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Nov 1988

Volume 84, Issue S1, pp. S2-S224

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back to top Session W. Engineering Acoustics III: Modern Acoustic Transducers
Invited Papers
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New developments in the field of acoustic transducers (A)

G. M. Sessler

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S66-S66 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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New developments in the field of acoustic transducers based on the silicon, fiber‐optic, and polymer technologies are reviewed. The silicon transducers are produced on silicon wafers with micromechanic methods utilizing anisotropic etching procedures. Subminiature transducers of this kind, based on the capacitive or piezoelectric principles, were made with membranes of about 1‐mm2 area and less than 1‐uμm thickness. Their use as “intelligent sensors” with on‐chip signal‐processing capability is presently under study. Fiber‐optic sensors consist of glass fibers and detect modulations of either phase or amplitude of the transmitted light waves, caused by interaction of the sound waves with the fiber. Such transducers have initially been used as hydrophones and accelerometers but are now also under study as microphones. Polymer‐electret transducers are finding widespread applications as microphones, earphones, and ultrasonic devices. New developments include the use of electret biasing in silicon transducers and the design of electret‐microphone arrays and antennas. Piezoelectric polymer transducers were recently improved by the use of better materials and by innovative poling techniques, such as methods to produce single‐film monomorphs and bimorphs.
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Fiber‐optic sensors for vibro‐acoustic measurements (A)

S. L. Garrett

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S66-S66 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Optical fiber sensors of sound and vibration offer potential advantages over conventional (electronic) sensing techniques in applications where there is strong electromagnetic interference, large sensor/receiver separations, or restrictive (remote) power requirements. The rapid acceptance of optical fibers as a communications medium over the past decade has stimulated much research in fiber‐optic sensing but has generated very few commercially available vibro‐acoustic sensors of the intensity [J. W. Berthold, “Overview of fiber‐optic intensity sensors for industry,” in Fiber Optic and Laser Sensors V, Proc. SPIE 838, 2–8 (1987)] or the interferometric [C. M. Davis, “Fiber optic sensors: An overview,” Opt. Eng. 24(2), 347–351 (1985)] types. This presentation will review the basic fiber‐optic sensor types (amplitude/phase and intrinsic/extrinsic), present examples of microphone, hydrophone, and accelerometer designs, and speculate on the factors that will influence their commercialization over the next decade. Issues of minimum detectable signals, electro‐optic demodulation, and multisensor array multiplexing will be addressed as time permits.
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Piezoelectric acoustic transducers for electronic telephone sets (A)

Juro Ohga

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S66-S66 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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The development of piezoelectric ceramic transducers for the transmitter, receiver, and tone‐ringer sounder of recent electronic telephone sets is reviewed. As the first development stage, the working attenuation (WA) was studied as a measure to compare efficiencies of various types of electroacoustic transducers. The conclusions were that WA for electroacoustic transducers, which was defined in a similar way to WA for electrical circuits, could be estimated easily from the specific response for a microphone, and that WA was almost independent from transducer size. Then, it was shown by comparison of WAs that the efficiency of the simplest piezoelectric ceramic transducer using a unimorph diaphragm was as high as that of ordinary electromagnetic or electrodynamic ones. Moreover, a piezoelectric transmitter and receiver needed the same resonant frequency of the diaphragm, because both diaphragms should work in the stiffness‐controlled region. Fortunately, their optimum resonant frequency was close to that of the tone‐ringer sounder. Therefore, the main problem of the second development stage was how to design acoustical circuits for the piezoelectric transmitter, receiver, and tone‐ringer sounder using the same diaphragms. This was solved by introducing a circuit with four degrees of freedom. As a related problem, simplification of transducer structure was also studied.
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A new piezoelectric microphone with divided electrodes (A)

Nobuomi Imai

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S67-S67 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A new type of piezoelectric ceramic microphone is proposed. Its merits are as follows. (1) High output, high efficiency—A high output voltage was obtained by dividing the piezoelectric ceramic electrodes and connecting them in series. The output voltge is proportional to the number of divisions. However, since the impedance is high, an FET circuit is required to decrease output impedance. At the same time, an improvement of the signal‐to‐noise ratio was also realized. (2) Reduction of diaphragm mechanical impedance—To improve the matching to the impedance of air, a mechanical transformer was introduced. The center of the diaphragm was lightened and its rigidity was increased by combining a dome‐type diaphragm and ring‐shaped piezoelectric ceramic. This made it easy to control the frequency response. (3) Stability—The diaphragm consists of a Y2O3⋅PSZ ceramic thin film formed into a dome shape and laminated to a ring‐shaped ceramic. This structure showed a higher temperature stability and noticeably lower aging change than the conventional flat‐type diaphragm. The specifications of the prototype microphone are the following: diameter, 1 in. (23.8 mm), nondirectional; sensitivity, −59 dB/uμbar at FET source output; frequency response, 20 Hz–12.5 kHz −+ 1.5 dB; S/N ratio, 59 dB A‐weighted.
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Use of thin PVDF film for measuring small single‐crystal samples of high temperature superconductors (A)

J. D. Maynard, J. H. Mather, Stewart Brown, and Albert Migliori

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S67-S67 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Although polyvinylidene fluoride (PVDF) piezoelectric film has been adopted in commercial applications for some time, its use in basic research has been less extensive. Recently, there have been found significant advantages in using PVDF to study the ultrasonic properties of the new high‐temperature oxide superconductors. In order to probe the anisotropy of the oxide superconductors, it is necessary to study single crystals, but currently available crystals are very small, on the order of the millimeter square and only about 100uμm thick. Measuring the temperature dependence of the ultrasound in such a small sample without having the transducer (or its leads or substrate) intrude on the measurement is a difficult problem that is not readily solved with conventional transducer materials. However, PVDF films as thin as 9 μm are commercially available and easily adapted to small samples and low‐temperature operation. Small active areas and leads are produced with metalization patterns on each side of the PVDF film. For resonance measurements, electrical crosstalk across the small sample is processed by frequency modulating the drive and using phase sensitive detection. In our experiment, resonances in ≃ 100‐uμm samples are measured with large signal‐to‐noise ratios. [Work supported by the Office of Naval Research and NSF Grant DMR 8701682.]
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Pulse discharge sound source and its application to acoustic measurement (A)

Hideo Shibayama and Ken'iti Kido

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S67-S67 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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This paper describes a pulse discharge sound source without any diaphragm, and its application for acoustic measurements. The waveform of the sound pulses radiated from this source is simple and reproducible. The power spectra of these pulses have no zero points up to 90 kHz. The duration of the sound pulse is about 50 uμs, and is dependent on the gap length between the discharge electrodes. The directivity function of the sound source is the same as that of a line source that is equal to the gap length. The sound generating spot is small, and the sound source requires no baffle board. As a result, no reflection from the sound source itself is produced. The timing of sound generation is stable and controllable. As examples of acoustic measurements utilizing the sound source, the estimated results of acoustic impedance characteristics of materials and their frequency responses of diffraction are shown. The estimated results agreed well with the calculated ones. A measuring system can be considerably simplified by use of the pulse discharge sound source and the digital signal processing technique. Measuring time can also be reduced.
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Directional microphone systems for teleconferencing (A)

G. W. Elko and J. E. West

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S67-S68 (1988); (2 pages)

Online Publication Date: 13 Aug 2005

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Many factors have contributed to historically unrealistic predictions for the growth of audio teleconferencing. Aside from the sociological issues, there exist many acoustic problems that can severely degrade the performance of present technology. A few of these problems are: the effects of reverberation, room noise, feedback, undesired speakers, and echo. At present, the most common solution for some of these problems has been to locate a microphone(s) in close proximity to the desired speaker(s) with gain switching to eliminate feedback problems. While this solution can sometimes be effective, it also introduces some compromises that are easily spotted and are considered annoying by teleconferencing users. The most notable problems are with gain switching and reverberation. These detrimental effects can be reduced by the use of highly directional microphones systems tailored to the problems of teleconferencing. The development of directional microphones and their present use in audio teleconferencing will be reviewed. Some of the highly directional microphone arrays that have been worked with will be described. Finally, comments will be given on how these new directional microphone systems address the problems of teleconferencing in small and large acoustic environments.
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Modern techniques for frequency response measurement of loudspeakers (A)

Shokichiro Yoshikawa and Takashi Wakuri

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S68-S68 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Usually, the frequency responses of loudspeakers are measured by the point‐by‐point method in a free sound field. In this method, accuracy of the measurement depends on the precision of the free sound field. Since the wavelength of sound is quite long in the low‐frequency range, an anechoic room of a large size with high accuracy is required to obtain good results in the low‐frequency range. To overcome this difficulty, several new measuring methods have been proposed [e.g., J. Merhaut, J. Audio Eng. Soc. 30, 882 (1982); and IEC84/WG8 (Chairman) 1 April (1984)]. In this study, a comparison and evaluation of new measuring methods were carried out. Factors that affect the accuracy of measurement were investigated. The accuracy and the applicable frequency range were clarified. The possibility of measuring the frequency response of loudspeakers without using an anechoic room will be discussed.
Contributed Papers
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Application of piezoelectric composite for large‐area hydrophone arrays (A)

Fred G. Geil and Robert Y. Ting

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S68-S68 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Large‐area hydrophone arrays using flexible materials are subject to high levels of interelement coupling arising from the material in‐plane sensitivity coefficient g31, in the presence of flexural energy within the array module. A “0—3” type piezocomposite offers a solution to this interelement coupling problem without introduction of fabrication complexity such as stiffening of elements. A simplified array fabrication concept will be presented, as well as results from small array tests using commercially available “0—3” piezocomposites based on lead titanate powder dispersed in a rubber matrix. These test results show a near‐zero level of acoustic contamination, shown by directivity patterns and relative phase responses. Besides low interelement coupling, cost, waterproofing, and electrical interfacing are also important in the design considerations of affordable, realistic arrays for the seawater environment. The advantages of using “0—3” piezocomposites in meeting these additional requirements will be briefly discussed. [Work supported by ONT.]
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A balanced, centrally stiffened design for PVDF hydrophone arrays (A)

Robert Y. Ting and Fred G. Geil

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S68-S68 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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As PVDF thick films become more readily available, there has been a growing interest in the development of a large‐area hydrophone for passive sonar applications. These new PVDF materials offer the advantages of having a lower density, greater mechanical strength and flexibility, and better impedance match to water than conventional piezoelectric ceramics. Furthermore, they are now available in large sheets, from which arrays can be developed for acoustic detection covering large surface areas. Prototype PVDF hydrophone arrays were fabricated and tested to show that in‐plane flexural motion greatly affected hydrophone responses due to the in‐plane anisotropy of the piezoelectric properties of PVDF. To circumvent this difficulty, a balanced, centrally stiffened configuration was adapted by sandwiching two pieces of PVDF to a common damped plate, while aligning in parallel the stretched “3—1” directions of the PVDF sheets. With this design, constant receiving sensitivity and directivity patterns in agreement with theoretical predictions were obtained. Further refinement of this design was also achieved through mathematical modeling. [Work supported by ONR.]
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Comparison of ceramic and thick‐film PVDF hydrophones for large surface arrays (A)

B. Tocquet, B. Fromont, and M. Josserand

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S68-S68 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A special process allows the manufacturing, in industrial conditions, of thick, nonvoided PVDF films (up to 2 mm) with excellent piezoelectric properties. The characteristics of PVDF are compared with those of some classical PZT ceramics. From such films, very simple and reliable large surface sensors have been designed and developed. Compared to classical ceramic hydrophones, these sensors show a great stability versus temperature and pressure and a very good sensitivity to self‐noise ratio for low‐frequency passive listening systems. Furthermore, these large surface sensors fit very well with hard baffles and are less sensitive to flow noise than small ceramic hydrophones. Some examples of results obtained with such new hydrophones are given. Clusters of such PVDF hydrophones can be encapsulated with viscoelastic “pρc” materials in flat panels and fixed, with decoupling layers or directly on the hull of the ship, in direct contact with the hydrodynamic flow, leading to a very compact and reliable technology for large flank arrays.
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On the use of terfenol D in underwater acoustic projectors (A)

Mark B. Moffett, Jan F. Linberg, and James M. Powers

J. Acoust. Soc. Am. Volume 84, Issue S1, pp. S68-S69 (1988); (2 pages)

Online Publication Date: 13 Aug 2005

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Terfenol D, an alloy of iron, terbium, and dysprosium, is a highly magnetostrictive material that can provide significantly more power than lead zirconate titanate (PZT) under field‐limited conditions. Field‐limiting at resonance, however, requires a mechanical quality factor Qm, less than unity for terfenol D and less than four for PZT. Transducers that are a small fraction of one wavelength in size will have to be elements of a large array in order to achieve the requisite low Qm's. If the Qm is too high, the transducer will be stress‐limited, rather than field‐limited, and the use of terfenol D is not particularly advantageous. [Work supported by NUSC's IR/IED program and by ONT.]
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