounded by a frame 13 formed by four sides 15 at right angles to the front face 11 . A single transducer 3 is provided, connected to an amplifier via an audio connector 9 . The side faces 15 are both acoustically coupled to the front face 11 . The resonant bending wave mode at the front face 11 is not only maintained at the front face, but also coupled to the side faces (15).

A box can also be implemented in the form of a sealed enclosure (FIG. 6), with front and rear end faces 11, 17 and four sides 13 joining the front face 11 to the rear end face 17 to form a sealed enclosure containing a volume . Two transducers 3,5 are provided, one at each end face 11,17.

Also shown in Figure 6 is a circuit 19 which is switchable between inverting and non-inverting drive of the two transducers. A double pole double throw switch 21 switches between parallel and antiparallel drive of the transducers.

As shown in Figure 7a, the transducers or the front and rear end faces can be decoupled. Or the magnet assemblies of two traditional moving coil transducers can be coupled together as shown in Fig. 7b. As another alternative, a single transducer could have its voice coil attached to the front face 11 and the magnet assembly attached to the rear face 17, the magnet assembly being much heavier than the voice coil and thus the assembly would preferentially couple low frequencies to back face. Therefore, this configuration can be used to increase the bass response of the speaker. The front and rear end faces can be reversed.

A finite element calculation of the response of the acoustic device was performed for a five-sided device similar to that shown in FIG. 5 . For convenience, this configuration will be referred to as an open box. Figure 8 shows how the box responds to excitation at 178 Hz (Figure 8a), at 348 Hz (Figure 8b) and at 1000 Hz (Figure 8c) without air. Figure 9 shows the conditions in the presence of air at the same frequencies, namely at 178 Hz (Figure 9a), at 348 Hz (Figure 9b), and at 1000 Hz (Figure 9c). As can be seen, the response is not limited to any planar surface, but is coupled across the entire five surfaces of the tank. Also, the presence of air advantageously increases the complexity of the shape.

The velocity response of the transducer as a function of frequency is shown in Figure 10: Figure 10a shows the results without air, and Figure 10b shows the results with air. A large value indicates a high speed achieved by excitation at that frequency. Particularly large velocities occur at resonance. As can be seen, the response with no air shows a smaller number of larger spikes. This is specific to the smaller number of resonant modes. The response in the presence of air encapsulation shows a larger number of peaks and smaller peaks. This is specific to a larger number of weaker modes. As can be seen, the modes in which the plate couples to the enclosed volume increases the number of resonant modes and thus improves the acoustic device. Unexpectedly, this effect is quite pronounced even in the open box.

Figure 11 shows the air pressure inside the chamber at 348 Hz. The asymmetrical air pressure pattern can be clearly seen. It is this air pressure distribution that results in the complex mode shape of the air-coupled resonant modes.

Similar calculations have been carried out for an acoustic device similar to that shown in Figure 6, namely a closed six-sided box with a front, a rear and four sides. Some results are shown in FIGS. 12 and 13 . All calculations include air. Also, coupled modes are coupled on all surfaces of the box, front and rear, left and right sides, and top and bottom.

Figure 12a shows the resulting pattern at 178 Hz driving the enclosure at in-phase speed. Since the front and rear end faces face in opposite directions, this is accomplished by the transducers on the front plate moving the plate outward while the transducers on the rear plate move the plate inward. This can be achieved by electrically connecting the transducers out of phase, for example using the switches shown in FIG. 6 . Figure 12b shows oscillations at 1000 Hz.

Figures 13a and 13b show that as a monopole, the front and rear plates of the box are moving in opposite phases, ie electrically connected in phase to the same frequency mode of the transducer. As can be seen, complex responses are again obtained.

Figures 12c and 13c show the air pressure inside the chamber when driven at 1000 Hz with dipole and monopole respectively, corresponding to the response of the chamber shown in Figures 12b and 13b. Figure 12c clearly shows an asymmetric response even though the drive and tank are symmetrical. Figure 13c shows the very different pressure responses resulting from driving the same tank in different ways.

Several transducer speed versus frequency plots obtained with a closed box are provided in FIG. 14 . Figures 14a and 14b show the response at the matched front and rear facets (driven with unipolarity) of a symmetrical closed box, which can be expected. Figure 14c shows the significantly less uniform and thus poorer results obtained for dipole drive of the same tank.

Of course, all of the above results are only calculations, but they do show the improvements possible using a shell bent back on itself, to enclose a volume.

Figure 15 shows the velocity response for a box whose front face has a different stiffness than the rear face, driven by two transducers, one at the front (shown in Figure 15a), and one at the rear face (Figure 15b) Curved circle. As can be seen, the response of the front face is advantageously different from that of the rear face. Therefore, the use of asymmetry can be beneficial to increase the modal density at frequency.

It should be noted that there is a difference between a device such as that shown in FIG. 5 with many end faces, and a device such as that in FIG. 1 in the form of a continuous curve. The bond 32 between the end faces acts like a hinge, and thus not only does the resonant bending wave mode not travel from one facet to the next. Instead, a more complex coupling of patterns occurs.

Other faceted structures are also possible. Figures 16 to 20 show various forms, namely truncated square pyramids, tetrahedrons, dodecahedrons, cylinders and cone segments. Each of these forms may be open or closed; for example a cylinder may or may not have an end face, and a cone segment may or may not have a rear end face. Individual end faces may be formed separately and then joined, or groups of end faces or even the entire structure may be integrally formed.

Good aspect ratios for an isotropic rectangular plate are 0.707:1 and 0.882 to 1, as discussed in WO97/09842. The characteristics of the enclosure can also be adjusted according to the invention, thereby optimizing the acoustic device to maximize the distribution of resonant modes at frequencies and to position the transducers correctly on the board, thereby providing good uniformity of the transducers to the modes coupling.

This can be done using techniques discussed in various distributed mode patent applications. In particular, a sequential approach is used to find the optimum aspect ratio and transducer position to provide the best possible result.

In order to find the proper properties for the vented box, a first step was carried out to model the variation of the aspect ratio of the central panel of the vented box in FIG. 5 . The aspect ratio was varied from 1 to 2.25, and the corresponding frequencies of the modes were calculated by finite element analysis. The rms central difference of the mode frequencies is plotted against the aspect ratio (see Figure 21). The central difference of the mode frequencies of the nth mode is the frequency of the (n+1)th mode plus the (n−1)th mode minus twice the frequency of the nth mode. If the patterns were equally spaced, this measure would be equal to zero. Thus, the root mean square (rms) central difference provides a map of the advantages of various aspect ratios. The smaller the rms central difference, the better.

From Figure 21 it can be seen that good results are obtained for aspect ratios between 1.6 and 2.2, especially good results are obtained between 1.95 and 2.05. An aspect ratio of 2 was taken as a convenient value for further study.

The next stage is to find the optimal driving point at the end face. Computes the velocity response as a function of frequency for several drive point locations. Figure 22 yields three instances, at the center 22a, at the standard drive point of a flat distributed mode plate 22b, and at the optimum drive point 22c. For graphs such as these, the standard deviation is plotted as a function of position in FIG. 23 . The best result is shown in black with the least deviation. Please note that the edge of the board is not shown. Such edges are bad driving points.

As can be seen upon inspection of the figure, the optimal actuation points occur at four locations, at about 30% of the distance along the long side, and 30% of the distance along the short side, and three other locations found around the central axis of symmetry at the first location, To a position about 70% along the major axis and 30% along the minor axis, 30% and 70% along the respective axes, and 70% and 70% along the respective axes. These points differ from the optimal actuation points for a simple rectangle whose location occurs near the center at coordinates about (3/7, 4/9) expressed as distance ratios along the sides.

The position is quite tolerant and good results can be obtained at positions from 14% to 42% along the long side, and from 22% to 34% along the short side, along with a point of symmetry for this value.

Although the above calculation was performed without accounting for the effect of air, it provides a good indication of the proper aspect ratio and transducer location even for a real device. Of course, features such as air coupling or slight anisotropy of the end faces can shift the optimal aspect ratio and drive position slightly.

The embodiments that have been described above relate to loudspeakers, ie devices that convert electrical energy into sound. The method of the invention is equally applicable to microphones in which incident sound energy is converted to electrical energy by a transducer.

Claims (17)

1. an acoustics comprises

One actual shell (1) continuously, shell gives bending, so that small part is surrounded an air volume (2),

One transducer is coupled to shell (1), it is characterized in that, described shell (1) is supported equally distributed bending wave pattern, and described bending wave is coupled to this volume (2), so that the coupled resonance pattern to be provided, one signal of telecommunication is coupled with this coupled mode in transducer, thereby is coupled to ambient sound again.

2. acoustics as claimed in claim 1, wherein mode of resonance is crossed over shell.

3. the described acoustics of arbitrary as described above claim, wherein transducer (3) is an exciter, for the excitation mode of resonance, so that acoustics effect such as loud speaker.

4. acoustics as claimed in claim 1 or 2 wherein, has an aperture (7,9) on shell.

5. acoustics as claimed in claim 4, wherein this aperture (9) comprise a pipeline in shell stretches into volume (2).

6. acoustics as claimed in claim 1 or 2, wherein said shell comprise a plurality of end faces (11,13,15,17).

7. acoustics as claimed in claim 6, wherein each end face (11,13,15,17) has a natural resonance frequency, and natural resonance frequency has different value.

8. acoustics as claimed in claim 7, wherein different natural resonance frequencies are selected to cause the frequency of ten to 20 low-limit frequency modes of resonance for interlocking.

9. acoustics as claimed in claim 6, wherein shell comprises a front end face (11), and front end face has aspect ratio 1.6 to 2.2.

10. acoustics as claimed in claim 6, wherein shell comprises a rectangle front end face (11), and transducer (3) from one along the one side on long limit from 14% to 42%, and from one along one side of minor face from 22% to 34% position, the contact front end face.

11. acoustics as claimed in claim 6, wherein shell comprises opposed front end face (11) and rear end face (17).

12. acoustics as claimed in claim 11 wherein is provided with one first transducer at front end face, and at rear end face one second transducer (5) is set.

13. acoustics as claimed in claim 12, wherein first and second transducer (3,5) gives mechanical couplings.

14. acoustics as claimed in claim 11, wherein a single transducer (3) is coupled to before and after end face (11,17).

15. acoustics as claimed in claim 6, wherein shell (1) is a butt tetragonal pyramid body.

16. acoustics as claimed in claim 6, wherein shell (1) is a tetrahedron.

17. a method that drives acoustics comprises

One actual shell (1) continuously is provided, and shell is bent into to small part surrounds an air volume (2),

Two transducers (3,5) are provided, and two transducers are coupled to shell;

It is characterized in that shell is supported bending wave, causes bending wave to be coupled to this volume, so that the coupled resonance pattern to be provided, with described two transducers of common signal of telecommunication driven in phase (3,5), the coupled mode that causes transducer (3,5) to drive shell and volume becomes a kind of one pole configuration; And

Acoustic energy is radiated to surrounding air from coupled mode.

Images