David C. Greene

Comments on “Perception of the Range of a Sound Source of Unknown Strength” [H. R. Hirsch, J. Acoust. Soc. Am. 43, 373–374 (1968)]

The relative error to be expected in human range perception is quite large, even under good conditions, and should be considered in designing the experiment suggested by Hirsch in his Letter to the Editor entitled “Perception of the Range of a Sound Source of Unknown Strength” [J. Acoust. Soc. Am. 43 373–384 (1968)]

Vibration and Sound Radiation of Damped and Undamped Flat Plates

Because of the many resonances, plates have an extremely small driving‐point impedance, add accurate measurements are difficult to perform. Some of the fundamental difficulties that accompany such measurements are illustrated by experimental results. Measurements performed with thin, rectangular plates show that the asymptotic laws of vibration and sound radiation predict, within the range of experimental error, the characteristic impedance and the sound radiation of plates. The theoretical prediction that damping would have no appreciable effect on either the characteristic impedance or the background level of the radiated sound pressure was found to be accurate.

Use of Acoustical Holography for the Imaging of Sources of Radiated Acoustic Energy

It is experimentally demonstrated that scanned‐receiver acoustical holography is useful for mapping or “imaging” those parts of a complex vibrator responsible for the energy radiated to the far field.

Method for study of range‐dependent time‐differences‐of‐arrival between underwater sound eigenrays

PASCAL language computer programs are described, which allow the calculation of eigenrays between an underwater source and receiver and the study of the range‐dependence of time‐differences‐of‐arrival (TDOAs) between eigenrays. The method uses a sound velocity profile from a disk file and user‐supplied source and receiver depths. First, characteristics of rays which graze the surface and the bottom are calculated and presented to the user. Using this information, the user estimates eigenray angles. The program refines the users estimate until the range error of the eigenray is smaller than a threshold value (typically 0.1 ft). Time of arrival for the eigenray is noted and the process is repeated for a set of range values. The process is repeated for other eigenrays. The range histories of arrival times of the eigenrays are used to calculate the range‐dependent TDOAs. Using an 8‐bit, 4‐MHz microcomputer, less than 10 s are required to refine an estimate to an eigenray determination. Calculation of TDOA‐ve...

A PASCAL language facility for the design and analysis of acoustic line arrays

A PASCAL language facility for the design and analysis of acoustic line arrays is described and sample outputs of the facility are presented. The facility consists of two types of programs. The first type is used for designing arrays (e.g., generating element positions and shading coefficients) and the second type is used for analyzing the performance (e.g., array gain, side‐lobe levels) of the designs generated. Flexibility is maintained by leaving the programs intact and introducing all changes for new cases via input textfiles. Outputs available are automatic plots of universal beam pattern functions, polar directivity patterns, directivity index as a function of beam steer angle or frequency and array gain as a function of beam steer angle in a directional noise field.

Use of Acoustical Holography for the Imaging of the Sources of Radiated Sound on a Complex Vibrator

A demonstration has been made of the application of acoustical holography to the imaging of the sources of radiated sound on a complex vibrator. Demonstration was made in a small water tank using a submerged glass disk as a test object. The disk was driven to flexural vibrations by a force of frequency 2.5 MHz, applied at a point. The radiated sound field was sampled by a mechanically scanned probe hydrophone, the output of which was compared with a synthetic reference “beam” to form a hologram. Optical reconstruction of a hologram provides a picture of the (acoustically) self‐luminous disk. Those parts of the disk responsible for the radiated sound may be readily identified and assigned a semiquantitative share of the responsibility for the radiated energy. The principles demonstrated are applicable at lower frequencies and in other media.

Measurements of the Sound Radiated by a Large Glass Plate

A series of measurements was made of the sound radiated by a large glass plate which forms part of the wall of the air‐wall research building of The Pennsylvania State University. For most of the measurements, the plate was excited to flexual vibrations by a point force applied at its center. Measurements included the frequency dependence of the radiated sound level, the directional characteristics of the radiated sound, and the noise‐transmission properties of the compound glass walls of the building. Sound‐radiation efficiencies of the glass plate are determined at several frequencies in the audio range. The experimental results are compared with those predicted by theory.

Vibration of a Plate at Low Frequencies

The normal mode method of analysis correctly predicted the high‐frequency driving‐point impedance and background sound radiation of point‐excited flat plates for both damped and undamped conditions. Evidence now has been obtained that the same methods also correctly predict the behavior of the plate in the lower frequency range, wherein the resonances are well separated. Effects resulting from the position of the driving point are examined theoretically and experimentally.

Vibration and Sound Radiation of Plates

A plate, suspended on thin wires, has been vibrated by an electromagnetic vibration generator. From this experiment, these measurements were obtained: the amplitude distribution; the driving‐point velocity as a function of frequency; and the sound pressure radiated as a function of frequency. The measurements were repeated after the plate had been heavily damped with an asphaltic coating. The velocity was found to decrease with distance from the driven point. The rate of decrease became greater when either the damping or the frequency was increased. The driving‐point impedance and the radiated sound pressure proved to be practically independent of the frequency and of the damping of the plate. These results are in agreement with the theory.

Balanced Hydrophone with Isolated Shield

A balanced hydrophone intended for use at frequencies below 1000 cps was designed, constructed and tested. The design is believed to be novel in that the shield surrounding the sensitive elements is isolated electrically from the hydrophone case and from the water. The hydrophone has an open‐circuit sensitivity of approximately −75 db reference 1 v per μbar, a source capacity of 530 μμf, and no directionality below 1000 cps. It may be used at depths to 500 ft. The same principles may be used in designing hydrophones for higher frequencies.