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Journal of the Acoustical Society of America

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Oct 1946

Volume 18, Issue 2, pp. 257-504

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Jungle Acoustics

Carl F. Eyring

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 257-270 (1946); (14 pages) | Cited 3 times

Online Publication Date: 17 Jun 2005

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The study of jungle acoustics was carried out during the wet season in Panama. Measurements permit the following conclusions to be drawn: Within a jungle the temperature and wind velocity gradients are so small that the sound refraction they produce may be neglected for all practical purposes. Humidity increases the transmission loss at high frequencies and field measurements of the loss agree with laboratory values reported by others. Terrain loss, measured in db, between any two specified distances from the sound source is defined as the transmission loss between these points less that caused by the geometrical divergence of the sound beam. Terrain loss in the jungle was found to increase linearly with distance. The terrain loss coefficients, measured in db per foot, were measured for various types of jungle and were found to be a function of frequency and of the density of the terrain, the density of terrain being measured by the difficulty of penetration and the distance a foreign object may be seen. The level of the ambient noise in the wet season jungle is very low especially for the quiet periods between animal calls. At night the low frequencies decrease as the light breezes cease and the high frequencies increase as the insects begin their nocturnal chorus. A jungle is a difficult place in which to judge the direction of a sound—a probable error of 20° is to be expected. The error is found to be smallest when the sound comes from a direction near the axis passing through the two ears, and in the range studied the error decreases as the sound source moves farther away. Reverberation and scattering are the cause of part of the error of judgment, but an improved technique of listening suggested may increase the observer's accuracy.

Ultrasonic Ambient Noise in Tropical Jungles

John S. Saby and Howard A. Thorpe

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 271-273 (1946); (3 pages)

Online Publication Date: 17 Jun 2005

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Reflection of Sound Signals in the Troposphere

G. W. Gilman, H. B. Coxhead, and F. H. Willis

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 274-283 (1946); (10 pages)

Online Publication Date: 17 Jun 2005

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Experiments directed toward the detection of non‐homogeneities in the first few hundred feet of the atmosphere were carried out with a low power sonic “radar.” The device has been named the sodar. Trains of audiofrequency sound waves were launched vertically upward from the ground, and echoes of sufficient magnitude to be displayed on an oscilloscope were found. Strong displays tended to accompany strong temperature inversions. During these periods, transmission on a microwave radio path along which the sodar was located tended to be disturbed by fading. In addition, relatively strong echoes were received when the atmosphere was in a state of considerable turbulence. There was a well‐defined fine‐weather diurnal characteristic. The strength of the echoes was such as to lead to the conclusion that a more complicated distribution of boundaries than those measured by ordinary meteorological methods is required in the physical picture of the lower troposphere.

Theory of the Propagation of Sound in Scattering and Absorbing Media

M. P. Givens, W. L. Nyborg, and H. K. Schilling

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 284-295 (1946); (12 pages)

Online Publication Date: 17 Jun 2005

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A theory for the transmission of sound waves through a medium which is both scattering and absorbing is presented. The theory predicts that in such a medium curves of physical transmission loss against distance should be convex upward if the sound is detected by means of a directional microphone directed toward the source. The curve should be concave upward if the microphone is directed perpendicular to a line from the source. The theory also predicts the directionality of sound in the medium. The theory is applied to transmission through forests and through open air containing inhomogeneities.

Theory of Propagation of Sound in a Half‐Space of Variable Sound Velocity under Conditions of Formation of a Shadow Zone

C. L. Pekeris

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 295-315 (1946); (21 pages) | Cited 10 times

Online Publication Date: 17 Jun 2005

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This paper treats the propagation of sound from a point source situated at some depth below the surface of a half‐space in which the sound velocity decreases with depth. There is then a “limiting ray” which forms the boundary of a “shadow zone” into which no rays penetrate. Our chief interest is in the intensity of the diffracted radiation that penetrates into the shadow zone and its dependence on distance from source and on frequency. In Part I a detailed study is made of propagation of sound in a medium in which sound velocity decreases at a constant rate a with depth. It is found that inside the shadow zone the intensity I of monochromatic sound of frequency f falls off essentially exponentially with distance from the shadow boundary:
math
, (A) where B denotes the point on the shadow boundary at the depth of the point of observation P, R0, and R the ranges of B and P, r = R−R0, and c the sound velocity. The intensity on the shadow boundary I(B) decreases with decreasing frequency on account of interference between the source and its image. As a result there exists at each point inside the shadow zone a frequency of optimum penetration. The solution inside the shadow zone is given in terms of normal modes. It is shown that in case of explosive sound the travel time for a point in the shadow zone is such as to indicate that part of the trajectory is covered near the surface with the surficial sound velocity. Details are given for computing the pressure‐time curve in case of an exponentially decaying pulse at the source. In order to study the effect of a variable sound velocity gradient with depth, an investigation is made in Part II, using asymptotic methods, of propagation of sound in media in which c = c0/(1+αz+βz2)½. It is found that Eq. (A) is still valid for extremely high frequencies, where a now denotes the surficial velocity gradient. For moderate and low frequencies the decrement may be less or greater than that given by Eq. (A), depending on whether the velocity gradient decreases or increases with depth. In general, for high frequencies the decrement is largely determined by the surficial velocity gradient, while for low frequencies it is increasingly dependent on the velocity distribution in the deep layers. The general theory of propagation of sound in media with arbitrary vertical variation of sound velocity is developed from the stand‐point of normal modes and asymptotic solutions. It is emphasized that for certain media the normal mode solution needs to be supplemented by branch‐line integrals.

Ray Computation for Non‐Uniform Fields

John S. Saby and Wesley L. Nyborg

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 316-322 (1946); (7 pages)

Online Publication Date: 17 Jun 2005

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A formula is derived which simplifies the computation and tracing of rays in non‐uniform sound fields. In applying it, a field is first assumed to be equivalent to an array of horizontally homogeneous strata each with a uniform gradient of the speed of sound. Then the “range” of the ray through the whole array can be computed by the formula in one step, rather than several, to a degree of approximation considered adequate for many experimental situations. Special problems encountered in applying the formula under certain types of conditions are discussed. The method is particularly useful in the study of the propagation of ultrasonic waves through the atmosphere near the ground, where micrometeorological conditions often produce rather complicated gradient conditions.

The Propagation of Sound in an Inhomogeneous and Moving Medium I

D. Blokhintzev

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 322-328 (1946); (7 pages) | Cited 12 times

Online Publication Date: 17 Jun 2005

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In this paper the wave equations for the propagation of sound in an inhomogeneous and moving medium are established (Section I). In Section II special cases are considered and a generalization of Kirchhoff's theorem (Huygens' principle) is given for a moving medium. In Section III the general equations of acoustics are considered in the approximation of geometrical acoustics. And finally, in Section IV the equations are generalized for the case of a medium containing a salt solution (sea water).

The Propagation of Sound in an Inhomogeneous and Moving Medium II

D. Blokhintzev

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 329-334 (1946); (6 pages) | Cited 1 time

Online Publication Date: 17 Jun 2005

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In the present paper several applications of the theory developed in Part I are set forth. Section I deals with the propagation of sound in a turbulent medium, Section II with its propagation through a shock wave.

Propagation of Plane Sound Waves in Rarefied Gases

Hsue‐Shen Tsien and Richard Schamberg

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 334-341 (1946); (8 pages) | Cited 1 time

Online Publication Date: 17 Jun 2005

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If the density of gas is very small, the conventional Navier‐Stokes equations are not accurate enough. The present investigation includes the effect of the so‐called third approximation to the solution of Boltzmann‐Maxwell equation as obtained by D. Burnett. The results of this more accurate calculation show that, even under extreme conditions, the velocity of propagation deviates from its usual value by only 2 percent.

On the Absolute Pressure Calibration of Condenser Microphones by the Reciprocity Method

A. L. DiMattia and F. M. Wiener

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 341-344 (1946); (4 pages) | Cited 2 times

Online Publication Date: 17 Jun 2005

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Through cooperation among the Bell Telephone Laboratories, the National Bureau of Standards, and the Electro‐Acoustic Laboratory at Harvard University, the technique of obtaining absolute pressure calibrations of condenser microphones by the reciprocity method has been perfected and standardized to a gratifying degree of accuracy. Numerical data are given for a Western Electric Type 640‐AA instrument which was calibrated at the three laboratories in succession, showing the accuracy to be expected at the present state of the art.

Violation of the Reciprocity Theorem in Linear Passive Electromechanical Systems

Edwin M. McMillan

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 344-347 (1946); (4 pages) | Cited 3 times

Online Publication Date: 17 Jun 2005

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A simple energy argument is used to show that the theorem of reciprocity is not necessarily valid for linear passive electromechanical systems. It is found that in the case of crystal or electrostatic transducers the theorem is satisfied, while in the case of magnetic or electrodynamic transducers it is satisfied in magnitude but not in sign. It is further shown that by combining the two types of transducers, systems can be constructed that violate reciprocity in magnitude.

Development of Midget Earphones for Military Use

H. A. Pearson, A. B. Mundel, R. W. Carlisle, W. F. Knauert, and M. E. Zaret

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 348-354 (1946); (7 pages)

Online Publication Date: 17 Jun 2005

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In order to meet the military requirements for a headset that could be worn under the MK‐1 helmet, midget hearing aid receivers were redesigned for this application. Data are presented showing characteristics of the original design adopted and of subsequent improvements resulting in improved performance. Some of the methods for securing adequate performance in quantity production and methods for 100 percent inspection of frequency characteristics are described.

The Directional Characteristics of a Free‐Edge Disk Mounted in a Flat Baffle or in a Parabolic Horn

Frank H. Slaymaker, Willard F. Meeker, and Lynn L. Merrill

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 355-370 (1946); (16 pages)

Online Publication Date: 17 Jun 2005

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In the attempt to obtain a sharp beam of supersonic sound from a parabolic horn, it became apparent that the sharpness of the beam was a function of the mode of vibration of the diaphragm exciting the horn. Experiments showed that the sharpest beams with the smallest side lobes were obtained when the diaphragm was radiating most of the energy toward the side walls of the horn rather than toward the mouth. Experiments with a free‐edge disk for a diaphragm related the wide‐angle radiation to the two‐nodal‐circle mode of vibration of the disk. Comparison between directional patterns measured with the disk in a flat baffle and the corresponding calculated patterns confirmed the correlation between the wide‐angle radiation and the two‐nodal‐circle mode. Further analysis indicated that similar radiation patterns could also be obtained from one‐ and three‐nodal‐circle modes of vibration. In the analysis, the differential equation for the vibration of a thin plate is solved for a free‐edge circular disk assuming circular symmetry and considering a driving force applied at an arbitrary circle concentric with the disk. The dynamic curve of the disk is given for any frequency in the form of a series of the normal modes of the disk, and the directional pattern of the radiation from the disk mounted in an infinite baffle is calculated. The calculations have been so systematized and tabulated that the dynamic curve and directional pattern for any disk and frequency can be calculated quite readily. The analysis is sufficient to predict the disk diameter and thickness required for the one‐, two‐, or three‐nodal‐circle modes for any frequency, when the disk is mounted in a flat baffle and when the maximum radiation is placed at an arbitrary angle from the axis. On the basis of the analytical predictions, a disk has been made to give the required wide‐angle radiation when vibrating with one nodal circle instead of with two nodal circles as discovered experimentally.

A Fifty Horsepower Siren

R. Clark Jones

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 371-387 (1946); (17 pages)

Online Publication Date: 17 Jun 2005

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This paper contains a description of the theory, construction, and testing of a siren with an acoustic output of fifty horsepower, at an efficiency of seventy percent. The siren is of the traditional type in which the air stream from a compressor passes through an orifice which is opened and closed at an audiofrequency. Part I contains a development of the theory on which the design was based. The theory is developed with the aid of an equivalent circuit diagram (Fig. 1) in which the various acoustic impedances are represented as lumped circuit elements. The theory is for the most part based on the customary linear equations for sound propagation, but the non‐linear nature of the impedance of the port is taken into account. Mere inspection of the equivalent circuit diagram is sufficient for the understanding of the considerations which are important for the obtaining of a high efficiency. According to this theory the best efficiency can be realized in a siren when the orifice is provided with a suitable acoustic horn, when the excess pressure of the air supplied by the compressor is small compared with the absolute pressure of the atmosphere, and when the operations of opening and closing the port occupy a small fraction of the period of one cycle. Expressions are obtained for the theoretical efficiencies of several types of sirens, and are found to be near one hundred percent for a good design. Part II contains a description of the siren which was constructed to operate with a compressor that supplied 2500 cubic feet of air per minute at a pressure of five pounds per square inch. The siren contains six ports with a total area of 22 square inches, which are opened and closed by a rotary chopper at a frequency of about 500 cycles per second. Each of the ports is provided with an exponential horn whose cut‐off frequency is 125 cycles per second. The theoretical efficiency of this siren is computed to be between 70 and 90 percent. Part III is devoted to a description of the testing of the siren and a discussion of the results obtained. The tests indicated that at full output the siren produces an average level of 138 db at a distance of 100 feet. The measured level at the mouth of the horns is 174 db, and the calculated level in the throat of the horns is 184 db above 10−16 watt per square centimeter. The level in the throat of the horns corresponds to a sound intensity of 260 watts per square centimeter. The most reliable method of determining the efficiency, based on thermodynamic principles, indicates that the efficiency of the utilization of the available energy in the air from the compressor is about 72 percent.

Measurement of Recording Characteristics by Means of Light Patterns

B. B. Bauer

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 387-395 (1946); (9 pages)

Online Publication Date: 17 Jun 2005

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See Also: Erratum

Abstract Unavailable

Analyses of the Tones of a Few Wind Instruments

F. A. Saunders

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 395-401 (1946); (7 pages)

Online Publication Date: 17 Jun 2005

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Abstract Unavailable

The Pressure Distribution in the Auditory Canal in a Progressive Sound Field

Francis M. Wiener and Douglas A. Ross

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 401-408 (1946); (8 pages) | Cited 12 times

Online Publication Date: 17 Jun 2005

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The variation of the sound pressure along the auditory canal was determined experimentally on a number of subjects, male and female, placed in a progressive sound field. This was accomplished by insertion of a small flexible probe microphone at various positions along the auditory canal. The subjects were placed in front of a loudspeaker in a room free from acoustic wall reflections. The free sound field at the subjects' location was essentially that of a plane progressive wave. The measurements were carried out over the significant range of audiofrequencies for various orientations in azimuth of the subjects with respect to the sound source. The sound pressure at the eardrum is found to be greater than the free‐field pressure. The average ratio of these two quantities is a function of frequency, and reaches values of about 20 db in the vicinity of 3000 c.p.s. The human ear is thus an effective acoustic “amplifier.” The increase in sound pressure at the eardrum over the free‐field pressure is caused by a combination effect of diffraction by the head and pinna and resonance in the auditory canal. The measurements of the sound pressures at several other positions along the auditory canal serve to separate these two phenomena to a certain extent and to furnish additional information about the pressure distribution. Most of the data were obtained with a group of male subjects, but measurements on a few women did not show any marked discrepancies.

Physics and Physiology of Acoustic Trauma

L. Rüedi and W. Furrer

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 409-412 (1946); (4 pages)

Online Publication Date: 17 Jun 2005

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Acoustic strains that are physically quite different always produce similar functional damage to the ear. The well‐known so‐called “4000 c/sec. dip” is based on a functional peculiarity of the cochlea. There ensues the possibility of constructing a universally efficient ear defender which does not essentially impair the auditory capacity.

Effects of Ear Protective Devices on the Intelligibility of Speech in Noise

K. D. Kryter

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 413-417 (1946); (5 pages) | Cited 4 times

Online Publication Date: 17 Jun 2005

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Articulation tests were conducted to determine the intelligibility of speech in the presence of noise when listeners did not wear earplugs and when listeners wore earplugs (NDRC Ear Wardens). It was found that with a reverberating signal from a public‐address system and in the presence of noise that raises the open‐ear speech threshold by 60 db or more, the wearing of Ear Wardens increases the intelligibility of speech. But, with direct person‐to‐person speech, the ambient noise must be of sufficient intensity to raise the speech threshold by 80 db or more before Ear Wardens may be used without interfering with the reception of speech. Since in some military and industrial situations noise is generated that raises the threshold for hearing speech by more than 80 db, the use of suitable earplugs, under those conditions will: (a) maximize the reception of speech and (b) afford protection against the deafening, fatigue, and annoyance effects commonly attributed to sustained intense noise.

The Masking of Speech by Sine Waves, Square Waves, and Regular and Modulated Pulses

S. S. Stevens, Joseph Miller, and Ida Truscott

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 418-424 (1946); (7 pages) | Cited 2 times

Online Publication Date: 17 Jun 2005

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The ability of three standard wave forms to mask speech, by raising its threshold of perceptibility, was measured as a function of the intensity and frequency of the masking signal. Depending on the other parameters of the interfering signal, optimal masking is produced when the fundamental frequency lies in the range 100 to 500 c.p.s. Intense sine waves mask best when their frequency is about 300 c.p.s., whereas for weaker sine waves the most effective frequency is about 500 c.p.s. Square waves are less critical as to frequency and intensity; fundamental frequencies between about 80 and 400 c.p.s. mask with approximately equal effectiveness. Pulses of 10‐microsecond duration are most effective at a prf of about 200 p.p.s. When the time interval between successive pulses is made irregular (random interval‐modulation), their ability to mask speech is dramatically increased. The increase in masking is a function of the original prf, the rate of modulation (determined by the spectrum of the modulating voltage), and the range of modulation. By a proper adjustment of the parameters of its random interval‐modulation the train of pulses can be made to sound like a band of white noise.

Ultrasonic Absorption in Copper Acetate and Ethyl Acetate

Robert T. Beyer and Myron C. Smith

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 424-428 (1946); (5 pages)

Online Publication Date: 17 Jun 2005

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A study is made of ultrasonic acoustic absorption in ethyl acetate and in various concentrations of an aqueous solution of copper acetate. The balance method has been used, in which the radiation pressure on a detector suspended in the sound field is measured by a sensitive balance. In neither case is the theoretical variation of the absorption coefficient with the square of the frequency observed. The relation is more nearly linear over the frequency range 5–20 megacycles. In the case of copper acetate, the absorption increases rapidly with the concentration for very dilute solutions. No transition is observed from an abnormal absorption coefficient at low frequencies to the theoretical value at higher frequencies.

Effects of Amplitude Distortion upon the Intelligibility of Speech

J. C. R. Licklider

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 429-434 (1946); (6 pages) | Cited 4 times

Online Publication Date: 17 Jun 2005

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Abstract Unavailable

On the Intelligibility of Bands of Speech in Noise

James P. Egan and Francis M. Wiener

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 435-441 (1946); (7 pages)

Online Publication Date: 17 Jun 2005

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Articulation tests were conducted with a large number of communication systems having band widths ranging from about one‐half octave to a system covering the entire range of speech frequencies. The systems were linear and their responses were approximately uniform over the pass band, with sharp cut‐offs at either end. The acoustic gain of the systems was expressed relative to the transmission of speech through one meter of air between talker and listener. Two spectra of masking noise were used, and each system was tested over a wide range of speech‐to‐noise ratios. In one group of experiments the speech was filtered before mixing with noise and in the other group both the speech and the noise were passed through the same filter. For each of the band‐pass systems a relation between syllable articulation and level of received speech was obtained. From these gain functions, families of equal‐articulation contours may be derived. These contours show, for example, how the gain must be changed for a given change in the band width of a system in order to maintain a constant articulation score.

Training for Voice Communication

John W. Black and Harry M. Mason

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 441-445 (1946); (5 pages)

Online Publication Date: 17 Jun 2005

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A training program was devised for improving the intelligibility of Army Air Force personnel in talking over the interphone and radio. The contents of the course, amount of training required, and the training requirements were determined experimentally. This paper summarizes the results of the over‐all program and compares them with the improvements that accompanied “partial” training, particularly in voice loudness and precision of articulation.

Underwater Noise Due to Marine Life

Donald P. Loye and Don A. Proudfoot

J. Acoust. Soc. Am. Volume 18, Issue 2, pp. 446-449 (1946); (4 pages)

Online Publication Date: 17 Jun 2005

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The widespread use of underwater acoustical devices during the recent war made it necessary to obtain precise information concerning ambient noise conditions in the sea. Investigations of this subject soon led to the discovery that fish and other marine life, hitherto generally classified with the voiceless giraffe in noisemaking ability, have long been given credit for a virtue they by no means always practice. Certain species, most notably the croaker and the snapping‐shrimp, are capable of producing noise which, in air, would compare favorably with that of a moderately busy boiler factory. This paper describes some of the experiments which traced these noises to their source and presents acoustical data on the character and magnitude of the disturbances.
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