摘要:

Underwater sound transmission has been studied as a function of range at frequencies of 0.2, 0.6, 1.8, 7.5, and 22.5 kc, under a variety of thermal conditions. At the three lower frequencies, marked interference patterns are observed. These result from the combination of surface reflected sound (as from an image source) and directly transmitted sound. Refraction modifies the interference pattern. When the sound velocity increases with increasing depth, the interference pattern is contracted; when the velocity increases with depth, the pattern is extended. An easily used formula is developed to predict this influence of refraction on the propagation of sound emitted from a point source adjacent to a reflecting plane. The theory is in good agreement with the experimental data obtained with the source at a depth of 14 feet from the surface and the receiver as deep as 300 feet.

ISSN:0001-4966    

DOI:10.1121/1.1916395

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1916396

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

This paper presents data on the properties of a very simple, low power underwater noise generator—a wire heated by a heavy electric current. The dependence of the sound generated upon frequency, input power, wire size, material, and wire history has been determined. The sound has a very broad frequency distribution. For small wires a large part of the sound is supersonic. About one part in a million of the input electrical energy appears as sound. With increasing electric power the sound power passes through a broad maximum. This paper can also be considered as a study of cavitation noise, the cavities being produced by heat rather than by reduced pressure. Thus a new approach to the problem of cavitation noise is provided. As a study in heat flow, this report provides quantitative data on the acoustical concomitants of boiling.

ISSN:0001-4966    

DOI:10.1121/1.1916397

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

The properties of an electrically heated wire in various liquids have been investigated in an effort to explain qualitatively the properties of the hot‐wire sound generator of the preceding paper. The sound from the hot wire is primarily a cavitation noise—its properties can all be ascribed to the continuous formation and collapse of vapor cavities. Many of the phenomena of cavitation, including corrosion, have a counterpart in boiling. It is shown that under certain conditions the hot wire has two stable forms of operation for the same input power. In one the wire is relatively cold, and the sound emitted has a broad frequency distribution. In the other the wire is red hot, and for some liquids the sound emitted is a single pure tone a whistle of 1000–2000 cycles/sec. Swishing the wire through the liquid increases the intensity of this whistle.

ISSN:0001-4966    

DOI:10.1121/1.1916398

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

Several observers have found that the absorption coefficient for supersonic radiation in water is inversely proportional to the square of the frequency, in the range 3 to 80 megacycles. This agrees with classical theory except in the size of the proportionality constant, but when extrapolated to lower frequencies disagrees sharply with earlier measurements which suggested a strong absorption peak near one megacycle. An experiment is described here which employed a small microphone detector and attempted to measure absorption in a divergent supersonic beam by comparison of the measured and theoretical acoustic pressure curves along the beam axis. The results, between 1.09 and 1.30 mc, are negative in that the computed absorption coefficients fall below the experimental error. However, a fairly reliable upper limit can be set on the coefficient. This limit is consistent with the value predicted by extrapolating the frequency‐squared relationship down to one megacycle. There seems little chance of a strong absorption peak in the near neighborhood of one megacycle.

ISSN:0001-4966    

DOI:10.1121/1.1916399

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

A line source underwater transducer may be used in conjunction with a parabolic cylinder reflector to produce a highly directive sound ray similar to that obtained from a shaded plane source. Poor patterns result from the construction of a unit where the element is at the focus of a parabolic cylinder reflector because the finite size of the element is so great as to block a large proportion of the reflected sound rays. To circumvent this difficulty an ideal surface is derived which will allow placing the line source out of the direct path of the reflected rays. This derivation and the construction and acoustic characteristic of the consequent transducer are discussed.

ISSN:0001-4966    

DOI:10.1121/1.1916400

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

An analysis is made of the problem of intelligence pick‐up from anatomical vibration. Methods for measuring the actual and effective response‐frequency characteristics of the throat are described. Measurements on the variation of effective response among different subjects are discussed in connection with the concept of “oral efficiency.” Design and performance data are given for several magnetic throat microphones of relatively high sensitivity.

ISSN:0001-4966    

DOI:10.1121/1.1916401

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

In Part I of this paper, it was shown that the reciprocity theorem for passive linear electroacoustic transducers could be established on the basis of a pair of general linear relations connecting the voltage and current at the electrical terminals of the transducer with the pressure and velocity distribution over the transducer surface provided certain “reciprocity relations” existed among the parameters entering these equations. In Part II it is now shown that on considering a transducer to be comprised of media characterized in the usual manner by appropriate linear constitutive relations between stress, strain, electric and magnetic polarization, charge and current density, and electric and magnetic field intensity, one can establish the validity of the linear relations and the “reciprocity relations” assumed in Part I, provided that certain sufficient conditions are met. These conditions are: (l) That the coefficients in the constitutive relations satisfy certain “symmetry conditions.” (2) That no magnetostrictive media and no static magnetic field are present in the transducer (that is, that the coupling is purely electrostatic or piezoelectric or both), or that no piezoelectric media and no static charge density are present in the transducer (that is, that the coupling is purely electromagnetic or magnetostrictive or both). (3) That the transducer does not radiate electromagnetic waves from its surface.The validity of the “symmetry conditions” on the coefficients of the constitutive relations is established for frequencies sufficiently low that the harmonic changes in the variables can be regarded as adiabatic in the thermodynamic sense and under certain circumstances at higher frequencies by means of a general argument based on a principle of microscopic reversibility. One may, however, anticipate the possible breakdown of these conditions when appreciable losses due to “relaxation” phenomena in the transducer media are present. The conditions for the validity of the reciprocity theorem derived in this paper are only sufficient, but they may be considerably extended in their generality to include the presence of other phenomena (such as electrostrictive effects) by the application of the same general methods.

ISSN:0001-4966    

DOI:10.1121/1.1916402

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

The “front‐to‐back discrimination” represents the ability of an underwater hydrophone to favor reception of a desired signal originating in front of the hydrophone by discriminating against a usually undesired signal originating behind the hydrophone. Under normal conditions in a confined measurement area, the value of the front‐to‐back discrimination is influenced to a considerable extent by such measurement factors as conditions of the medium, the directionality characteristics of the projector, and the distance separating the projector and the tested hydrophone. It is reported that reverberation appears to mask the true back‐side response of the hydrophone, and that thermal gradients and possibly unusual natural effects oftentimes limit the validity of some underwater measurements. Data are presented to show that a hydrophone measured with highly directional projectors exhibits a higher front‐to‐back discrimination than if measured with non‐directional projectors. It is also shown that the front‐to‐back discrimination appears to decrease with increasing distance between the projector and the tested hydrophone. These effects are attributed to the absence of a strictly free‐field in a bounded volume of water.

ISSN:0001-4966    

DOI:10.1121/1.1916403

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

Methods of acoustical measurement and of data presentation are reviewed, with emphasis on relating headphone data to the requirements of the communications and monitoring fields of application. Data are shown to demonstrate the importance of statistical analyses of the performance of headphones on different wearers. Some effects of presenting too “artificial” an acoustical load to the receiver during measurement are illustrated. An experiment in artificial ear development is described. The requirements for an improved artificial ear are outlined, based in part upon the experimental work.

ISSN:0001-4966    

DOI:10.1121/1.1916404

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP