verberation time and absorption characteristics of an underwater chamber with and without a pressure-release wall covering. The reverberation times were found to be faster than the transient response times of conventional analog equipment. Therefore a digital computer was employed to filter the data and to determine the slope, and thus the reverberation time by using a linear, least-squares curve fit. The pressure-release wall covering was found to reduce the average sound absorption coefficients to values acceptable for a reverberation chamber based on the standards for air acoustics.

Thesis Advisor: R. W. Farwell

The Prediction of Transducer Element Performance from In-Air

Measurements [43.30.Yj]--Mark Evan Schafer, Graduate Program in Acoustics, The Pennsylvania State University, University Park, PA 16802 (M.S.) A technique has been developed which accurately predicts the per- formance of underwater acoustic arrays prior to array construction. The technique is based upon the measurement of lumped-parameter equivalent circuit values for each element in the array, and is accurate in predicting the array transmit, receive, and beam pattern response. The measurement pro- cedure determines the shunt electrical and motional circuit elements from

electrical immittance measurements. The electromechanical transforma-

tion ratio is derived from in-air measurements of the radiating face velocity and the input current of the transducer at resonance. The equivalent circuit values of a group of Tonpilz-type transducers were measured, and the self and mutual interaction acoustic loadings for a specific array geometry were calculated. The response of the elements was then predicted for water-load- ed array conditions. Based on the predictions, a selection scheme was devel- oped which minimized the effects of interelement variability on array per- formance. The measured transmitting, receiving, and beam-pattern characteristics of a test array, built using the selected elements, were com- pared to prediction made before the array was built. The results indicated that the technique is accurate over a wide frequency range.

Thesis Advisor: W. J. Hughes

Experimental Method to Measure Low-Frequency Sound Radi- ation--Nearfield Acoustical Holography [43.60.Gk, 43.35.Sx, 43.7õ.De]--William Y. Strong, Jr., Graduate Program in Acoustics, The Pennsylvania State University, University Park, PA 16802 (M.S.) The tech- nique called Nearfield Acoustical Holography (NAH) has been developed at the Pennsylvania State University. The technique, a significant improve- ment over conventional holography, is not subject to the wavelength resolu- tion limitation associated with conventional techniques. An exact formula- tion of the Green's function propagator allows reconstructions of the entire pressure, particle velocity, and vector intensity fields. The vector intensity may be plotted to show the flow of acoustic energy around and away from a source. On some sources, tl•ese mappings reveal areas of nonradiating, cir- culating energy flow. These areas may have large pressure or velocity ampli- tudes and negative intensity regions that previously led to misconceptions about the production of acoustic energy. This work presents details of the development of the NAH system. Included are a review of the theory, de- tails of the experimental apparatus design, accuracy verification, and the use of the system to study a relatively simple source(a homogeneous rectan- gular plate) and a complex source (an intact, string excited guitar). The guitar research• revealed some interesting results such asthe rose is an impor- tant source of low-frequency energy; the motion of the back plate is very important; and similar plate modes occur at different frequencies due to a complex relationship among the strings, ribs, body, and neck.

Thesis Advisor: J. D. Maynard

Microphone Placement Factors for 1/2-1n.-Diam. Microphone [43.88.Kb]--John P. Seiler, University of Pittsburgh Graduate School of Public Health, Dept. of Industrial and Environmental Health Sciences, 5th Ave. and DeSoto St. Pittsburgh, PA 1526• 1982 (M.S.) A series of tests was conducted to compare the measurement of an undisturbed acoustical field with a «-in.-diam microphone to the measurement of the acoustical sound field with a •-in.-diam, shoulder-mounted, man-worn microphone. One- third octave band spectrum measurements were made ,and the positional center head microphone placement factor was determined for three shoul- der positions and five microphone orientations. The acoustical tests were performed in a reverberation room and in an anechoic chamber on five test subjects. The free-fieldmeasurements were conducted for azimuthal angles from 0ø-360 ø in increments of 45 ø . Averaged microphone placement factors are presentedfor a •-in.-diam microphone,in 1/3-octave bands, from 200 to 8000 Hz, for all test locations, microphone orientations, and acoustical environments. Where possible, comparisons are made between the results of this study and earlier work which determined the microphone placement factors for a 1-in.-diam microphone. A computation of the best microphone location/orientation was made for four defined types of noise spectra. This was done in order to predict the optimum position for the location ofa no;se dosimeter microphone under free and diffuse field conditions.

Thesis Advisors: Dr. Dv•ight Underhill, Dr. J. Alton Burke, and Mr. Den- nis A. Giardino

Copies of this thesis may be obtained from Ms. Nancy Goettman at the above address.

Laboratory Study of Sound Propagation into a Fast Bottom Medi- um [43.30.Bp]--James Arthur Bradshaw, Naval Postgraduate School, Monterey, CA, June 1981, 61 pp. (M.S.) An experimental study was per- formed to determine the feasibility of a laboratory experiment to test an existing theoretical model describing sound propagation into a fast bottom underlying a wedge-shaped medium. Sand under fresh water was found to satisfy the constraints of the theoretical model and to simulate the continen- tal shelf. In a laboratory experiment, accuracy of density, sound speeds, and attenuation were shown to be sufficient to allow quantitative comparison to the predictions of the beam angle.

Copies of this thesis are available from the National Technical Information Service, Springfield, VA 22151. [Order No. AD-A104596.]

Acoustic Boundary Wave Generation and Shadowing at a Sea- mount [43.30.Bp, 43.30.Pt]--Edgar Alvin Jordan, Naval Postgraduate School, Monterey, CA, June 1981, 97 pp. (M.S.) The interaction of a sound field and a seamount has been studied by physically modeling the ocean surface over two-dimensional and three-dimensional models of Dickens

Seamount. By using a smooth ocean surface, and one with a surface of scaled Rayleigh roughness to model a 35-kn wind, the relative contributions are determined for off-axis scattering elements, multiple reflection from the ocean surface, and diffraction over the crest of the seamount. Boundary wave generation over a randomly rough plane surface is studied experimen- tally. The ratio of boundary wave amplitude to volume wave amplitude is found to be proportional to (frequency) to the • power and (range to the « power, and the ratio of the empirical scattering parameter to the rms height of the roughness elements is found to be approximately 0.3. The spatial correlation length of the randomly rough surface takes the place of the center-to-center separation of the hemispherical bosses used in Tolstoy's theoretical treatment.

Copies of this thesis are available from the National Technical Information Service, Springfield, VA 22151. [Order No. AD-104594.]

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1334 J. Acoust. Soc. Am., Vol. 72, No. 4, October 1982 Notes and Briefs 1334

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