CN1317220A - Panel form acoustic apparatus using bending waves modes - Google Patents

Abstract

A panel form acoustic member capable of supporting bending wave vibration has the bending wave velocity varied in the region of coincidence to provide a range of coincidence frequencies, for causing the acoustic coupling of the bending waves in the panel to occur over a broader range of angles or for making the coupling more uniform. The member may be incorporated in a loudspeaker having a panel member (1) and an exciter (3) fixed to the panel member to excite bending waves in the loudspeaker to cause an acoustic output.

Description

Plate-like sound device using bending wave modes

The present invention relates to panel-shaped acoustic devices using bending wave modes, and more particularly to loudspeakers comprising such panels.

Distributed mode acoustic devices are known from a number of publications, eg WO97/09842. These devices don't work with a membrane that moves back and forth (piston action) like a typical loudspeaker, but instead have a transducer coupled to a rigid plate that vibrates in bending waves. The bending wave vibrations are distributed over the desired frequency range and coupled to the air. This technique is commonly used in loudspeakers, where the transducer is an exciter that excites bending wave vibrations of the plate, resulting in the output of sound.

In WO 98/39947, published after the priority date of the present invention, a distributed mode acoustic device is described which is also piston-like. In order to arrange the center of mass at a position suitable for placing the exciter, the distribution of the bending stiffness should be arranged so that the center of the bending stiffness and the position of the exciter are staggered.

It is often beneficial to use a large stiff board, which has good high and low frequency performance. At the coincidence frequency, the bending wave travels at a speed that matches the speed of sound in air, but, above the coincidence frequency, a strong beam of sound is produced.

On smaller boards, the effect of coincidence is less of a problem, but the reduced area attenuates low frequency performance and reduces modal density at low frequencies, making the frequency response uneven.

A less rigid plate could also be used here to reduce the coincidence effect, but this would hurt high frequency performance in two ways. First, the mass of the coil can have a strong effect, since its impedance is comparable to that of the board at low frequencies, affecting the frequency response rolloff at high frequencies. Second, the aperture resonance of the plate material inside the voice coil, which occurs when the plate wavelength matches the driver diameter, occurs at lower frequencies for less rigid plates. This effect becomes apparent as a sound pressure peak. Also, the low frequency performance of the less rigid large boards is relatively poor.

In accordance with the present invention there is provided a sheet-like acoustic member capable of supporting bending wave vibrations, wherein the bending stiffness and/or surface mass density of the sheet is varied over the area of the sheet to produce a range of coincidence frequencies with maximum, minimum The coincidence frequency ratio is 1.2:1 so that the sound power and/or directionality of the bending waves in the panel couple more evenly with the surrounding air than in an isotropic panel.

In various embodiments, a 1.2:1 coincident frequency range produced moderate effects. However, a larger range of at least 1.5:1, preferably at least 2:1, gives greater effect.

The control of coincidence is not the subject of theory or textbook teaching. Although the coincidence effect is known, it is considered a problem to be avoided. Additional methods teach adding mass or coupling layer damping to the member, these methods being essentially isotropic processes.

The plate-like sound member can be incorporated into any number of possible sound devices, so it can provide a sound absorber, a sound resonator for reverberation control, a sound box or a support for an auditory assembly comprising such a plate-like sound member .

Active devices couple a transducer to a plate-like sound member, converting an electrical signal to a bending wave in the member or converting a bending wave in the member to an electrical signal. Such active devices represent an important application. Accordingly, there can be provided a microphone having the above-mentioned plate-like sound member and a transducer for converting bending waves in the member into electric signals.

One particularly important application is loudspeakers. Accordingly, a loudspeaker can be provided comprising a plate member capable of supporting bending waves in the audible frequency range, an exciter positioned on the plate member for exciting bending waves in the plate to produce an acoustic output, wherein the bending stiffness of the plate member varies with The position of the landing member changes, so the effect of such overlap on the sound output of the board will be relatively smooth.

Effects of coincidence on sound output include discontinuities or peaks in the sound beam or output sound pressure or power above the frequency of coincidence, integrated over the entire frontal hemisphere and/or in specific directions as a function of frequency. Any or all of these effects are reduced using the present invention.

A change in the bending stiffness results in an additional change in the speed of sound within the panel, and thus a change in the coincidence frequency, with a corresponding change in the direction of sound radiation over the surface of the panel. Variations in bending stiffness can be arranged in this way, resulting in the distribution of sound radiation over a wider angle, thus reducing sound beam generation.

Further in flexural wave plates, the power output as a function of frequency often has peaks, steps, or discontinuities at the coincidence frequency, and this irregularity can be smoothed out by varying the coincidence frequency.

The coincidence frequency is inversely proportional to the bending stiffness and can normally be varied by varying the bending stiffness, which is achieved by varying the thickness of the plate.

The plate can be a little stiffer at the exciter location, since gap resonances caused by coil mass coupling over a limited area are beneficial at high frequencies for a stiffer plate.

In addition, there is a maximum bending stiffness near the exciter. For example: the plate can be made symmetrical about the central stiffness maximum so that the optimal eccentric exciter location for the distributed mode plate is near (not on) the minimum coincidence frequency which is usually the bending stiffness maximum. "Close" means close enough that the bending stiffness at the actuator is at least 70% of the maximum value; preferably 80% and further preferably higher than 90%.

In other embodiments, the edges of the panel are stiffer than the center, and the varying stiffness is also used to smooth the coincidence frequency.

The exciter can be located on a thin area of the board where the mechanical impedance is less, which helps to couple low frequency energy into the board.

The panel has a maximum bending stiffness in the central region (central longitudinal and transverse thirds of the panel) and decreases towards the edges. These panels are produced by injection molding, by casting from a thicker area in the center of the panel.

The present invention provides the benefits of a large rigid plate and reduces some of the disadvantages, particularly the effect of overlapping frequencies in the audible range.

But the invention is not only applicable to large rigid boards, but also gives good results on small boards as explained below.

In order to affect coincidence, the bending stiffness must vary over a linear dimensional plate area comparable to or greater than the wavelength of sound in the relevant frequency range, which is typically 3 to 4 cm for a frequency of 10 kHz. Therefore increasing the bending stiffness in a very small area is not suitable for smoothing the coincidence effect, it is more suitable to vary in the region of linear size by at least 1.5 times and preferably 2 times the coincidence wavelength. Variations in the area of at least 5%, preferably 10%, of the plate area are advantageous for reducing the registration effect.

As explained in the previous paragraphs, the variation in bending stiffness is concentrated at the actuator location. For example: the gradient of bending stiffness is large close to the position of the exciter, and decreases slowly on the straight line extending outward from the position of the exciter. In some embodiments, such contours produce a very useful smoothing of the coincidence effects. The gradient drops to zero at the edge of the actuator area, otherwise the variation can extend to the edge of the plate.

The bending stiffness can be colonized on the area of the plate away from the actuator, whereas all the variation in bending stiffness is concentrated in the area of the actuator.

The bending stiffness may also vary in a long strip around the edge of the panel member, with bending stiffness being greatest at the edge and decreasing towards the inside of the panel, or minimum at the edge and increasing inward. The edges of such panels are clamped within a frame: variations in the bending stiffness at the edges create a desired match or mismatch between the mechanical impedance of the panel and that of the clamp for further control of the sound output.

In particular, the bending stiffness can vary within an edge strip that does not exceed 10% of the plate length from the edge.

The reduction in stiffness near the edge of the plate reduces the mechanical impedance of the plate in the edge region, and if the reduced impedance is lower than that of the clamp frame, little energy will be transferred from the plate to the frame.

Similarly, an increase in surrounding stiffness increases the mechanical resistance of the plate in this area. If the board is supported on elastic supports, the increased impedance of the board creates a larger mismatch to minimize unwanted energy transfer to the frame. Conversely, if the panels are attached to a rigid clamping type frame, this provides a smoother transition from the panels to the clamping edges, thus contributing to the mechanical robustness of the final structure.

Also, in the case of rapid changes in bending stiffness near the edges, the sound vibration energy is reflected back into the panel so that little energy reaches the frame.

Bending stiffness can vary rapidly in the edge region and is relatively constant inside the plate. Additionally, the bending stiffness can vary over the edge region and inside, and also within the actuator region and near the edge, with a region between the edge and the actuator region having little or no variation in bending stiffness.

Other options are to vary the bending stiffness in a wave pattern or many steps on the plate.

The coincidence frequency f _c at which the speed of sound matches the velocity inside the plate changes as $f_c=\frac{{\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="A9981057400241.png,A9981057400311.png"\; state="\{\{state\}\}">c</figure-callout>}}^2}{2\mathrm{\&pi;}}\sqrt{\frac{\mathrm{\&mu;}}{B}}$

where c is the speed of sound in air, μ is the area density of the plate, and B is the bending stiffness.

In fact, any parameter can be varied to vary the velocity in the plate and thus the coincidence frequency, with or without varying the bending stiffness. Accordingly, the Young's modulus of the skin or the areal density of the skin or the core can be changed.

In another aspect, a method of fabricating an acoustic structure capable of supporting bending wave vibrations is provided wherein the wave velocity is varied specifically within the coincidence region to produce a range of coincidence frequencies.

The method further includes the steps of selecting the plate material and plate size, selecting the initial bending stiffness form of the plate, and recursively changing the plate profile or surface tension stiffness, so as to improve the frequency and angular response of the plate by changing the wave velocity in the plate near the coincidence frequency to produce The range of coincident frequencies. Within the step of recursively selecting the plate profile, the resonant mode distribution over frequencies within the plate can also be optimized.

In another aspect of the invention, there is provided a method of fabricating a loudspeaker system, comprising selecting a panel material, panel size, and type of driver, selecting an initial driver location on the panel, selecting an initial panel bending stiffness profile, and recursively varying the driver Position and plate shape, to select the position and profile with the best plate frequency and angular response, compared to a plate, to reduce the effect of coincidence, to provide a plate with recursive selection of the plate profile, and to attach the exciter to the recursive selection s position.

The size, profile and driver position can be selected to produce a distributed mode loudspeaker where low frequency modes can be well distributed across frequency and where gap effects at high frequencies are minimized.

Reference is now made to the accompanying drawings, by way of example only. Specific embodiments of the invention are described in which:

Figure 1 illustrates a loudspeaker according to the invention;

Figure 2 illustrates the board morphology used in a loudspeaker according to the invention;

Figures 3 to 6 show the sound velocity and sound output from speakers of consistent thickness for comparison.

Fig. 7 shows loudspeaker parameters according to the first embodiment of the present invention;

Figures 8 to 10 show the results obtained using the loudspeaker illustrated in Figure 7;

Fig. 11 shows the parameters of the second embodiment of the present invention;

Figures 12 to 14 show the results obtained using the loudspeaker illustrated in Figure 11;

Figure 15 illustrates a third embodiment of the invention;

Figure 16 illustrates the coincidence frequency variation on the plate of Figure 15;

Figures 17 and 18 are the results obtained using the loudspeaker illustrated in Figure 15;

Figures 19 to 21 illustrate the effect of gap resonances within the plate shown in Figure 9;

Figures 22 and 23 illustrate another method of achieving variation in bending stiffness; and

Figure 24 shows another plate configuration.

Figure 1 shows a loudspeaker comprising a board 1 on which an exciter 3 is mounted. The exciter 3 excites resonant bending waves in the panel, causing the panel to emit sound, while the conductor 5 connects the exciter to the amplifier. In this embodiment the panel 1 is made of a core 7 and two skins 9 . Alternatively, the plates may be integrally formed.

The plates used in loudspeakers may be distributed mode plates, as described in WO97/09842 and other applications, which utilize an even distribution of resonant modes over frequency to achieve a useful frequency response, which is achieved when the modes are distributed over the plate Being on time has its benefits.

In order to obtain a good excitation wave mode distribution, the shape of the plate and the position of the exciter should be selected. WO 97/09842 gives some specific suitable shapes, for example a rectangle with an aspect ratio of 1:0.882 or 1:0.707 for an isotropic plate. Instead, some adjustments to these ratios may be made according to the thickness profile of the board.

The location of the exciter is also important, the exciter location should couple to the distributed resonant modes, some good exciters are located close to but not on the center of the plate. For an isotropic rectangular plate, the coordinates of such a location may be (3/7,4/9) side lengths, close to the plate center coordinates on (1/2,1/2). Of course, preferred coordinates will deviate from these values for panels having the aforementioned variation in bending stiffness of the present invention. But these values can still be used as a good starting point, and the best position can be found by trial and error. In addition, laser or computer analysis will help find effective actuator locations.

Injection molding is a cost-effective way to make curved corrugated panels. Not only does this have a moderate unit cost and yields a consistent finished product, but also some of the mounting features and fastening structures that mount the plate to the actuator and plate support frame can also be included in the mold as an integral part of the plate, saving parts and assembly cost. Injection molding is very effective for making the aforementioned boards that are thick in the middle and get thinner towards the edges.

Rather differently, a useful control parameter in distributed mode loudspeakers is the coincidence frequency, and primarily its location within the frequency spectrum. The reason for this is that above the coincidence frequency and below the coincidence frequency, the board will operate under different radiation principles. The coincidence frequency f _c of some practical curved wave plates is usually in the audible frequency range, with aurally negative effects. At the coincident frequency, the sound radiation is more intense for a wide angle emission, the angle decreases towards the normal axis with increasing frequency. From below the coincidence frequency to above the coincidence frequency, the change of the radiation angle will cause the spatial energy transfer, which is not what we want. Further, gap effects limit the high frequency performance of less rigid boards. Clamped distributed mode loudspeakers can use less rigid plates, but are generally reluctant to increase mass density because of the resulting loss of efficiency.

It would be contemplated to exploit the bending stiffness of the variable plate to spread over a frequency range to control the negative effects of coincident frequencies by reducing the energy content at a single frequency. The net effect is not a sudden shift to radiation patterns with high energy content, but a smooth shift over broad coincidences.

Due to the change in the bending stiffness of the plate, the bending wave wavelength in the frequency region near the coincidence will change. Example: In the case of increasing thickness from the center outward, the wave speed increases towards the edge of the plate. Conversely, the wave speed decreases as the thickness gradually decreases from the center to the outside. This will change the eigenvectors associated with the bending waves over the surface area of the plate.

The stiffness gradient of the plate can be varied in several ways, suitable methods include:

1. Variations in thickness across the board are produced by foaming in injection molding. Figure 2 shows some variations of this approach, and such an increase in thickness may result in an increase in mass. The added mass is relatively smaller than the added stiffness because stiffness increases rapidly with thickness (approximately the square of the thickness in the case of a sandwich) compared to a small amount of added mass (foam mass density is very small).

2. In the case of integral casting, stiffness gradients and surface density gradients can be used (see Figure 2). In this case, doubling the thickness provides 8 times the stiffness while only doubling the surface mass density, so this is still a viable unibody approach.

3. Stiffness gradients can be achieved by compression molding foamed materials into desired shapes, such as Rohacell boards or splints with skins covering the foam core. In this case the mass density is maintained on the plate surface.

4. The fabrication of stiffness gradients can be achieved using so-called "smart polymers", which have modulus gradients with size or area. Variable area or domain rigid polymers can be in sheet form for use in layered mixtures or for use in injection molding and other manufacturing processes. Here, the plate can maintain its consistent thickness, achieving the desired stiffness gradient without compromising quality.

5. The use of plate contours, curvature, or corrugations, this technique produces moderate stiffness gradients unless small radii of curvature are used. This method can be used in some applications where "shape" needs to be combined with style, see Figures 22 and 23.

6. The core can be ground or sanded to the desired shape and the already ground or sanded core can be coated on both sides.

Injection molding is well suited for producing curved corrugated panels in a batch, low-cost, and consistent manner. Whereas integrally formed radiators can be cast in a straightforward manner, but may not be suitable for some applications.

These processes solve the casting problem in a "no cost" fashion, since very little additional material is required for foaming. Stiffness variation by a factor of 2 can extend about 40% within the coincidence frequency (eg from 10kHz to 14kHz), which is quite helpful for most applications and is sufficient to extend the coincidence frequency/energy.

Some curved plates with this type of stiffness variation are shown in FIG. 2 . A preferred method for molding is to create a thickness that varies outward in a positive or negative gradient. By controlling the blowing agent in the core of the panel, a large gradient of stiffness can be created within the panel. In an integrally formed plate the stiffness varies as the cube of the thickness, whereas in a splint the stiffness varies approximately as the square of the thickness.

Where the stiffness gradient is decreasing towards the edge of the panel, one or more of the following advantages are obtained. The first is that due to the higher stiffness near the center of the plate, there will be less gap effects due to the limited size of the exciter coils. Again, a near-supportless panel can be achieved which can be beneficial in certain applications, with the advantage of smooth transfer to a panel support or frame. Thirdly, it is possible to use injection molding by starting the casting from the center of the plate, thus producing a low quality foamed core.

Conversely, when the stiffness gradient is increasing toward the edge of the plate, the design can be used to increase the stiffness to create a smooth transition to the clamped edge plate design, which increases the mechanical strength of the final structure.

The excitation of the plates can be achieved in any desired manner, such as described in our many previous patent applications. The goal is therefore still to excite the plate modes evenly, and to achieve a good degree of smoothness of mechanical impedance (for mechanical power input) and/or sound radiated power over the design bandwidth. Such optimal positions can be obtained analytically, such as FE methods or rules of thumb.

The behavior of a bending wave plate is characterized by bending waves at low frequencies, where the plate has a consistent bending stiffness and mass density. But at high frequencies, effects like coincidence and gap resonances can cause deviations from predictions based on static calculations. Because at these high frequencies the plate can operate with a greater degree of shear, it can be characterized by a bending stiffness that falls on high frequencies. The precise high-frequency behavior can only be determined with a complete knowledge of the shear behavior of the plate material, which is not always possible. Therefore, the results obtained from the basic equations for constant bending stiffness may not reflect the real behavior of some plate materials near the coincidence. Vibration analysis and testing are necessary to determine the acoustic characteristics of a panel in accordance with the present invention.

Sound power is the integral of the magnitude of the sound pressure at all angles. The characteristic of the frequency smoothing function is usually a factor of the sound quality. The thickness of the plate is gradually thinned, and the unevenness on the power output of the curved wave plate can be stretched and coincided within the frequency. Increasing stiffness away from the drive point extends the coincidence to low frequencies; conversely, decreasing stiffness extends the range to high frequencies.

If we consider large lightweight panels, the increased radiative coupling strength above coincidence means that most of the energy input to the panel is radiated from close to the excitation location. Further away from the exciter, the plate velocity gradually decreases and almost no energy is radiated. Therefore, the change in plate thickness should be fairly concentrated close to the actuator, otherwise a part of the plate will change without strong radiation and have only a slight effect. This has the further advantage that changes in plate thickness do not cause changes in low frequency performance: at low frequencies (at coincidence) the radiation efficiency is greatly reduced and the energy is distributed in frequency and in resonant bending modes entire board surface.

Also in heavy panels the coupling to radiation is lower and the sound will radiate over a larger area than the panel. Variations in plate stiffness/wave velocity are thus concentrated over a larger area.

If the structural material damping is greater than the radiation damping, then using the same characterization, the regions that should vibrate will be allocated according to the structural material damping.

In summary, the morphology required to disperse the coincidence effect depends on the mass density, bending stiffness, shear properties, and damping properties of the plate.

In principle, variations in the bending stiffness of the plate also broaden the directionality. For plates of decreasing stiffness from the exciter location, the sound radiation extends to larger angles to the plate normal. Conversely, for plates of increasing stiffness, sound radiation extends to smaller angles to the plate normal. On both counts, the angular range has increased.

In practice, the radiation directionality from the panel is more difficult to smooth than the sound power, as the following discussion illustrates.

Consider next a radiated sound beam from a partial panel above coincidence, with velocity V _panel . The beam angle from the on-axis position is determined by the following equation: $\mathrm{\&theta;}={\sin }^{-1}\left(\frac{c}{v_{\mathrm{panel}}}\right)$ Radiation in different directions on different plate sections with different bending wave velocities at a specific frequency results in a specific polar plot of the sound energy as a function of direction.

As the frequency increases, the velocity of the panel increases and the angle of the radiated sound decreases towards the orientation axis position according to the above equation. The velocity-dependent angular difference will be greater for large angles, and decreases towards the on-axis position of the direction. So as the frequency increases, the sound is emitted in a pattern that changes shape, and the energy is concentrated into narrower beams on the normal axis.

A polar plot changes shape as the angle increases. By specifically varying the bending stiffness of the board, a more consistent sound output can be achieved at a given frequency, but the sum of the outputs can produce a less smooth output at other frequencies.

In addition, the sound pressure level at a listening angle can be arranged to maintain a relatively constant value. But the sound pressure in other places is no longer constant and will show an enhanced sound beam effect.

In summary, it is not possible to achieve a maximally smooth polar plot at all frequencies, or a maximally smooth frequency response at a point. The designer will have to strike a useful compromise between a relatively flat response over a range of points and a relatively flat polar plot over a range of frequencies. Experimental results

The panels tested in Figures 4 to 10 consisted of uncompressed Rohacell sandwiched between 100 [mu]m thick glass skins. The tapered plates are formed by laminating the skins on Rohacell plates previously ground to the desired profile. Choose a fairly large board here to emphasize the coincidence effect, the following dimensions (midi demo dimensions):

Board length: 544mm

Board width: 480mm

Board area: 0.26m ²

The drive point was chosen in the center of the board to simplify the profile of the test board, and the driver was a 4 ohm NEC electric driver with a 13mm diameter voice coil.

The following measurements were performed on each board: 1) Sound pressure level as a function of angle around the board

Measuring distance=1m

The total area of the soundproof board-shaped board = 1m ²

The result is expressed in units of dB

Data is not smoothed. 2) A single-frequency polar plot to display directivity

Measuring distance=1m

The total area of the board in the shape of the sound insulation board = 1m ²

The result is expressed in units of dB

The data were third-octave smoothed to smooth the most detailed variation features of the curved plate radiation and thus highlight coincident aspects. 3) Sound power

Measuring distance=1m

The total area of the soundproof board-shaped board = 1m ²

The result is expressed in units of dB

Data is not smoothed. 4) Measure the speed of the driving point with a laser speed measuring system

The result is expressed in the unit mm/s/V

Data is not smoothed. 5) Scanning the distribution of plate velocity over its area, this is used to determine the wavelength within the plate at a given excitation frequency, and thus the resulting bending wave velocity.

First, results from a comparison plate with constant bending stiffness will be presented.

Figure 3a shows the bending wave velocities calculated from the plate material parameters for plates of three different uniform thicknesses of 4mm, 3mm and 2mm.

Figure 3b shows the experimentally determined velocities of these plates, obtained from images of the vibration patterns within the plates at a fixed frequency. The predicted values are close to the experimental results at low frequencies. But at high frequencies, the measured velocities are lower than predicted due to the influence of shear forces. At high frequencies, the rate of variation with frequency is slower than expected for the root-of-square relationship for pure bending.

The graph in Figure 3 also shows a line labeled "c", which represents the speed of sound in air. The frequency at which this line intersects each slab thickness velocity trace is the coincidence frequency. Predictions/calculations of static or low frequency bending stiffness indicate that the coincidence frequency increases from approximately 5kHz to 8.5kHz as the plate thickness decreases from 4mm to 2mm. In practice, however, this change in thickness results in a larger shift in coincidence frequency from 5 kHz to 14 kHz. The 4mm board has a 5kHz coincidence frequency, the 3mm has a 7kHz coincidence frequency, and the 2mm has a 14kHz coincidence frequency.

The tapered plate described later has a thickness of 4mm at the actuation point and a 2mm edge reduction.

Figures 4-6 illustrate the coincidence effect addressed by the present invention, showing measurements for a 4 mm flat panel.

FIG. 4 shows the on-axis and off-axis axes 40 . and 80. The single-point frequency response on . As the angle increases away from the on-axis position, low frequencies are attenuated due to some acoustic cancellation. The high frequency peak at 80° occurs near 5kHz, which is the coincidence frequency of the panel. The peak sound output at 80° reaches 80dB, which is 14dB higher than the on-axis response at this frequency. Response peaks followed by some attenuation are characteristic of coincidence effects in large stiff plates.

Figure 5 shows a polar plot of the sound pressure levels in different directions at 6kHz, 9kHz and 15kHz. The narrowing of the radiation is pronounced, starting at 90° at 6 kHz and decreasing to less than 60° at 15 kHz. This decrease in the beam radiated at an angle as frequency increases is characteristic of the coincidence effect.

Figure 6 shows sound power as a function of frequency. The power changes slowly at low frequencies, but as the frequency increases further, the power increases to a maximum near the coincident frequency and then decreases at higher frequencies. This maximum is much wider than the sound pressure level locus appears in Figure 5, because the power measurement is the integral of the sound pressure level over all angles, and thus does not reflect changing directionality, only rather slowly Total sound output as a function of frequency. The maxima in this power response are also characteristic of coincidence effects, as seen in large solid curved radiators.

A first embodiment of a loudspeaker according to the present invention will be described below. The loudspeaker has tapered plates.

Figure 7a shows the profile of a tapered plate as a function of the fractional distance to the edge of the plate. The plate is ground to this profile in the x and y planes, forming a pyramidal shape in the central area. Figure 7b shows the corresponding coincidence frequency pattern, with a greater variation at locations quite close to the exciter.

Figure 8 shows the single-point frequency response at increasing angles around the plate, the on-axis response is similar to the reference plate in high and low frequency extension.

In this embodiment, the coincidence maxima exhibited by the first plate can be attenuated by up to 10 dB. The maximum width can also be increased by a factor of two.

Large attenuation beyond the 80° on-axis sound pressure level associated with the on-axis response observed in the comparative example at coincidence is not exhibited in panels according to this embodiment, while high frequency off-axis response is significantly improved.

FIG. 9 shows a polar plot of sound pressure levels at the same frequencies as those shown in FIG. 5 at 6 kHz, 9 kHz and 15 kHz. It is evident from the comparison of these two sets of graphs that the polar plot according to the first embodiment significantly reduces bunching compared to the reference plate.

Figure 10 shows the sound power radiated by the panel according to the first embodiment, when compared with the reference panel response shown in Figure 6, it can be seen that the coincident maximum is attenuated by 5dB and broadened to high frequencies, with a decrease towards the edges The stiffness gradually decreases as predicted for thin plates.

All in all, the test panels were a significant improvement over the flat panels in terms of all the problems caused by the coincidence effect. This represents a good compromise with a significant improvement in every aspect of the coincidence problem and the profile of the board, while also maintaining a good frequency response.

Figures 11 to 14 show the results for loudspeakers according to a second embodiment of the invention with a very large gradient within the panel, very similar to the results for the panel of Figure 7 .

Both boards show a good compromise, improving all aspects of the coincident radiation characteristics, the second board shows a slightly worse set of single point frequency response and sound power traces than the first, however, its single frequency polar plot The result is a slight improvement. Although optimization is always a trade-off, the designer can choose according to the needs of the application situation.

The first two embodiments relate to medium size boards, the third embodiment below will discuss small sizes, (A5-210 x 148.5mm).

Figures 15 and 16 show the outline of the plate. As can be seen, its size is greatly reduced. The board is made of 14.5mm thick Rohacell compressed to 10.8mm in the center and 1mm at the edges, with the control plate compressing the entire surface to 9.8mm. The exciter is fixed behind the plate in an optimal position for isotropic plates. With this exciter position good results can be obtained even with progressively thinner plates. Of course, further optimization of the exciter is conceivable. Location.

Figure 16 shows a plot of coincidence frequency calculated as a function of position on the plate. Figure 17 shows the results for a 10mm flat plate of the same dimensions compared to a tapered plate. Sound power measurements are shown in Figure 18a (flat plate) and Figure 18b (gradually thinner plate).

As can be seen, the panel exhibits a rather excellent broad directivity: even at 13kHz it radiates evenly to the front hemisphere. The power response of the tapered plate also does not show a significant step near 5kHz; such a step is clearly visible in the reference plate response. Please note that these test boards are fixed on a very shallow box, which will form a maximum around 500Hz, which in the actual speaker needs to be controlled by electronic filters or other devices. This is caused by the box, not by thinning.

Numerous tests have also been carried out on other boards, the results of which confirm that in the board according to the invention many effects caused by the coincidence effect are improved. Contouring is important in order to smooth out the directional effect, but precise contouring is far less important in achieving smoothness of overall sound power.

Figures 19 to 21 show the results of drive point velocity measurements for three plates excited by a voice coil transducer having a diameter of 25mm. Figure 19 shows the results for a 4mm panel, Figure 20 for a panel from the first example and Figure 21 for a 2mm panel.

The steep peak between 10kHz and 20kHz in the velocity trace is evidence of gap resonance. The resonance occurs at 13.1 kHz for the 4 mm and tapered plates. For a 2mm plate the resonance occurs at 11.8kHz.

As expected, increasing the stiffness of the plate increases the resonant frequency of the plate. The resonant frequency of the tapered plate is determined by the plate thickness at the driving point, thus similar to a 4mm thick plate.

This shows that the gap resonance is determined by the thickness at the driving point. Therefore, having a stiff plate section at the exciter location minimizes this gap resonance.

Results (not shown) also indicate that when the variation in bending stiffness is concentrated near the plate edges, the effect on the radiation signature above the coincident frequency will be small. However, this treatment of the board has beneficial effects, as will be discussed below.

In order to make an actual loudspeaker, the board is usually fixed to some frame/support, the purpose of which is to keep the vibrational energy within the board, minimizing energy transfer to the frame. This is achieved by a greater impedance mismatch between the board and the frame. Varying the plate thickness at the edges controls the impedance at the plate/frame boundary without affecting the overall radiation signature. Some examples of the benefits of such approaches are provided below.

Tapering the board to a very thin thickness near the edges reduces the impedance of the board to very small values. If this impedance is lower than that of the clamping frame, only relatively low energy will be transferred.

Increasing the thickness of the edge of the board increases the impedance significantly, and if the board is connected to a frame with soft terminations (and relatively low impedance), the increase in the impedance of the board creates a large mismatch at the edge, making the transmission to the frame Energy is minimal.

In addition to the above two examples, sudden increases/decreases in plate thickness at boundaries reflect energy back into the bulk of the plate. For example: a sudden increase in thickness on the edge of a plate provides a similar situation to a clamped boundary, where energy incident on the boundary is reflected back to the plate and the edge can then be securely clamped or supported since it includes very little vibrational energy .

It is not necessary to vary the bending stiffness by varying the thickness of the plate. Figure 22 shows a plate with constant thickness but with varying radii of curvature over the area of the plate, which results in variations in bending stiffness. Figure 23 illustrates another way in which the panels are corrugated as shown to achieve a higher bending stiffness in the central region than in the outer regions.

Also the thickness of the plate need not be varied in the simple manner described above, for example the bending stiffness can be varied over a corrugated surface, or in a series of steps on the surface of the plate. Some available profiles are shown in Figure 24, which can be obtained with undulations or steps in the corresponding plate thickness or otherwise.

Claims (34)

1. but the tabular sound components of support bends ripple vibration, wherein bending wave and/or the mass surface density in the plate changes in plate, and the coincidence frequency of generation certain limit, wherein, the ratio of maximum coincidence frequency and minimum coincidence frequency is 1.2: 1 at least, like this, compares with isotropic plate, aspect sound power and/or directivity, bending wave is coupled to air more equably in the plate.

2. acoustic device comprises tabular sound components as claimed in claim 1 and is fixed in a exciter on this member.

3. loud speaker, comprise a plate (1) and be installed to this plate (1) and go up the exciter (3) that causes a voice output to evoke the bending wave in the member, wherein plate (1) is a tabular sound components as claimed in claim 1, and coincidence effect obtains smooth-going to the influence of voice output.

4. loud speaker as claimed in claim 3, wherein the bending stiffness of plate (1) can change on the zone of 10% plate area at least.

5. as claim 3 or 4 described loud speakers, wherein bending stiffness has a maximum, and exciter (3) is coupled on the plate (1) to have on the peaked position of 70% bending stiffness at least.

6. as each described loud speaker of claim 3 to 5, wherein the thickness of plate changes on the zone of plate (1), with bending stiffness that certain limit is provided and therefore obtain the certain limit coincidence frequency.

7. as each described loud speaker of claim 3 to 6, wherein bending stiffness has a maximum in the middle section of plate (1), and successively decreases outward.

8. loud speaker as claimed in claim 7, wherein exciter (3) is coupled on the plate (1), near the position of maximum deflection rigidity value.

9. as each described loud speaker of claim 3 to 8, wherein exciter (3) is positioned on the plate (1), the position of maximum deflection rigidity value.

10. as each described loud speaker of claim 3 to 7, wherein panel stiffness has minimum value in plate (1) central authorities, and increases progressively toward the edge of plate (1).

11. loud speaker as claimed in claim 10, wherein exciter (3) is positioned near the central authorities of plate (1), in the also low zone of the mean rigidity of plate (1).

12. as claim 10 or 11 described loud speakers, an edge of clamping plates (1) at least wherein, and maximum bending stiffness is arranged at least one plate of clamping (1) edge.

13. as each described loud speaker of claim 3 to 12, wherein the gradient of bending stiffness concentrates near the actuator position.

14. loud speaker as claimed in claim 13, wherein the gradient of bending stiffness is quite high near exciter (3) position, and slowly reduces along the line that extends outward from actuator position.

15. as each described loud speaker of claim 2 to 14, wherein bending stiffness changes near the board member edge.

16. loud speaker as claimed in claim 15, wherein bending stiffness is the highest on the edge of plate (1), and descends towards the inside of plate (1) smoothly.

17. as claim 15 or 16 described loud speakers, wherein a supporter is entreated an edge at least.

18. loud speaker as claimed in claim 17, wherein the bending stiffness on plate (1) edge is that the mechanical impedance on plate (1) edge and the impedance of supporter (13) do not match.

19. as each described loud speaker of claim 3 to 6, wherein the bending stiffness of plate is by change in the wavy pattern, so the coincidence effect in the voice output is by smooth-going.

20. as each described loud speaker of claim 3 to 19, wherein plate is the distribution mode plate with many resonance mode of flexural vibration by frequency distribution.

21. a loud speaker comprises

One plate (1) can support the resonance bending wave in the audiorange;

Exciter (3) on one this plate (1), in order to evoking the resonance bending wave in the plate, producing a voice output,

Wherein the coincidence frequency in the plate (1) can be on overlapping the position change on the plate (1) on the length range of at least one wavelength, make coincidence effect smoothness in plate (1) voice output.

22. loud speaker as claimed in claim 21, wherein bending stiffness can go up change at plate (1), so coincidence frequency change.

23. a loud speaker comprises

One board member (1) can support the resonance bending wave in the audiorange;

Exciter on one this board member (1), in order to evoking the resonance bending wave in the plate, producing a voice output,

Wherein the bending stiffness of board member (1) changes in the panel edges zone.

24. loud speaker as claimed in claim 23, wherein the edge of plate is clamped and the bending stiffness of plate (1) can rise rapidly towards panel edges.

25. an acoustic absorption body comprises tabular sound components as claimed in claim 1.

26. the acoustic resonance body of the control that is used to echo comprises tabular sound components as claimed in claim 1.

27. an audio amplifier comprises the tabular sound components as claim 1.

28. the supporter of an audio components comprises tabular sound components as claimed in claim 1.

29. an acoustic device as claimed in claim 2, transducer is converted to the signal of telecommunication with the bending wave in the plate, so this device is as microphone.

30. but a method of making the sound components of support bends ripple vibration, wherein velocity of wave carries out specific variations in overlapping the zone, to produce the coincidence frequency of certain limit.

But 31. the method for the sound components of manufacturing as claimed in claim 30 one support bends ripple vibration further comprises step:

Option board material and plate size;

The initial bending Stiffness Distribution of option board; And

The change plate bending stiffness of pulling over distributes, and near the velocity of wave the utilization change coincidence frequency in the plate improves the frequency and the angular response of plate, to produce the scope of coincidence frequency.

32. method as claimed in claim 31, wherein in the step of option board profile of pulling over, the distribution of resonance mode on frequency in the plate also is best.

33. method of making a resonance pattern speaker system, comprise an initial bending Stiffness Distribution of an initial excitation device position, option board on option board material, plate size and exciter kind, the option board, change actuator position and web wheel exterior feature with pulling over, with select one can be with the frequency of plate and the position and the profile of angle voice response smoothness, a plate that this web wheel exterior feature of pulling over and selecting is arranged is provided, and on this pulls over the position of selecting, an exciter is installed.

34. method as claimed in claim 33, wherein in the step of selection actuator position and web wheel exterior feature of pulling over, the distribution of resonance mode on frequency in the plate be optimization also.

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