JPH0134000B2 - - Google Patents

Abstract

A broadly tuned directional acoustic transducer system for transmitting and receiving airborne sound, which provides enhanced efficiency and reduced cost without undue narrowing of bandwidth, makes use of an acoustic transducer element (2) coupled to a vibratable member (10), e.g. a plate, having a higher order flexural mode resonance at approximately the desired frequency of operation, the vibratable member being coupled to the air through low-hysteresis acoustic propagation material having an acoustic impedance much less than that of the vibratable member and much greater than that of the air. The material is disposed so that in the desired direction of propagation there is no substantial reduction of sound intensity in the far field resulting from cancellation occasioned by interaction of sound radiated from adjacent antinodal zones. Preferably the thickness of the material is such that it acts as an efficient acoustic impedance matching transformer. Preferably, the transducer element is piezoelectric and coupled to the centre of a disc-shaped vibratable member (10) to which the coupling material is applied in rings (16, 18, 20).

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to acoustic transducers, and more particularly to acoustic transducers having improved coupling means between a transducer element and a low impedance medium for propagating sound waves to or from the transducer. It concerns a converter. In particular, the present invention relates to an acoustic transducer suitable for use in pulsed acoustic sounding, where it is desirable to use a transducer with high coupling efficiency and high directivity characteristics, and with a relatively low Q.

This type of transducer is utilized for converting acoustic energy to and from another form of energy, usually electrical energy and acoustic energy and vice versa. The amount of electrical energy converted depends on the magnitude of the vibration of the mechanical element, which has a relatively high acoustic impedance. In practical systems, the vibrations of mechanical elements are coupled to a medium such that acoustic energy is transferred to and from the medium. This medium is typically air, which has a very low acoustic impedance. The degree of this coupling determines the efficiency, frequency response, and directionality of energy propagation through the medium of the energy conversion system.

One widely used acoustic transducer assembly uses an axially resolvable cylindrical element such that a piezoelectric crystal is held within the open end of a cylindrical support, such as in a tube. ing. When the outer end surface of the element vibrates in response to excitation of the element, such as by electrical stimulation, sound waves are emitted from the end or radiating aperture of the tube. Such transducer assemblies are commonly utilized for transmitting and receiving sound in gaseous media. Typically, it is a high frequency sound wave such that the wavelength of the sound in the medium is smaller than the dimensions of the emitting aperture.

The sound radiation pattern produced by such a transducer approximates that of a planar circular piston operating within an infinite baffle. It is well known that the directivity of such transducers is a function of the ratio of the diameter of the radiator and the wavelength of the sound in the propagation medium. Therefore, when propagating waves of the same wavelength enter the same medium, a radiator with a larger diameter exhibits higher directivity than a radiator with a smaller diameter. Therefore, for a given directivity, the transducer elements need to be larger when the frequency of the sound is low.

The acoustic power and bandwidth of the sound emitted from a transducer such as those described above are of particular importance. The transmission of sound between low acoustic impedance gaseous media and high impedance transducer assemblies such as those described above presents significant and well-known problems. The problem is
Sound is present regardless of whether the sound is radiated from the transducer assembly to the medium or received from the medium into the transducer assembly. This means that the coupling and bandwidth of the acoustic energy transmitted between the radiation source and the medium is substantially reduced.
In the case of a piezoelectric crystal and an air medium, the impedance difference is quite large, on the order of 10000:1 or more.

The essence of the degree of coupling problem is that the low-impedance gaseous medium has very little reaction to the motion of the high-impedance piezoelectric crystal, and there is almost no reaction from the piezoelectric crystal to the gaseous medium. This means that the effect of

A well-known means of transferring more energy from a piezoelectric crystal to a gaseous medium is to excite the piezoelectric crystal at one of its natural resonant frequencies, thereby increasing the motion of the surface of the piezoelectric crystal by a factor of ten. or 20
The goal is to make it more than double the size. In this way, the acting force moves against the same area of the gas medium as the piezoelectric crystal surface, but the piezoelectric crystal surface moves a much greater distance each time through each cycle of its motion. Therefore, more movement of the piezoelectric crystal surface takes place and more energy is transferred to the gaseous medium. Even in such cases, the bandwidth is very narrow because relatively little energy is transferred to the medium and the piezoelectric crystal vibrations are only slightly damped, and the resulting piezoelectric ringing effect causes the formation of a sharp It becomes impossible to transmit and receive pulses of sound energy with extremely short rise and fall times.

Another well-known means of giving a piezoelectric crystal more motion and thereby transferring more energy to the gaseous medium is the use of a rigid cone whose front dimension is larger than that of the piezoelectric crystal. Alternatively, an intermediate structure such as a diaphragm may be placed between the piezoelectric crystal and the gas medium. With such an arrangement, suitably constructed according to well-known principles, a larger area of the gaseous medium is absorbed by the piezoelectric crystal (according to the ratio of the area of the front surface of the cone or diaphragm to the area of the front surface of the piezoelectric crystal). Vibrate due to movement.

Therefore, for the same reaction force exerted by the gaseous medium, there is a larger area moving over the same distance, and as a result, more energy is transferred to the gaseous medium than if the piezoelectric crystal were to operate without an intermediate structure. Given. Depending on the size and stiffness of the cone or diaphragm, the performance of the device will be affected differently. When highly directional output is required, the diaphragm dimensions are limited by the need to maintain a coherent wavefront, and the overall impedance mismatch remains, resulting in a somewhat limited output. Only improvements are made.

A third known means of making the vibrating element perform more motion and thereby transfer more energy to the gaseous medium is to connect one or more impedance-transforming transmission lines between the piezoelectric crystal and the gaseous medium. It is to be placed in between. Such impedance matching methods are fully described in Hands, US Pat. No. 3,674,945, issued July 4, 1972, entitled "Acoustic Matching System." The operation of this latter method depends on the acoustic properties of the impedance matching section placed between the high impedance piezoelectric crystal and the low impedance gaseous medium, and on the acoustic properties of the piezoelectric crystal and the gaseous medium themselves. A suitably manufactured matching structure of this type allows the motion at the interface between this matching structure and the gaseous medium to be
The motion at the interface between the matched structure and the piezoelectric crystal surface can be greater than the motion at the interface. Therefore, a short strong stroke at the high impedance end of the matching structure at the piezoelectric crystal surface is converted into a long stroke and small energy movement at the low impedance end of the matching structure at the interface with the gaseous medium. .

It is easy to demonstrate how large the impedance mismatch between a piezoelectric crystal and a medium such as air is. For example, if a piezoelectric crystal is used to transmit sound output into the air without a matching structure, it must be driven with a pulsating amplitude so large that the piezoelectric crystal is likely to fracture, but some form of matching structure cannot be When inserted between the crystal and the gaseous medium,
The same sound output can be transmitted into the air by driving at a much lower pulsation amplitude that does not cause fracture of the piezoelectric crystal.

A portion of the acoustic transmission line is configured to provide an impedance match between a high impedance piezoelectric crystal and a low impedance air medium. Although the use of structures such as those described above improves sound transmission compared to the absence of such matching structures, the efficiency of such structures is nevertheless very low. Also, the degree of coupling to the medium is not high enough to significantly attenuate the vibrations of the piezoelectric crystal, so if bandwidth needs to be increased, the vibrations of the piezoelectric crystal may be damped by other means. It should be noted that it must be attenuated.

Various proposals have been made to match the impedance of a high impedance vibration source, such as a piezoelectric crystal, to a lower impedance medium, such as air, by using intermediate structures using diaphragms or discs. Until now, it has not been possible to achieve such impedance matching without sacrificing either directivity or bandwidth.

One example of such a proposal is provided by Scarpa US Pat. No. 3,891,869, issued June 24, 1975. This patent discloses an acoustic device including a vibrating element comprising a piezoelectric vibrator having a structure in which a disk-shaped, high-mass backing element is fixed to one side of a piezoelectric crystal, and an acoustic transducer is fixed to the other side of the piezoelectric crystal. A transducer assembly is disclosed. The acoustic transducer element comprises a plurality of disks of varying axial cross-sectional area, the generator surface being the largest dimension of the emitting surface.
The specification, column 2, line 48, states that a transducer containing a disk is designed to reduce impedance by increasing the contact area on a moving radiating surface while performing small arc-shaped oscillations at high speed. It is said that it works.

In the device described in the Scarpa patent, it is not required that the generated sound be highly directional. The main feature of this device is that the central lobe of radiation is suppressed by the phase difference on both sides of the vibrating disk, and the main part of the radiated energy is
It is located at a point where it is radiated as a sidelobe at an angle of approximately 45 degrees to the main axis of the device.

Another prior art proposal is described in an article by JA Galliego-Juarez, G. Rodríguez-Corral, and L. Gaetegearreton published in the November 1978 issue of Ultrasonics.

This paper describes a transducer that utilizes a stepped diaphragm. A transducer is described herein that creates an impedance match between an ultrasonic vibration source and a gaseous medium and provides highly directional radiation. Although this device provides effective impedance matching, its bandwidth is narrow and typically corresponds to a Q of about 2000.
A bandwidth of about 10 Hz for devices operating at about 20,000 Hz. Although these high-Q devices are suitable for continuous sound generation at a constant frequency, it is necessary that the transducer exhibit a much lower Q with a bandwidth of at least 5 to 10 percent of the resonant frequency. Therefore, it is not suitable for use in pulse acoustic sounding.

Conventionally, transducer systems suitable for application in pulsed acoustic sounding in gaseous media have been described in U.S. Pat.
It is disclosed in No. 3674945. This type used simpler and less efficient coupling methods used with mechanical or electrical means to damp the vibrations of the vibrating element. Other problems with such systems arise in applications where long measurement distances are required. The absorption of sound energy by a gas medium increases as the frequency increases, so in order to increase the measurement distance, it is necessary not only to increase the output energy but also to lower the frequency. This means that larger transducer elements must be used to obtain the required directivity and power. The piezoelectric materials widely used in such devices are expensive and heavy. 10kHz according to US Pat. No. 3,674,945
Although it is possible to manufacture transducer systems that work satisfactorily, the mass and cost of such systems would be too great for typical commercial use.

A broadly tuned highly directional acoustic transducer system according to the present invention comprises a plate having a high bending mode resonance at the radiating surface and substantially at the operating frequency of the system; a transducer element connected to the surface to excite or respond to said high bending mode resonance, wherein at least every other antinode zone of the radiating surface of the plate has a lower acoustic impedance than the plate for low-loss acoustic propagation. desired in the far field of sound formed from the material and radiated into the gaseous medium from adjacent antinode zones of the plate with a difference in at least one of the relative phase and relative amplitude of the radiation from the adjacent antinode zones; are coupled to the gaseous medium by means applied to at least every other antinode zone of the radiating surface of the plate with a thickness selected to sufficiently reduce substantially mutual cancellation in the radial direction of the radiating surfaces of the plates. Preferably, the plates are axially symmetrically resonant, and in a presently preferred form of the invention, a disk-shaped plate is used axially coupled to the transducer element. The axes of the plates and discs are also the axes that indicate the directionality of sound from the system. For disc-shaped plates, the coating material is arranged as concentric rings covering adjacent antinode zones, with the thickness of adjacent rings being different to produce coherent radiation at far axial locations. There is. In one embodiment of the invention, the thickness of the material covering every other antinode zone is zero. That is, every other antinode zone is uncovered. Therefore, the impedance matching from the coated zone into the gaseous medium is much better than from the uncoated zone, so that virtually no phase cancellation occurs in the axial far field. Either the sound emitted from the covered zone may undergo a sufficient phase shift to substantially reduce cancellation. Alternatively, the entire radiating surface of the plate is coated with a material of such thickness that there is both a phase shift of the radiation from every other antinode zone and a matching of the acoustic impedance between the plate and the propagation medium, usually air. You may do so. The coating material need not be uniform; adjacent zones may be coated with different materials, or the material may consist of different layers of material if the desired phase and/or amplitude modulation is achieved. It could also have a graded character. As the system damps and its Q decreases by improving the coupling of the system to the medium,
It can be used in acoustic sounding techniques without external attenuation.

The invention will be further described with reference to the accompanying drawings.

Referring first to FIG. 1, a directional transducer system suitable for transmitting and receiving pulses of sound at a predetermined frequency consists of a pair of piezoelectric crystal elements 2 mechanically operated in an axial compression mode. The electrical contact with the end of the element is made by a lug of a conductive brass washer 4. The element can be made of lead zirconate titanate or other suitable piezoelectric material,
It is also connected to the windings of a suitable electrical matching transformer (see FIG. 3) 6. Via this transducer 6, electrical signals are transmitted to and from the acoustic transducer.

The element 2 and its connecting washer 4 are sandwiched between a mounting block 8 and a disk-shaped plate 10 shaped like an upside-down cone and thickened in the center, so that the entire structure is bolt 12 and nut 1
4 are held together. The diameter of the plate 10 is much larger than the diameter of the element 2, and the material and dimensions of the plate are such that it exhibits a higher bending mode resonance exhibiting a single nodal circle at frequencies very close to the desired operating frequency. has been selected. In this resonance mode, the antinode zones of the inner and outer plates of the nodal circle move in opposite phases to each other.

In the antinode zone of the upper surface of the plate outside the nodal circle there is a ring 16 made of a less dense elastic material, for example closed cell polystyrene or other synthetic plastic or cellular rubber, or a cell-free elastic synthetic plastic such as polyurethane. is installed. The material should be such that it allows sound waves to propagate with low loss. That is, it should exhibit low hysteresis as an acoustic propagation medium. The thickness of this ring, discussed further below, is such that the sound waves transmitted from the plate to the ring are in opposite phase compared to sound waves passing through air of a similar thickness. (While it may be convenient, and usually is, for the system to operate in air, it will be appreciated that the invention may also be applied to systems operating in other gaseous media.) In a preferred arrangement, another ring 18, 20 of a low density, low hysteresis acoustically transmitting material, which may be the same or different from the material of ring 16, is located at a node on the plate above ring 16. attached inside the circle. These rings have an equal thickness that is an odd multiple of a quarter wavelength of sound at the operating frequency in the material of the ring so as to provide an acoustic impedance transformation between the plate and the adjacent air. .

In the embodiment shown in FIGS. 2 and 3, parts that correspond in function to parts of FIG. 1 have the same reference numerals. Plate 10 is of uniform thickness, which is easy to manufacture and greatly helps in predicting its resonant properties. The plate is operated in such a way that three nodal circles are formed in the higher resonant bending mode, so that the rings 16,1
The numbers 8 and 20 have been increased accordingly. Additional mounting and drive blocks 22 are provided for coupling the transducer elements to plate 10. Although four transducer elements 2 are shown, this number depends on the required operating frequency, the piezoelectric material utilized,
and system dimensions. The system, except for the radiating surface of plate 10, is enclosed within a housing 24 within which the system is sealed by a peripheral polyurethane seal 26 and a felt seal 28. Space 30 below the board
is filled with a foam rubber sound absorber, the transducer and drive block are wrapped in cork 32 and surrounded by compound padding 34. Those skilled in the art will appreciate that the system packaging can be varied widely to suit various requirements and environments, provided that the proper functioning of the system is not substantially interfered with.

The operation of the embodiments described thus far will be better understood by reference to experiments conducted by the inventor. In all of these experiments, the transducer was driven to peak amplitude in bursts of approximately 2 ms duration by a square wave voltage source with a peak of 800 volts. Measurements were made in microbar peak-to-peak at 8 feet (2.4 meters) from the transducer using a condenser microphone, type 4133.

The system illustrated in Figure 1 is constructed using an aluminum plate 10 having a diameter of 12.5 cm. First, the system is set at three different resonant frequencies.
The test was conducted with the rings 16, 18 and the disc 20 omitted. At the lowest frequency tested, 7.09kHz,
The plate essentially behaves like a piston and the directional pattern is observed to have fairly good directional properties, with the axial lobe having a 3dB beamwidth of about 20° and the side lobes all It was more than 12dB down, but the coupling into the air was poor. The maximum sound pressure level measured occurred on the axis of the transducer and the measured value was 120 microbar peak to peak. The Q of the system was unacceptably high for pulsed acoustic sounding applications. Next, at resonance of the plate at 15.56kHz, the plate radiated essentially in a bending mode, and the directional pattern simply showed much larger side lobes and a smaller central lobe; is inappropriate for many sounding techniques. The on-axis sound pressure level was only 87 microbars, and the first sidelobe sound pressure level was 250 microbars.
All the energy of the gun was concentrated in the first and second side lobes. At a frequency of 33.5 kHz, the directivity pattern deteriorates even further, corresponding to a higher degree of bending mode, and the first
The sound pressures of the second side lobe were 140 microbar and 125 microbar, respectively.

Made of insulating polyurethane foam with a thickness of 14.3 mm
As a result of attaching the ring 16 with a width of 19 mm and a width of 19 mm, the second of the two resonance frequencies mentioned above changed slightly to 16.7 kHz, but a significant change in the directivity pattern was observed, and the directivity pattern The pattern is
3dB beamwidth is 10°, all sidelobes are
It turned out to be an excellent result, with a reduction of more than 12dB.
The maximum sound pressure level is again on the axis of the transducer and is 550
increased to microbars. The ring 16 is calculated to cause a 180 degree phase reversal of the sound radiated from the outer part of the nodal circle of the disc, the position of which can be visually determined by conventional means, as discussed below. did.

When disks 18 and 20 are added, they (corresponding to a quarter wavelength at the resonant frequency)
Made of low-density polyurethane foam with a thickness of 6.7 mm, the performance of the system was found to be significantly improved. The resonant frequency changed slightly to 15.83kHz, and the 3dB beamwidth widened slightly to 12.5°, but the sidelobes were all
More than 18dB down, maximum sound pressure level is 1700
increased to microbars. The coupling to the medium has been improved to the point where the attenuation is more than adequate for pulsed acoustic sounding techniques. In the echo tests, the amplitude of the electrical signal output from the transducer system resulting from receiving the echoes returned from the hard target at various distances was: That is, from 1.5 meters it was 2.5 volts peak to peak, from 2.5 meters it was 1.60 volts peak to peak, and from 3 meters it was 1.15 volts peak to peak. The system operated at the same frequency and provided the same beamwidth, but was much lighter and used much less piezoelectric material than a system constructed according to the teachings of Hands US Pat. No. 3,674,945.

Based on the success of the above experiment, a further test was conducted using a plate 10 having a uniform thickness as shown in FIG. I didn't. In some tests, an effect similar to seal 26 was obtained with an external cork damping ring. The actual effectiveness of seal 26 was determined by subsequent testing, and some loss of efficiency was observed;
This was not unreasonably overly serious. This minor problem can be alleviated if necessary by engaging a seal in the nodal circle on the back side of the disc. It has been found that the use of a plate 10 of uniform thickness allows the resonant frequencies and nodal circles of the various bending vibration modes to be calculated with a fair degree of accuracy using commonly known formulas. According to this formula, the optimal parameters could be easily determined by test adjustment. Also,
It has also been found that the nature of the adhesive used to secure the various rings is not important as long as it does not introduce undue discontinuity in the acoustic properties of the structure.

One of the objects of this invention is U.S. Patent No. 3,674,945
By providing a transducer system for applications in pulsed acoustic sounding that provides a narrow beamwidth and a significantly higher acoustic power at frequencies lower than are economically viable by known industrial techniques such as be. Therefore, as already mentioned, in the system configuration shown in Figure 3 excluding the housing, the aluminum plate 10 with a diameter of 27.3 cm and a thickness of 7.6 mm is used.
The experiment was conducted using The piezoelectric element and mounting block assembly, excluding the plate, was first tuned to approximately the desired resonant frequency (initial experiments were set at 11.8kHz).
The outermost ring 20 was omitted, leaving the circumference of the plate 10 undamped. The phase correction ring 16 is made of 20.6 mm thick polystyrene foam and has an impedance matching disk 18.
and ring 20 were made of 8.5 mm thick polyethylene foam, and these parts were positioned so that their edges coincided with the nodal circles. The directivity pattern from the system after optimizing the ring is:
It was found to exhibit a 3dB bandwidth of 7.5°, an axial sound pressure level of 830 microbar and sidelobes down more than 20dB. When operated in a pulsed acoustic sounding system, a transducer output of 1.9 volts peak to peak was obtained from a hard target at a distance of 2.15 meters.

The electrical impedance of the transducer system with and without plate 10 was measured over a frequency range including the resonant frequency, with reactive and resistive components recorded in both cases. The difference between the two sets of numbers represents the impedance through the plate, which is quite large at the resonant frequency and is essentially resistive in nature, with resistive and reactive components of 1150 ohms and -1600 ohms, respectively, of the overall system. approximately 850 ohms, indicating substantial coupling to the medium and low system Q.

Another experiment was carried out using a plate 20 cm in diameter and 2 mm thick, again with a configuration similar to that shown in Figure 3, with three rings 16 11.5 mm thick to accommodate even higher bending modes. I did it. 22.2kHz
At the operating frequency, the 3dB beamwidth was 7.5°, the sidelobes were down at least 13.7dB, and the sound pressure level was 1500 microbar. The acoustic power returned from the hard target at a distance of 2.15 m was 3.3 volts peak to peak.

For comparison, it has a radiation surface of 19 cm in diameter and
A transducer constructed according to the teachings of Hands US Pat. No. 3,674,945 operating at 21 kHz was tested under identical conditions. This transducer exhibits a 3dB beamwidth of 9°,
The side lobes were down 12dB. The acoustic output of this transducer is more than 2250 microbar,
It was peak peak. However, the echo power was quite low, with only 1.9 volts peak-to-peak being measured for the echoes coming back from the hard target at a distance of 2.15 meters. Furthermore, such transducers use 8.4 kg of piezoelectric material, whereas the test transducer used only 70 g of piezoelectric material.

In another test, a transducer operating at 21.5 kHz with a plate 14 cm in diameter and 4.9 mm thick and in a configuration smaller than that shown in Figure 3 had a 3 dB beamwidth of 8° and a sidelobe. is down by 14dB, and the sound pressure output is
It was 1600 microbar peak to peak.

In view of the very large increase in coupling efficiency obtained with the embodiments already described and the good results obtained with the embodiment of FIG. We conducted an experiment using In order to reduce undesirable sidelobes in the antenna pattern, more resonant modes were rigorously tested to increase the number of nodal circles. Operating in such a mode allows the use of thinner disks and provides resonance at the desired frequency with disks of any size. The transducer thus modified is shown in axial cross section in FIG. 4, in which case the same reference numerals used in FIG. 3 are used to designate similar parts. There is. Only the differences will be explained in detail.

The plate 10 is considerably thinner than that shown in FIG. 3, and the polyurethane seal 26 and felt seal 28 of FIG. 3 have been omitted. Rings 18 and 20 are similarly omitted, with ring 16 being one of the plates.
It is attached to the antinode zone of Tsuoki. Although in the illustrated embodiment the even numbered antinode zones (counted from the center) are shown covered by rings 16, the opposite arrangement could also have been used. However, it is preferred that the entire outermost antinode zone be coated in order to ensure that the ratio of covered to uncoated area of the plate is the highest reasonably achievable. In the illustrated embodiment, there are 10 antinode zones and 5 rings 1
6, but this number may vary provided that whatever degree of sidelobe suppression is desired is achieved. The thickness of the ring relative to the material of the ring 16 depends on the radiating surface of the plate and the gaseous medium from which this surface radiates, as discussed elsewhere.
Usually chosen to provide the best match for air.

In the embodiment of FIG. 4, somewhat different coupling means are employed between the transducer element 2 and the plate 10. A mounting block 22 is connected to the plate via posts 23. A foam fill 25, formed in chips or in situ, is used to prevent reflections within the housing cavity and is separated from the compound fill 34 by a polyurethane sealing membrane 27 cast in situ. Separated.

A transducer constructed according to the embodiment of FIG. 4 was tested under the same conditions as described above with respect to the embodiment of FIGS. 1-3.

The transducer has a diameter of 24 cm and a thickness of 1.3 cm according to Figure 4.
It was constructed using a plate 10 made of 6061-T6 grade aluminum with a diameter of 6061 mm. Ring 16 has a density of 0.025gm/
Made of CC low density closed cell polyethylene foam.
It was 5.3 mm thick, which is the quarter wavelength of sound in the material at 21 kHz, the operating frequency of the transducer.
The drive structure of the transducer element, ie the mounting block and column, was tuned to resonate at this frequency. 21.0kHz after optimizing the arrangement of ring 16
The system's directivity pattern at the test frequency was found to exhibit a 3 dB bandwidth of 4.9°, a 12 dB bandwidth of 8.3°, an axial sound pressure level of 3000 microbar, and sidelobes down at least 18 dB. When operated in a pulsed acoustic sounding system, a transducer output of 5.5 volts peak to peak was obtained from a hard target at a distance of 2.25 meters. The 3dB bandwidth of the acoustic sounding system is
It was 1.9kHz, corresponding to System Q of 11.2. It has been found that even wider bandwidths can be obtained by offsetting the resonant frequency of the disk from that of the drive assembly. Set the resonance frequency of the disc to 1.2kHz
Offset by 0.05 kHz will result in a corresponding increase in bandwidth.

For an operating frequency of 13kHz, the plate diameter is 33cm.
Another transducer was constructed in which the thickness of ring 16 was increased to 7.6 mm to provide quarter-wavelength matching. In this case, the plate had 11 antinode zones, and the odd antinode zones counted from the center were covered by rings 16. At the test frequency of 13.03 kHz, the system's antenna pattern showed a 3 dB beamwidth at 4.9°, a 12 dB beamwidth at 9.1°, sidelobes down at least 15 dB, and an axial sound pressure level of 5600 microbar. 2.25 m when activated in a pulsed sounding system
14.6 volts from a hard target at a distance of
A peak-to-peak converter output was obtained. The 3 dB bandwidth of the acoustic sounding system was 1 kHz, corresponding to a system Q of approximately 13.

From the tests described above, it can be seen that, despite the simplification of the ring system, and the mass of the transducer elements in the experimental transducer is even more significant than that constructed and tested according to Figs. It will become clear that, despite the fact that it is reduced, even higher outputs can be obtained from a converter according to FIG. In the embodiment of FIG. 4, no attempt is made to use phase reversal to prevent cancellation in the far field by reversing the phase of the sound emitted from every other antinode zone. Instead, far-field cancellation is prevented by making the sound power radiated from every other antinode zone negligible compared to the sound power radiated from the interference zone. Because the mismatch between the plate and the medium in the zone not carrying ring 16 is so great, very little energy is radiated, while ring 16 is an effective radiator. Therefore, the energy radiated from the ring on the axis of the transducer is not substantially cancelled. However, interference between the radiation from different rings 16 prevents the generation of significant side lobes in the directional pattern. Although the effective radiating area of the plate decreases proportionally to the area not covered by the ring 16, the dimensions of the plate may be scaled in such a way that the proportion of the plate's energy transferred to the medium in each vibration cycle actually increases. This reduction can be easily compensated for by making it larger.

One drawback of prior art transducers, such as that of Hands' U.S. Pat. The point is that it can be seriously damaged. This is because the deposited material loads the matching material and changes its tuning, thereby reducing the effectiveness of the matching material until the deposited material is evaporated or otherwise removed, or completely removed. This is because it will disappear. Similar disadvantages also occur with transducers constructed according to FIGS. 2 and 3 of the present invention. However, in the case of the embodiment of FIG. 4, the magnitude of the failure is much smaller, and even if the radiating surface is sprayed with water, the transducer will still function satisfactorily, albeit at a reduced output. I understood. This surprising result is thought to be caused by a change in the relative functions of the two sets of antinode zones. The water loads the alignment ring 16, detuning it and impairing its alignment function. However, the ring operates to phase shift the sound emitted from the zone covered by the ring with respect to the sound emitted from the uncovered zone;
The radiation from this covered zone becomes more important as the radiation from ring 16 decreases. The phase shift is due to the difference in the speed of sound in the ring material relative to the air and the null characteristics of the detuned matching section.
The result of the phase shift is that the sound radiated from adjacent zones is almost quadrature rather than out of phase.
Mutual cancellation is greatly reduced and effective output is maintained. This logic of operation was tested by measuring the output of the radiating surface while gradually wetting the radiating surface of the transducer. The power decreased as moisture build-up increased, but no zero or minimum power was recorded. Although the matching provided by the ring 16 is eliminated, the coupling to the medium can still be enhanced due to the large radiation surface relative to the effective area of the transducer element allowed by the invention. This in itself effectively provides a significant degree of impedance transformation.

Transducers with reduced sensitivity to moisture are carefully detuned from the matching ring sufficiently that moisture, dirt, or other surface loads do not significantly alter the phase shift imparted by the ring or its radiation efficiency. We were able to provide this by: Such an arrangement would not normally be advantageous since it would not improve the output of the system in humid conditions. Another approach tested was to make the ring of lossy material so that the radiation from the ring was significantly reduced compared to the uncoated antinode zone. This again sacrifices the alignment that could be provided by the ring, but also reduces the efficiency of the system since the ring absorbs a much larger portion of the energy applied to the plate. No satisfactory results were observed with this arrangement. The reasons for this are low efficiency and the difficulty of achieving effective sound absorption in thin materials. Yet another disadvantage is that when effective sound absorption is obtained, the surface of the ring remains immobile and the acoustic self-cleaning properties evident in the transducer according to the invention are lost. For similar reasons, rings 16 of low-loss material are placed in every other antinode zone.
It is not believed to be particularly advantageous to apply sound-absorbing material to the interference zone when the interference zone is applied.

Instead or in addition to making the thickness of material applied to every other antinode zone in the embodiment of FIG. The thickness may be chosen to provide a phase to avoid cancellation in the axial far field.

A number of additional experiments have been conducted, some using different materials for plate 10 and rings 16, 18, 20, including synthetic plastic materials. The suitability of materials for these rings may be determined by the same criteria discussed in detail with respect to the impedance matching layer used in the system of Hands US Pat. No. 3,674,945. The requirements for the material used for plate 10 are high impedance, low mechanical losses during vibration and high elastic modulus. The uniform thickness of the plate 10, which is small compared to its diameter, effectively behaves as a zero rest mass object at resonance, as just described, as far as the vibration drive system is concerned, independently tuning the plate and the transducer elements. be able to. This feature aids in system design and offers the possibility of staggered tuning of the drive system and transducer elements to further increase the overall bandwidth of the transducer system. Plate diameter to thickness ratios of 25:1 to about 500:1 have been found to yield good results, but this range should not be considered limiting. Plates with a large ratio are usually preferred because the spacing between nodal circles is reduced and higher grade resonances can be used for a given plate diameter. A large number of nodal rings makes it easier to avoid unwanted sidelobes in the transducer response. Second
Although the embodiment of FIGS. and 3 has three nodal rings, more is preferred, and the embodiment of FIG. 4 has ten rings. The experimental results obtained are
If we apply the modified material according to the same principle, then these zones will not necessarily be ring-shaped, but symmetrical or It is shown that similar results would be obtained using non-circular plates operating in asymmetric bending modes. Although the plate must have a large radiating surface compared to the transducer to realize the advantages of the invention, the principle of the invention is that the plate itself transforms the impedance by increasing the area of contact with the propagation medium. It is permissible to enlarge the plate to such an extent that the degree of coupling to the medium can be greatly improved without utilizing the alignment techniques of US Pat. No. 3,674,945 to act as a vessel.

It has been found that a wide variety of elastic materials can be used for rings 16, 18 and 20. Materials ranging from polystyrene foam having a density of only 1.2% of the density of the board material to solid polyurethane elastomer having a density of 43% of the density of the board material have been tested with good results.
The use of this polyurethane elastomer results in slightly lower efficiency and greater overall mass, which has a greater effect on the resonant frequency of the plate. On the other hand, its use simplifies manufacturing and allows larger irregularities to be provided. Polyurethane elastomers also allow for phase modification by rings that are much less thick, since the difference in the speed of sound propagation through them compared to the speed of propagation in air is greater than for the foamed materials tested. It has the advantage that it can be done. Furthermore, it exhibits very low hysteresis at the frequencies of interest. Some material is used in rings 16, 18 and 20.
It is of course recognized that suitability for use in a system depends on its properties as a low-loss acoustic propagation medium at the operating frequency of the system, its acoustic impedance, and the relationship between the acoustic impedances of the plate and the gas medium. There will be. Ideally, the ratio of the acoustic impedance of the plate material to the acoustic impedance of the ring should be the same as the ratio of the acoustic impedance of the ring material to the acoustic impedance of the gaseous medium, but even if it is less than the ideal relationship, This can be compensated for by other properties of the ring material. Therefore, if the plate is aluminum, the gaseous medium is air, and the ring material is polystyrene foam, the ratios defined above are each approximately
400 and about 85. When the material of one ring is a solid polyurethane elastomer, the ratios are about 8 and about 4000, respectively. The material should not exhibit significant hysteresis in the propagation of acoustic waves at the operating frequency, since this will prevent proper operation and reduce efficiency. Materials with small closed cells appear to provide the best results among foamable materials.

If the mass of the material forming the rings 16, 18 and 20 is significant compared to the mass of the plates, the resonant frequencies of the plates will shift significantly and, of course, appropriate allowances must be made for this.

Although the various rings have been described as being formed separately, when the rings are attached to adjacent antinode zones, it is possible to make the rings as a single integral molding, especially when they are all formed of the same material. It is clear that it can be done. Additionally, although in the embodiments described with respect to FIGS. 1-3 the overall thickness of material applied across adjacent antinode zones of the plate is shown alternating up and down; This need not be the case if the thickness complies with the requirements discussed below.

As discussed in US Pat. No. 3,674,945, the material used for matching purposes should be an odd number of quarter wavelengths thick. In transducer embodiments in which adjacent antinode zones are to be matched to the medium, the antinode zones are alternately further matched to the sound passing through the same thickness of air (including any other gaseous medium). It is necessary to coat the material (not necessarily the same material) to a thickness such that it provides a phase shift of approximately 180°. This thickness is n/2f
It can be expressed as (1/C ₀ −1/C ₁ ). where n is an odd integer, f is the operating frequency, C ₀ is the speed of sound in air, and C ₁ is the speed of sound in the material used. The more this thickness can be selected to meet matching conditions, the better. It is clear that there should be a significant difference between C ₀ and C ₁ to keep the thickness reasonably small. Understand that experimentation is usually required to optimize the thickness of material applied to the plate, especially since the speed of sound in foamed materials is a function of operating frequency and configuration and sometimes material orientation, among other things. Should.

The phase modifying and matching material (rings 16, 18, 20 in the described embodiment) is a low density material with a lower acoustic impedance than the plate, so it typically adds little mass or stiffness to the plate. , and thus also has a relatively little influence on the resonance properties of the plate. This allows the use of relatively thin disks such that the surface area of the disk is very large compared to its volume or the energy stored in the disk. Since the rate of energy transfer from the plate to the surrounding medium is proportional to the area of the radiating surface, during each cycle the proportion of energy stored in the plate that is transferred to the medium increases;
Therefore, the Q of the system decreases. Furthermore, since the ratio between the area of the plate and the effective area of the transducer elements is very large, much smaller transducer elements can be used to achieve any desired energy transfer. Thus, the transducer elements utilized in the various embodiments described, which typically contain about 70 to 150 grams of lead zirconate titanate, were constructed in accordance with U.S. Pat. No. 3,674,945. A 10 kHz transducer with comparable performance would probably require 50 kilograms of expensive piezoelectric material and would be less efficient.

Although prior art transducers, such as those according to U.S. Pat. It is an advantage of the present invention that it can be used. Although the lead zirconate titanate material can be readily fabricated into annular elements suitable for use in the present invention, it is difficult to fabricate elements suitable for transducers such as the transducer of U.S. Pat. No. 3,674,945. is not easy.

The coupling between the transducer element and the diaphragm may be modified in various ways. As already explained with respect to FIG. 4, the transducer elements are mounted between similar mounting blocks, and this structure is connected to the plate by short posts that are attached to the structure at one end and to the plate at the other end. Good results have been obtained with the arrangement shown. 1978 of ultrasound instead of this note
Mechanical amplifiers could also be used, such as those described by Gariego-Juarez et al., November 2005, page 268.

Although all of the specifically described embodiments have involved arrangements in which the edges of the plates are substantially unconstrained, it is of course possible to use plates that are clamped or otherwise secured around the perimeter. is possible, in which case the nodal circle coincides with the circumference of the plate. Such an arrangement may be advantageous in some cases, particularly where it is desired to provide a fire-resistant system for use in environments with a risk of fire or explosion.

As described above, the acoustic transducer of the present invention has high conversion efficiency as a result of providing a low acoustic impedance material having the following functions so as to cover every other antinode zone on the radiation surface of the plate. It has become directional.

1. Changing the relative phase of sounds emitted from adjacent antinode zones in antiphase to reduce cancellation due to antiphase.

2. Varying the degree of coupling between each adjacent antinode zone and the propagation medium air to reduce cancellation by each antiphase radiation.

3. Improving the damping performance of a plate with a radiating surface. That is, one low acoustic impedance member
By installing it in the antinode zone of the tsuoki, the conversion efficiency of sound radiated energy will be improved.
As the energy remaining in the plate is quickly released, the damping performance of the plate increases, and the plate has particularly good response characteristics to high-frequency sound pulses.

Generally, the plate can be provided with damping elements, but this also results in a reduction in the radiation amplitude. In the present invention, by increasing the coupling with the propagation medium using a low acoustic impedance member, damping performance can be improved without reducing conversion efficiency.

As used herein and in the appended claims, the term "higher-order bending mode resonance" means at least two antinode zones separated by a node and mutually exclusive (without modification). It is to be understood to include any form of bending mode resonance in the plate that produces out-of-phase radiation.

[Brief explanation of drawings]

Figure 1 shows the first part of the invention without the protective housing.
FIG. 2 is a plan view of a second embodiment of the invention supported within a protective housing; FIG.
The figure is an axial sectional view of the embodiment of FIG. 2, and FIG. 4 is an axial sectional view of the third embodiment of the present invention. 2... Piezoelectric crystal element, 4... Conductive brass washer,
6...Electrical matching transformer, 8...Loading block, 1
0...Plate, 12...Through bolt, 14...Nut, 16, 18, 20...Ring, 22...Loading/driving block, 23...Column, 24...Housing, 25...Filling material, 26 ... Surrounding polyurethane seal, 27 ... Polyurethane seal membrane, 28
...Felt seal, 30...Space, 32...Cork, 34...Compound filling.

Claims (1)

[Claims] 1. A radiation surface and a plate having a bending mode resonance substantially at the operating frequency of the acoustic transducer, and a plate having an effective area smaller than the radiation surface of the plate and connected to the plate for exciting or said bending. a transducer element responsive to modal resonance, wherein every other antinode zone of the radiating surface of the plate has a lower acoustic impedance than the plate and a relative radiation of sound from an adjacent antinode zone to the gaseous medium. at least one of phase and relative amplitude
The mutual cancellation of the sound radiated into the gaseous medium from the adjacent antinode zones of the plate in the field far from the plate and in the desired radiation direction is sufficiently reduced, and the damping effect of the plate is further reduced. 1. An acoustic transducer, characterized in that it is successively covered by members having an increasing selected thickness. 2. The members attached to every other antinode zone shall be of a low-loss acoustic propagating material and have a thickness of an odd number of quarter-wavelengths of sound in the material at the operating frequency of the system, and shall be 2. The acoustic transducer according to claim 1, wherein no such member is attached to the antinode zone. 3. The thickness of each low-loss acoustic propagation member attached to every other antinode zone is such that the sound reaching a far field from the every other antinode zone is compared to the sound radiated from the remaining antinode zones. 2. The acoustic transducers of claim 1, wherein the acoustic transducers are formed to differ from one another by an amount such that they undergo a phase shift sufficient to substantially reduce cancellation. 4. Acoustic transducer according to claim 1, 2 or 3, characterized in that the plate is axially symmetrically resonant and axially symmetrically coupled to the transducer element. 5. Claims 1 or 2, or 5, characterized in that the plate is a circular plate of uniform thickness and that the plate and the transducer element are tuned to a resonant frequency very close to the operating frequency of the system. The acoustic transducer according to item 3. 6. Claim 1 or 2 or 3, characterized in that the plate is a circular plate of uniform thickness, and the ratio of diameter to thickness of the plate is from 25:1 to 500:1. Acoustic transducer as described in section. 7. Each antinode zone of the plate is coated with said material to a thickness that differs from the material coating the next adjacent zone by n/2f (1/C ₀ -1/C ₁ ), where n is an odd integer, f is the operating frequency of the system, C ₀
4. The acoustic transducer according to claim 3, wherein C1 is the speed of sound in a gaseous medium and _C1 is the speed of sound in the material. 8. At least some of the antinode zones of the plate are covered with a low-loss acoustic impedance matching member to a thickness equal to an odd multiple of a quarter wavelength of sound at the speed of sound in the matching member. An acoustic transducer according to claim 3 or 7. 9. Claims 1, 2, or 3, characterized in that the low-loss acoustic propagation member is selected from foamable synthetic plastics with closed cells and non-foamed elastomers. acoustic transducer. 10 Claim 1, characterized in that the plate is a disk having at least three nodal rings.
The acoustic transducer according to item 1 or 2 or 3.