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Dec 1977

Volume 62, Issue S1, pp. S1-S102

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back to top Session EE. Engineering Acoustics V: Devices and Calibration
Contributed Papers
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High‐amplitude/low‐frequency impulse calibration of microphones: A new method (A)

Alan Hunt and Paul D. Schomer

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S71-S71 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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This paper describes a new method to measure the peak amplitude response and the droop (low‐frequency response) of a microphone and its associated amplifiers. The requirements for the new method was that it be simple, inexpensive, and accurate. An acoustic “step function” would be an ideal source for testing the large‐amplitude and low‐frequency response of microphones, especially for measurements of impulsive noise. Unfortunately, a positive‐going acoustical step‐function source is difficult to obtain. This method involves the use of a negative step‐function input which is applied by closing a microphone inside a thin membrane which is inflated and then burst. The sudden decrease in pressure represents a negative‐going step‐function input to the microphone. The peak output voltage produced by the microphone must be proportional to the initial static pressure within the membrane and the return (decay) of the voltage output of the microphone to zero directly represents the droop of the microphone and system. The low‐frequency cutoff of the system is easily calculated from the time constant of the decay curve.
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Shape optimization of pressure‐gradient microphones (A)

T. D. Norum and J. M. Seiner

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S71-S71 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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Recently developed finite‐element computer programs were utilized to investigate the influence of the shape of a body on its scattering field with the aim of determining the optimal shape for a pressure‐gradient microphone. Circular cylinders of various aspect ratios were evaluated to choose the length to diameter ratio best suited for gradient microphone application. Alterations of the basic cylindrical shape by rounding the edges and recessing at the centerline were also studied. It was found that for a ±l‐dB deviation from a linear pressure‐gradient response, a circular cylinder of aspect ratio near 0.5 was most suitable, yielding a useful upper frequency corresponding to ka = 1.8. The maximum increase in this upper‐frequency limit obtained through a number of shape alterations was only about 20%. An initial experimental evaluation of a single‐element cylindrical pressure‐gradient microphone of aspect ratio 0.14 utilizing a piezoresistive type sensor was also performed and is compared to the analytical results.
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Comparison of various techniques for the determination of the random incidence response of microphones and sound level meters (A)

Myroslav R. Sorbyn and Edward B. Magrab

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S71-S71 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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The measurements necessary for the determination of the random incidence responses of microphones and sound level meters were made from 500–20 000 Hz in an anechoic chamber using both single‐frequency tenes and 1/3‐octave bands of noise. The random incidence responses were computed from these data using formulas given in the relevant IEC and ANSI standards documents and according to a Gaussian‐type numerical integration formula. This Gaussian formula appears to introduce less numerical error due to the approximation procedure used to compute the random incidence response than either the IEC or ANSI formulas. The random incidence responses of the microphones and sound level meters tested in the anechoic room were also measured in two reverberation rooms using both discrete sampling of the reverberant field and two types of continuous sampling: linear and circular. The results from the various methods and procedures are compared to the exact values (determined by a numerical procedure that used at least 30 points) and the minimum number of points (angles) necessary to achieve a prescribed level of accuracy in the random incidence responses are determined.
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Phase shift due to scattering by a microphone (A)

Richard K. Cook and Thomas M. Proctor

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S71-S71 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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The electrical output of a microphone in a plane wave of sound in air, harmonic in time, will have its phase shifted relative to the phase of the sound wave. Part of the phase shift is due to scattering by the finite bulk of the microphone, and part may occur in its electronic circuit. Information on the amounts of the phase shifts is needed whenever the outputs of an array of microphones are used to measure such quantities as cross‐correlation coefficients, the Umov (intensity) vector, the local phase velocities for various directions, etc., in a sound wave field. We present the theoretical analysis and some experimental results for the phase shift due to scattering. The theoretical analysis leads to the concept of a “phase plane” associated with the electrical output of a microphone. Results will be presented, mainly at long wavelengths, (1) for a solid spherical microphone, (2) for a probe‐tube microphone, and (3) for a Helmholtz resonator at low frequencies.
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Improved design of spherical multimode hydrophone (A)

S. H. Ko, H. L. Pond, and F. A. Alatalo

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S71-S72 (1977); (2 pages)

Online Publication Date: 11 Aug 2005

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An earlier model for a spherical multimode hydrophone [S.H. Ko, G.A. Brigham, and J.L. Butler, J. Acoust. Soc. Am. 56, 1890–1898 (1974)] is able to predict the hydrophone response curves in which resonances are present. In order to eliminate these resonances, the surface area of a given hydrophone is divided into selected areas which are separated from one another and such that the sum of voltages from the elements of an area is zero for the resonant mode, while the sum of voltages due to the nonresonant modes still provides the essential information about the direction of the incoming acoustic wave. This method was used successfully to remove the lowest (n = 3) resonant mode [H.L. Pond, F.A. Alatalo, and S.H. Ko, J. Acoust. SOc. Am. 60, Suppl. (1976)]. However, the elimination of the n = 3 mode is not sufficient. For higher modes, the technique developed here is to keep only two zones instead of adding more zones and to select the position of the line of division between zones by using a weighting factor, so that the, n = 3 mode resonance is eliminated and the effect of the resonance for the other modes is greatly reduced. Experimental results are in agreement with the theoretical results. [Work supported by CNM.]
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Electroacoustic sensitivity of ceramic cylinders (A)

W. D. Wilder

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S72-S72 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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The low‐frequency voltage/pressure sensitivity of piezoelectric right circular cylinders is derived for three common polarization schemes. The acoustic pressure on the three major surfaces is considered variable, thereby allowing for any configuration of shielding. Derived sensitivities are illustrated for a common piezoelectric ceramic material (Clevite PZT4).
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Acoustical compound reflex resonator system (A)

G. J. Sebesta, A. Hofer, and R. W. Carlisle

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S72-S72 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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A miniature paging‐tone loudspeaker is described. The reproducing range is centered around 2 kHz and the size is 18‐mm cube (just over ⅝ in.). The structure utilizes a conventional button earphone, loaded acoustically by a structure resembling a folded horn having two traverses. There is a first‐traverse chamber, a second‐traverse chamber which functions as a conical frustum horn, and suitable orifices coupling the chambers. The analysis requires the use of both resonator and conical horn theory. Mass (inertance) and compliance elements are calculated relative to a diaphragm of definite area, in both MKS and cgs units. The ratio of impedance in the two systems is shown to be exactly 10, demonstrating the feasibility of facilitated transformation. The nickname is “Beeper Cube” and current applications are concerned with paging intercoms for medical or service personnel.
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High‐sensitivity acousto‐optic system (A)

James H. Cole, R. L. Johnson, and P. G. Bhuta

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S72-S72 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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A Raman‐Nath imaging system was operated over a frequency range of 50 kHz to 10 MHz with an experimental sensitivity of −137 dB re 1μ Pa (approximately 10−16 W/cm2) at 500 kHz. An optical heterodyne technique was employed to obtain these sensitivities. Theoretical calculations project that additional laser power can provide sensitivities of approximately −184 dB re 1μ Pa (3 × 10−21 W/cm2) at 500 kHz. Improved resolution measurements as well as the results of the utilization of acousto‐optics to analyze specular acoustic reflections from thin metal plates in water is presented. For several materials tested there is strong agreement between theory and experiment in the 1–3 MHz frequency range. [Work supported by DARPA.]
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Optical fiber hydrophone (A)

J. A. Bucaro, E. F. Carome, and M. R. Layton

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S72-S72 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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Recent studies [J.A. Bucaro, H.D. Dardy, and E.F. Carome, Appl. Opt. 16, 1761 (1977); J.H. Cole, R.L. Johnson, and P.G. Bhuta, J. Acoust. Soc. Am. to be published] have shown the viability of obtaining sensitive, versatile acoustic sensors utilizing long, low‐loss optical fibers. In such devices an optical beam propagating in a length of optical fiber which is immersed in a fluid is modulated by the presence of a sound field in that fluid. We have studied such effects in a variety of fiber systems and have quantified various coupling mechanisms which lead to the optical modulation. Examples include phase retardation, acoustically induced birefringence, and intermode beating effects. These results will be discussed and future directions will be indicated. [This work has been supported in part by the Office of Naval Research.]
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Optical hydrophone (A)

P. Shajenko and J. P. Flatley

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S72-S72 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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Progress on optical hydrophone operating in conjunction with 2‐km‐long optical fiber is reported. The hydrophone is a pressure‐sensing light modulation device, where the Doppler frequency shift is multiplied by multireflections increasing the hydrophone's sensitivity. The light beam to and from the hydrophone is guided along optical fibers. Coherent detection is made by employing homodyne or heterodyne detection technique. Experimentally tested sensitivity and signal‐to‐noise ratio of the optical hydrophone are comparable to the piezoelectric hydrophone. Using a multiple‐interference technique, successful operation of the optical hydrophone with 2‐km‐long multimode optical fiber was obtained. Test results are presented. [Work supported by CNM and NAVSEA.]
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Operation of an ultrasonic flowmeter on a sodium loop of a fast breeder reactor (A)

H. B. Karplus and G. A. Forster

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S72-S72 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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The flow of fluid in a pipe is determined from the upstream and downstream flight times of ultrasonic (4 MHz) pulses traveling obliquely across the pipe. Temperature effects on dimensions, sonic velocity, refraction angles, etc., are compensated from the measured mean‐pulse travel times. This mean‐pulse travel time also yields mean fluid temperature. This permits a pair of instruments on the hot and cold leg of a heat transfer loop to give total power flow. Total time delays across the pipe were measured over the temperature range 350°–70 °C and compared with values computed based on expansion coefficient and elastic modulus data from the Nuclear Materials Handbook. Ultrasonically measured flow rates were compared with the installed electromagnetic and hydrodynamic flowmeters as well as with a special neutron activation flow calibration technique. The transducers for the ultrasonic system were specially designed to be clamped to the outside of the hot pipe using gold foil between optically flat surfaces with a clamping force of 2000 N.
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