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Journal of the Acoustical Society of America

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Oct 1939

Volume 11, Issue 2, pp. 169-253


Remaking Speech

Homer Dudley

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 169-177 (1939); (9 pages) | Cited 30 times

Online Publication Date: 16 Jun 2005

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Speech has been remade automatically from a buzzer‐like tone and a hiss‐like noise corresponding to the cord‐tone and the breath‐tone of normal speech. Control of pitch and spectrum obtained from a talker's speech are applied to make the synthetic speech copy the original speech sufficiently for good intelligibility although the currents used in such controls contain only low syllabic frequencies of the order of 10 cycles per second as contrasted with frequencies of 100 to 3000 cycles in the remade speech. The isolation of these speech‐defining signals of pitch and spectrum makes it possible to reconstruct the speech to a wide variety of specifications. Striking demonstrations upon altering the pitch of the remade speech stress the contribution of the pitch to the emotional content of speech. Similarly the spectrum is shown to contribute most of the intelligibility to the speech.

Filtration of Oblique Elastic Waves in Stratified Media

R. B. Lindsay

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 178-183 (1939); (6 pages) | Cited 1 time

Online Publication Date: 16 Jun 2005

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Previous work has been done on the problem of the transmission of plane compressional elastic waves normally through a stratified medium consisting of alternate plane parallel layers of two different substances. Such a structure acts as a low‐pass elastic wave filter. The present paper extends the analysis to the case of oblique transmission. This proves to be comparatively simple when the layer substances are both fluid. The transmission and attenuation (i.e., reflection) bands are found to depend on the angle of obliquity in a characteristic way. When one of the layer substances is solid, the problem is more difficult. Account must here be taken of the fact that compressional waves in the fluid layers give rise to both dilatational and shear waves in the solid layers. The result is somewhat more complicated than in the previous case but the structure still turns out to be a low‐pass elastic wave filter for angles of obliquity less than the critical angle. As before the transmission and reflection bands are a function of the angle. In the special case in which the thickness of the solid layers is small compared with that of the fluid layers one obtains the elastic wave analog of the Bragg reflection law for x‐rays, that is, one gets reflection approximately for the wave‐lengths λn  =  2l/n ⋅ sin θ, where l is the distance between successive solid layers and n is integral.

Normal Modes of Vibration in Room Acoustics: Experimental Investigations in Nonrectangular Enclosures

Richard H. Bolt

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 184-197 (1939); (14 pages) | Cited 3 times

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Existing theories of room acoustics have long been recognized as incomplete. The inadequacy is particularly disturbing in the lack of agreement between absorption coefficients measured in different laboratories. Also, the acoustic behavior of small rooms cannot always be prescribed with certainty. Several investigators have shown the necessity of analyzing a room as an assemblage of normal modes of vibration in a three-dimensional continuum. The present paper reports preliminary studies of one aspect of the “wave” approach: the influence of boundary shape on normal modes of vibration. An experimental method reported previously, (reference 25) has been improved and further developed for exploring the distribution of sound in small reflective enclosures of various shape. Standing wave patterns have been plotted for the lowest four or five modes of vibration in each of three models: a parallelogram in plan, a trapezoid in plan, and a model with an “alcove” or coupled space. A number of normal frequencies have been measured in each model, and the nature of the frequency distributions has been studied. The results have been summarized with respect to their significance for room acoustics. In particular, it appears that rooms of irregular shape still possess discrete normal modes and standing wave patterns as pronounced as those which occur in simple shapes.

The Effect of Rotatory and Lateral Inertia on Flexural Vibration of Prismatic Bars

William T. Thomson

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 198-204 (1939); (7 pages)

Online Publication Date: 16 Jun 2005

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The problem of the flexural vibration of prismatic bars neglecting the rotatory and lateral inertia is treated in various texts on sound and vibrations. The effect of neglecting these two terms introduces no appreciable error when the cross section of the bar is small in comparison to its length. For bars of large cross section the error introduced in neglecting these terms is sufficient to warrant attention. In his text on sound Rayleigh offers an approximate solution introducing a correction term for the rotatory inertia, under certain boundary conditions. In a more recent paper Mason gives an exact solution for the frequency of vibration, where the rotatory and lateral inertia are taken into account. The disadvantage of this solution lies in the fact that his Eq. (14) for the eigenvalues of the frequency is extremely complicated, thereby throwing up a barrier to further analysis. Also no mention is made by previous investigators of the effect of the rotatory and lateral inertia on the node positions or the shape of the deflection curve. It is the purpose of this paper to present a simplified exact solution and to determine the effect of the two terms mentioned on the position of the nodes and the relative amplitude of vibration. Curves applicable to the solution of any type of constant cross section are included.

The Transmission of Sound Inside Pipes

Philip M. Morse

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 205-210 (1939); (6 pages) | Cited 8 times

Online Publication Date: 16 Jun 2005

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Methods developed in a previous paper are used to obtain an exact solution for the transmission of sound inside a rectangular pipe with absorbing material on its inner walls. Plots are given whereby it is possible to determine the phase velocity and attenuation of the simpler type waves in terms of the specific acoustic impedance of the walls and the frequency of the wave. Several examples are given of the effect of the absorbing surfaces on the distribution of the sound pressure amplitude over a plane perpendicular to the axis of the pipe. The effect of the reactive part of the material's acoustic impedance on the attenuation of the sound is also discussed. Formulas are developed for the transmission of sound in tubes of circular cross section. The plots for rectangular pipes can also be used in calculations of the acoustical properties of rectangular rooms.

Sound Insulation Characteristics for Ideal Partitions

Keron C. Morrical

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 211-215 (1939); (5 pages)

Online Publication Date: 16 Jun 2005

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A Design for a Keyboard Instrument in Just Intonation

Chas. Williamson

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 216-218 (1939); (3 pages)

Online Publication Date: 16 Jun 2005

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In a keyboard instrument, the just scale may be closely approximated by setting up two equally tempered scales, one on the basis of A440.0, the other on the basis of A436.3, and mechanically selecting from them those frequencies which most nearly satisfy the appropriate ratios. Within one octave above middle C, no scale‐step is in error by more than 0.6 vibration per second. The keyboard is exactly the same as that now used, and the instrument is playable in equal temperament when the performer so desires.

A Sound Source for Investigating Microphone Distortion

William D. Phelps

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 219-221 (1939); (3 pages)

Online Publication Date: 16 Jun 2005

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Microphone Efficiency: A Discussion and Proposed Definition

Frank Massa

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 222-224 (1939); (3 pages)

Online Publication Date: 16 Jun 2005

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This paper proposes a definition for microphone efficiency, together with a discussion for justifying the points of view presented. Up to the present time, the question of microphone efficiency has been generally ignored and in its place has grown the use of microphone sensitivity, which is the voltage output for a particular value of sound pressure actuating the unit. Although this latter quantity may be quite satisfactory for commercial use, the author feels that from a scientific point of view, we should be interested in the absolute ability of a microphone to absorb acoustic energy from a sound field and convert it into electrical energy: this ability he defines as efficiency. A relation for microphone efficiency is derived and a family of curves are computed which show the efficiency of a microphone as a function of its size, impedance, and sensitivity. One obvious conclusion that can be deduced from the paper is that only a tiny fraction of the acoustic energy intercepted by an average microphone is actually converted into electrical energy. This means that we are yet very far from the theoretical limit and, consequently, great possibilities still exist for fundamental research in the improvement of microphone efficiency.

The Degenerative Sound Analyzer

H. H. Scott

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 225-232 (1939); (8 pages) | Cited 1 time

Online Publication Date: 16 Jun 2005

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The Propagation of Shock Waves in Air. I

L. Thompson and N. Riffolt

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 233-244 (1939); (12 pages)

Online Publication Date: 16 Jun 2005

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From a consideration of conditions at the boundary of a source of shock in air, and of conditions at great distances, a formula of the Riemann type has been derived for the velocity of propagation of a finite pulse. The formula includes an additional constant which provides the necessary flexibility to represent data obtained throughout the fields of sources consisting of detonating charges of explosives, and of other sources. The observations of wave displacements have been summarized in terms of an integral for “reduced” times, from which the constants of the function for velocity are immediately available. All distances are defined with reference to an equivalent dimension of the source, and the characteristics throughout the velocity field are obtainable from the characteristics of the source. A table is given for the velocity of a condensation pulse, for representative boundary velocities, at various distances from the source out to points at which the velocity has decreased approximately to its asymptotic value (the normal velocity of sound). Results obtained by Wolff and Burlot for very large sources are shown for comparison on a plot of reduced times. Preliminary results obtained with piezo‐gauges of experiments to determine relative pressures at the head of the wave are given in comparison with theoretical gauge pressures obtained by formulas derived in Part II.

The Propagation of Shock Waves in Air. II

L. Thompson

J. Acoust. Soc. Am. Volume 11, Issue 2, pp. 245-253 (1939); (9 pages)

Online Publication Date: 16 Jun 2005

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Using the Rankine‐Hugoniot equation of condition and Hugoniot's velocity of a discontinuity in a gas, formulas are obtained for density and pressure at the head of a shock wave which are referred to velocity of the wave as a parameter. Gauge pressures are defined in a form considered to represent the observations of pressure obtained in Part I. The function is used to calibrate the gauge, and a comparison of results by Rayleigh's pressure function is included. A discussion is given of the appropriate ratio of specific heats for condensation cycles so extremely short in duration as those of intense shock waves, with references to the literature bearing on the subject of the accumulation of molecular excitational energies in short intervals of time.
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