JP2000050392A - Ultrasonic transducer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extend the band of a transducer by forming cavities to resonate in prescribed frequencies on a dielectric spacer. SOLUTION: A spring 30 for a electrostatic transducer module 29 successively supports a conductive electrode unit 32, a dielectric spacer 34 having many apertures 36 and a metallic coat polymer film 38. A part of the film 38 on the upper part of respective apertures 36 and a part of the unit 32 on the lower part of the apertures 36 are respectively functioned as single transducers and provided with resonance characteristics to be functions for the tension and surface density of the film 38, the diameter of each aperture and the thickness of the polymer layer 34. Respective parts of the thin film 38 are deflected to a direction approaching the unit 32 or a direction separated from the unit 32 by an electric field to be changed between respective parts of the film 38 and the unit 32 and the frequency of the movement coincides with the frequency of the electric field. Although the same signal is applied to all the cavities 36, the band width of the module 29 is extended as a whole by the different resonance peaks of respective cavities 36.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to the transmission of acoustic signals, and more particularly, to transducers for transmitting such signals in air.

[0002]

2. Description of the Related Art An ultrasonic signal has an audible range (typically 20 kHz).
A sound wave with a frequency exceeding z). Many, if not most, applications involving ultrasound require the generation of a well-defined beam. Therefore, an ultrasonic transducer for converting an electric signal into a corresponding acoustic signal must have transmission characteristics with extremely good directivity, in addition to high conversion efficiency. Further, the mechanical impedance of the transducer must be as close as possible to the impedance of the transmission medium.

[0003] Ultrasonic transducers for aerial transmission fall into two important categories: electrostatic crystal devices and piezoelectric crystal devices. In an electrostatic transducer, the thin film vibrates due to the capacitive effect of an electric field, while in a piezoelectric transducer, an acoustic signal is generated by changing the shape of the piezoceramic material according to the applied potential. Both types of transducers have various operational limitations which greatly limit their usefulness in some applications. In particular, these operational limitations have hampered the development of parametric loudspeakers (ie, devices that produce audible sound with very good directivity through non-linear interaction of ultrasonic waves).
In a parametric system, a high intensity ultrasound signal modulated with an audio signal is demodulated as it passes through the atmosphere (ie, a non-linear transmission medium), thereby producing a highly directional audible sound. Generated.

[0004]

Piezoelectric transducers typically operate with high efficiency over a limited bandwidth. In parametric applications, the degree of distortion appearing in the audible signal is directly related to the available bandwidth of the transducer, and consequently poor sound quality when using narrow band transducers (eg, piezoelectric transducers). Will occur.
Further, piezoelectric transducers tend to have high acoustic impedance, which results in poor radiation efficiency in low impedance atmospheres. Due to this mismatch, most of the energy applied to the transducer reflects back into the amplifier (or the transducer itself), generating heat and wasting energy. Finally, conventional piezoelectric transducers tend to be fragile, expensive, and difficult to electrically connect.

[0005] Conventional electrostatic transducers use a metallized polymer film held on a conductive backplate by a direct current (DC) bias. The backplate has a depression that creates an acousto-mechanical resonance at the desired operating frequency. An alternating current (AC) voltage applied to a DC bias source alternately increases or decreases its bias, thereby increasing or decreasing the force pulling the membrane into the backplate. This change has no effect on where the surface is in contact, but will cause the membrane to oscillate over the depression. Without sufficient damping, the resonant peak of the electrostatic transducer would be very sharp and would operate efficiently at the expense of limited bandwidth. (Eg roughening the surface of the membrane that comes into contact with air)
By damping, the bandwidth will be somewhat increased, but the efficiency will be poor.

Sensors and Actuators by Mattila et al.
A, 45,203-208 (1994), other techniques for increasing the bandwidth of electrostatic transducers include:
Changing the depth of the depression across the surface of the transducer to create different resonances that add up to a wide bandwidth.

The maximum drive power (and maximum DC bias) of the transducer is limited by the strength of the electric field that the film can withstand, as well as the magnitude of the voltage that the air gap can withstand. The maximum electric field occurs where the film actually makes contact with the backplate (ie, outside the recess). Because membranes are generally very thin polymer films, even materials with sufficient withstand voltage can be damaged by charging or punch-through when subjected to very high voltages. Will wake up. Similarly, when using thin films, the metallized surface of the thin film is very close to the backplate, so the electric field across the thin film, and hence the capacitance of the device, is extremely high, resulting in the need for large drive currents. Become.

[0008] Piezoelectric thin film transducers are made of polyvinylidene fluoride (PV) that change shape in response to an applied potential.
DF) Utilize a lightweight, flexible membrane material, such as a thin film. This film can be made very light to improve its acoustic impedance matching to air, resulting in efficient ultrasonic transmission. In one known configuration, a PVDF film is coated on both sides with a conductive material and placed on a perforated metal plate. The plate is the top of an otherwise closed space, and the vacuum applied to the space draws the membrane into the holes. An AC voltage source connected between the two metallized surfaces of the membrane, which acts as electrodes separated by a dielectric, causes the PVDF material to expand and contract, reducing the degree of depression into the holes. Change, thereby generating sound waves. In a related configuration (also known), the membrane is placed below, rather than on, a perforated plate, and the pressure source replaces the vacuum. In this version, the alternating current source alters the degree to which the membrane protrudes into or through the hole, and similarly produces sound.

The electro-acoustic properties of these transducers make them suitable for parametric applications, but their practicality remains questionable. In a real environment from a commercial point of view, it is very unlikely that a vacuum or pressure can be sufficiently maintained for a long period of time, and a slight leak will cause the transducer to lose sensitivity and eventually Will break down.

[0010]

According to a first aspect of the present invention, the maximum output power of an ultrasonic transducer is not limited by the withstand voltage of the transducer membrane. Rather than placing the film directly on the surface of the conductor as in conventional devices (which results in a very large electric field across the film), the film is held on a dielectric spacer. Ultrasound transmission does not depend on the presence of strong electric fields. Thus, relatively large bias and drive voltages can be applied across the film and spacer without risk of failure. This is because the spacer greatly reduces the electric field applied to the film. Furthermore, the spacers also reduce the capacitance of the transducer, so that the required drive current is correspondingly reduced, simplifying the power amplifier design.

An acoustic transducer according to this aspect of the invention has a conductive membrane, a backplate with at least one electrode, and a series of depressions arranged between the membrane and the backplate and arranged in a pattern. A dielectric spacer can be provided, wherein the depressions form cavities that each resonate at a predetermined frequency. The depressions can take any suitable form, for example, concentrically arranged annular grooves, cylindrical depression distribution patterns, etc.
Or it can extend completely through. In addition, the depressions can vary in depth through the spacer to form cavities that resonate at different frequencies. That is, a different electrode can be assigned to each set of depressions forming one depth.

In a second aspect, the present invention combines a piezoelectric mode of operation with an electrostatic mode of operation. An acoustic transducer according to this aspect of the invention includes a substantially non-conductive piezoelectric film having a pair of opposedly disposed conductive surfaces, a backplate comprising at least one electrode, and a film and electrode ( Or a plurality of electrodes). For example, the cavity may be formed by a dielectric spacer having depressions (cylindrical recesses or openings, grooves, etc.) and the membrane and the electrode (or electrodes)
Can be placed between them. A DC bias pushes the membrane into the resonant cavity, and an AC source connected across the membrane provides a drive signal.

The transducer is preferably driven by a circuit in which the capacitive transducer resonates with the inductance of the circuit at the transducer's acoustic-mechanical resonance frequency. In this way, electrical energy can be transmitted to the transducer very efficiently, and therefore a relatively high carrier (carrier).
The frequency can be used easily. Due to the high efficiency and versatility of the transducers described herein, these transducers can be used for ranging, flow detection (flow d) as well as parametric applications.
and other ultrasound applications such as non-destructive testing. In parametric applications, multiple transducers can be incorporated into a transducer module, and the module is arranged and / or electrically driven to form a substantially non-linear interaction region with a large radiating surface.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the accompanying drawings. As shown in FIG. 1, an electrostatic transducer module 29 incorporating the present invention can include a conical spring 30, which in turn comprises a conductive electrode unit 32 and a number of openings 36. It supports a dielectric spacer 34 provided and a metalized polymer membrane 38. Components 32 to 38 include a film (thin film) 3
8 supporting the spring 30 and supporting the spring 30
Is pressed against the spring 30 by the upper ring 21 which is inserted into the spring 30. Module 29
Is composed of a plurality of electrostatic transducers corresponding to each opening 36 of the dielectric spacer 34. Specifically, a portion of the film 38 above each opening and a portion of the electrode unit 32 below the opening function as a single transducer, including, inter alia, the tension and areal density of the film 38, the diameter of the opening. , And a resonance characteristic which is a function of the thickness of the polymer layer 34. The changing electric field between each portion of the thin film 38 and the electrode unit 32 deflects that portion of the thin film toward or away from the electrode unit 32. Then, the frequency of the movement matches the frequency of the applied electric field.

As shown, the electrode unit 32 can be divided into individual electrodes 32a below each opening 36 by a suitable etching technique. These electrodes are
As will be described, it has individual leads extending from these electrodes to one or more drive units. Module 29 can be easily manufactured using conventional flexible circuit materials and is therefore low cost. For example, spacer 34 may be a polymer such as PYRALUX material sold by DuPont, and membrane 38 may be a metalized MYLAR thin film (also sold by DuPont). can do. If desired, the drive unit components can be located directly on the same substrate, for example, on the tab portion 32b. This structure is lightweight, easy to place, focusing and / or array configuration.
Or it can be flexible for orientation.

The geometry, in particular, the depth of the opening 36,
The resonance characteristics of the individual transducers of module 29 can be modified to cover the desired frequency range, thereby providing a single transducer with a single acoustic-mechanical resonance frequency, or an array of transducers. It will be appreciated that the overall response of the module can be broadened compared to. This can be achieved by using a dielectric spacer 34 consisting of two (or more) layers 34a and 34b, as shown in FIG. The upper layer 34a has a sufficient number of openings 36a. On the other hand, the lower layer 34b
Has a set of openings 36b that are aligned with only openings selected from the openings 36a of the layer 34a. Therefore, 2
Where the openings 36a and 36b are aligned, the depth of the opening is greater than the depth of the opening in layer 34a above the non-opening portion of layer 34b. Electrode unit 3
2 has an electrode 32b below the opening in the layer 34b,
An electrode 32c is provided below a portion of the layer 34a having only the opening (that is, excluding a portion of the layer 34b having the opening). This provides a first set of transducers having a higher resonant frequency (shallower aperture) and a second set of transducers having a lower resonant frequency (deeper aperture). Other geometries and configurations can be created by other processes, such as screen printing and etching.

FIGS. 3 and 4 show different aspects of the structure and operation of the module 29. FIG. In FIG. 3, module 29 comprises
Having one electrode 32, the cavities formed by the layers 34a, 34b have different depths d, d ', depending on whether the opening 36a is aligned with the opening 36b. FIG. 3 shows that the film 38 is
4 is not shown. A DC bias source 40 added to an AC source 42, which produces a modulated signal for transmission, is connected across the module 29, ie, the electrodes 32 and the metallized surface 38m of the membrane 38. You. The same signal is applied to all cavities 36, but their different resonance peaks increase the overall bandwidth of module 29.

Alternatively, as shown in FIG. 4, different sets of electrodes 32b, 32c may be connected to different AC drive signal sources 42b.
b, 42a. Each signal source 42a, 42b, it is electrically resonant at the mechanical resonance frequency f _1, f ₂ of the cavity to be driven. This "separated multiple resonance" arrangement optimizes response and maximizes power transfer by pairing each set of resonant cavities with an amplifier tuned to it. Resistors 43a, 4
3b separates electrodes 32b and 32c, but direct current can pass through them (inductors could be used instead).

As mentioned above, not only can the acoustic-mechanical resonance characteristics of the transducer be changed, but also the electrical resonance characteristics can be changed. For example, varying the capacitance of different areas of the transducer 29 (eg, by using materials having different dielectric constants for different areas of the spacers 34a, 34b) can create multiple electrical resonant circuits. . This electrical resonance affects the efficiency of the power transfer from the amplifier (ie,
As the impedance of the transducer more closely matches the impedance of the amplifier, more output power will couple into the transducer and the associated current draw will decrease), thus increasing the transducer's immunity to different amplifier configurations. The changing electrical resonance within a single transducer can be used regardless of whether the mechanical resonance is also changed.

The signal sources 42a and 42b can be realized as shown in FIG. The modulated output signal (modulated output signal) is sent to a pair of filters 44a and 44b. These filters split the signal into different frequency bands and combine them into a pair of tuned amplifiers 46a, 46b.
Send to Amplifier 46a is tuned to f _1. That is,
The inductance of amplifier 46a, in series with the measured capacitance across the cavities to which amplifiers 46a are connected, causes the electrical resonance frequency to equal the mechanical resonance frequency of those cavities. The amplifier 46b has f
Tuned to ₂ . Filters 44a and 44b provide f ₁ and f ₂
, Or a low-pass filter and a high-pass filter for dividing the modulated signal between the two.

The resonant cavity of module 29 need not have a circular cross section as shown. Alternatively, a different cross-section (e.g., square, rectangular, or other polygonal shape) can be used, and annular grooves (square, V-shaped,
Circular shape). Alternatively, it may have other volume shapes suitable for the selected application (or desired manufacturing method). The electrodes of the backplate for driving the array of concentrically grooved transducers are shown in FIGS. Here, the conductive pattern of the electrode unit 52 is composed of rings 53, 55 and 57, whereby the grooves of different depths can be individually driven. The spacing of the rings and the relative phases of the applied signals can be selected to shape the ultrasound beam emitted from the transducer module.

An appropriate groove depth for a desired operating frequency can be easily obtained without undue experimentation. In the case of a thin film having a cotton density σ (kg / m ² ) and a square groove having a depth h (m), the resonance frequency f ₀ is expected to have the following value.

[0023]

(Equation 1)

Where c is the speed of sound in the air.
ρ ₀ is the density of air. (The formula for non-square grooves is similar to the above formula). Resonant frequency is also affected by film tension, groove width, and DC bias. Therefore, for a transducer having a resonance frequency of 65 kHz using a thin film having an areal density of σ = 0.0113 kg / m ² as a substrate, the depth h of the hole / structure is 74 μm (3 mil).
It is. Depending on the depth of this cavity, for example, 50
If a capacitance of 0 pF is generated, an inductance of 12 mH (typically the secondary side of the transformer) is chosen to achieve a resonance of 65 kHz.

For this transducer, a reasonable bandwidth for efficient driving is 10 kHz (ie, 6 kHz).
0 to 70 kHz). Therefore, in order to widen the effective output bandwidth, the second having a resonance frequency of 75 kHz is required.
It may be desirable to utilize a set of transducers. 75 kHz using the same design approach
In order to realize the above resonance, a structure depth of 56 μm (2 mil) is required.

FIGS. 8 and 9 show alternative transducer module arrangements. In FIG. 8, each module has a hexagonal outer shape, thereby densely packing the modules. In FIG. 9, the modules have a square structure, and again the modules are tightly packed. This arrangement is well suited for the generation of multiple beams and the orientation of a phased-array beam. In all previous transducer implementations, the electrical crosstalk between the electrodes is
It should be noted that this can be reduced by placing so-called "guard tracks" between the electrodes. It should also be understood that transducers having multiple electrical resonances (not necessarily acousto-mechanical resonances) can be used to increase amplification efficiency over a wide band.

The transducer implementation described above is electrostatic in nature. As shown in FIG. 10, the dielectric spacer approach can be used with a piezoelectric film. In this case, the transducer module 60
Comprises a piezoelectric (eg, PVDF) film 62, a conductive back plate 64, and a dielectric spacer 66 having an opening 68 through which a resonant cavity is formed. Again, the cavities 68 may not have a single depth, but may vary in depth, and the back plate 64 may include a series of electrodes that match each cavity of the cavities 68. Can be composed of

The membrane 62 is dielectric in nature and
It is preferable that the upper surface and the lower surface are metal-coated. A DC bias provided by the circuit 70 is connected between the backplate 64 and the conductive upper surface of the membrane 62, thereby forcing the membrane into the cavity 68. This provides a reliable mechanical (mechanical) bias for the membrane 62 so that the membrane can be moved in a conventional piezoelectric transducer driving manner.
It can work linearly to generate an acoustic signal in response to the electrical output of a drive circuit 72 connected across the two ends. Thus, the membrane is held in place by electrostatic forces, but is driven piezoelectrically. As described above, the DC bias circuit 70 can include components that separate it from the AC drive circuit 72.

Alternatively, the mechanical shape of the membrane displacement can be used as a substitute or to increase the DC bias. For example, the membrane can be formed or mechanically tensioned such that the membrane is drawn into the cavity 68. Piezoelectrically induced contraction and expansion displaces the biased thin film, producing an acoustic signal.

As shown in FIG. 11, individual piezoelectric drivers and electrostatic drivers can be used. Therefore, the piezoelectric driver 72a is connected to both ends of the film 62 as described above, but the electrostatic driver 72b is connected between the metal-coated upper surface of the film 62 and the back plate 64, similarly to the DC bias circuit 70. Is done. As a result, a piezoelectric force and an electrostatic force (electrostatic force) are used together to drive the membrane 62. Depending on the orientation of the membrane 62, the drivers 72a, 72b can be driven in-phase or out-of-phase (whereby these forces build up rather than weaken each other). Therefore, when the driving voltage generated by the AC source 72a swings in the positive direction, the electrostatic force
2 is drawn toward the back plate 64 (which is preferably maintained at a high DC bias voltage as shown), while the piezoelectric driver 72b causes the membrane 62 to expand or contract (thin). Or Driver 72a
When the voltage generated by the piezoelectric element 72b swings in the negative direction, the attracted electrostatic force weakens, and the piezoelectric driver 72b
This process is assisted by contracting or expanding (thickening).

Conversely, these forces are not reinforced with each other, but rather, for example, by deactivating the selected portion of the transducer to change the direction of the signal and deliberately destructing them. The driver and the electrostatic driver can be operated.

As shown in FIG. 12, in another embodiment,
The use of an electric field replaces the vacuum utilized in prior art devices to draw the membrane through the holes toward the backplate. The piezoelectric module 62 of FIG.
The membrane further comprises a perforated top plate 82, which may be conductive or non-conductive.
Is in contact with As with conventional transducer modules, the top plate 82 is spaced above the back plate 64 by sidewalls 84. The DC bias provided by the circuit 70 is connected between the back plate 64 and the conductive surface of the membrane 62, thereby drawing the membrane 62 into the opening 86 in the plate 82. This provides a reliable mechanical bias to the membrane 62, whereby the membrane acts linearly to generate an acoustic signal in response to the electrical output of the piezoelectric drive circuit 72. be able to.

The structure shown in FIG. 10 is further simplified by using a conductive, grooved (eg, V-shaped) metal backplate instead of the spacer and backplate shown. Can be something. In this case, the groove serves the same role as the spacer gap, and a DC-biased backplate (or a mechanical arrangement as described above) draws the membrane 62 into the groove.

All of the above transducer embodiments can be used not only for transmission but also for reception.
It is also emphasized that it is often possible to mount the drive circuit and related circuits directly on the substrate of the transducer.

Thus, it will be appreciated that the inventor has developed an improved ultrasonic transducer which obviates the limitations found in the prior art. The terms and phrases used herein are used as terms of description and are not intended to be limiting, but the use of such terms and phrases, equivalent features illustrated and described, Nor is it intended to exclude any of its parts. However, it is clear that various modifications are possible within the scope of the claims of the present invention.

[Brief description of the drawings]

FIG. 1 is an exploded view of an electrostatic transducer module incorporating the present invention.

FIG. 2 shows a variation of the transducer module of FIG. 1 configured for multiple resonance frequency operation.

FIG. 3 is a partial schematic side view showing a configuration and an operation mode of the transducer module shown in FIG. 1 or FIG. 2;

4 is a partial schematic side view showing a configuration and an operation mode of the transducer module shown in FIG. 2 which are different from those shown in FIG. 3;

FIG. 5 is a schematic diagram of a drive circuit used in the embodiment shown in FIG.

FIG. 6 shows a typical electrode configuration.

FIG. 7 illustrates another exemplary electrode configuration.

FIG. 8 shows an exemplary transducer module arrangement.

FIG. 9 shows another exemplary transducer module arrangement.

FIG. 10 is a partial schematic side view of a hybrid transducer utilizing piezoelectric drive and resonance with a DC bias.

FIG. 11 is a partial schematic side view of a hybrid transducer driven by both electrostatic and piezoelectric.

FIG. 12 shows an improved piezoelectric transducer configuration.

[Explanation of symbols]

 29 Transducer module 32 Electrode 34 Dielectric spacer 36, 68 Opening (cavity) 38 Membrane 40, 70 DC bias source (DC bias circuit) 42, 72 AC source (AC drive circuit) 64 Back plate

Claims (22)

[Claims] 1. A sound wave transducer, comprising: (a) a conductive film; (b) a back plate comprising at least one electrode; and (c) a pattern disposed between the film and the back plate. An acoustic transducer comprising: a dielectric spacer consisting of a series of depressions arranged in a matrix, wherein the depressions form cavities that each resonate at a predetermined frequency. 2. The transducer of claim 1 wherein said backplate is comprised of a plurality of electrodes and wherein the depth of said depression through said spacer varies. Forming resonating cavities, each of said electrodes being aligned with a recess having a constant depth; 3. The method of claim 1, wherein at least some of said recesses extend completely through said spacer.
Transducer. 4. The transducer of claim 1 wherein said indentations are annular grooves arranged concentrically. 5. The transducer of claim 1, wherein said recess has a circular or polygonal cross section. 6. The apparatus according to claim 1, wherein said spacer has at least first and second spacers.
Wherein the depressions extend completely through the first layer, and wherein the second layer includes a second series of depressions less than the number of depressions in the first layer. Forming a first series of resonant cavities by aligning the depressions of the second layer with the depressions of the first layer, wherein the first series of resonance cavities are not aligned with the depressions of the second layer. The depressions in one layer form a second series of resonant cavities, wherein the first and second series of cavities are:
3. The transducer according to claim 2, comprising different resonance frequencies. 7. The transducer according to claim 1, further comprising: means for adhering said back plate, said spacer, and said conductive film. 8. The transducer of claim 1 wherein said conductive film is a polymer film having at least one surface metallized. 9. The conductive film has first and second opposed surfaces, wherein the first surface is not metallized and contacts the spacer. 9. The transducer of claim 8, wherein the second surface is metallized. 10. The conductive film has first and second opposed disposed surfaces, the first surface being metallized and in contact with the spacer. 9. The transducer of claim 8, wherein said transducer is absent extending completely therethrough. 11. The transducer of claim 1 wherein said conductive film comprises a non-conductive piezoelectric material sandwiched between first and second metallized surfaces. 12. The first metallized surface is in contact with the spacer, and further comprising: (a) the second metallized surface of the film and at least one backplate electrode. And (b) an AC source connected to the first and second metallized surfaces of the film to piezoelectrically drive the film. Transducer. 13. The membrane connected to the second metallized surface of the membrane and an electrode of the at least one backplate to reinforce each other in connection with the piezoelectric ac source. 13. The transducer according to claim 12, further comprising an AC source for electrostatically driving the. 14. An indentation of different depth comprising a cavity having a different mechanical resonance frequency, and for each of the different indentation depths an individual tuned to a frequency corresponding to said mechanical resonance frequency. 3. The transducer of claim 2, comprising: 15. The transducer of claim 14, wherein each drive circuit comprises an inductor coupled to the capacitance of the transducer to provide an electrical resonance corresponding to the mechanical resonance frequency. Transducer. 16. An acoustic transducer comprising: (a) a dielectric spacer having a pair of opposing surfaces and a series of apertures extending therethrough and arranged in a pattern; (B) a back plate having at least one electrode matching the pattern of the openings and means for coupling an alternating signal to the at least one electrode, the back plate being disposed on a first surface of the spacer; (C) a conductive film disposed on the second surface of the spacer; and (d) a conductive film for adhering the back plate and the conductive film to the first and second surfaces of the spacer. Means, wherein the apertures form cavities that each resonate at a predetermined frequency. 17. An acoustic transducer comprising: (a) a substantially non-conductive piezoelectric film having a pair of opposing conductive surfaces; and (b) at least one electrode. (C) means for creating a plurality of resonant cavities between the film and the at least one electrode; (d) means for forcing the film into the resonant cavity; (E) a sound wave transducer comprising an AC source connected to both ends of the film. 18. The method according to claim 18, wherein said means for creating a plurality of resonant cavities includes a dielectric spacer between said membrane and said at least one electrode, said spacer having a cavity forming a cavity. 18. The transducer according to claim 17, wherein 19. The apparatus of claim 1 wherein said means for creating a plurality of resonant cavities comprises a perforated plate spaced above said at least one electrode.
7 transducers. 20. The method of driving a transducer of claim 2, wherein the recesses of different depths form cavities having different mechanical resonance frequencies, the method comprising: (a) each of the different recess depths; Providing a separate resonant drive circuit tuned to a frequency corresponding to the mechanical resonance frequency; and (b) driving the cavity with the respective drive circuit tuned thereto. 21. The transducer of claim 20, wherein each drive circuit comprises an inductor coupled to the capacitance of the transducer to provide an electrical resonance corresponding to the mechanical resonance frequency. the method of. 22. A substantially non-conductive piezoelectric film having a pair of opposing conductive surfaces, at least one
A method of operating a sonic transducer comprising a backplate comprising two electrodes and means for creating a plurality of resonant cavities between said membrane and said at least one electrode, comprising: (a) Forcing the film into the resonant cavity; and (b) applying an AC signal to both ends of the film.