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Journal of the Acoustical Society of America

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Apr 1933

Volume 4, Issue 4, pp. 273-352


THE SUPERSONIC INTERFEROMETER AND ABSORPTION MEASUREMENTS

W. D. Hershberger

J. Acoust. Soc. Am. Volume 4, Issue 4, pp. 273-283 (1933); (11 pages)

Online Publication Date: 13 Jun 2005

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The previously developed theory of the supersonic interferometer is extended with the particular view of utilizing it for absorption measurements in gases. It is shown that the mechanical impedance of the interferometer is
math
where
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.
That is, if reactance is plotted against resistance we obtain an impedance spiral with center (Acρ/tanh 2αl, 0) and radius Acρ/sinh 2αl. The action of the interferometer is studied in terms of the properties of the spiral. Absorption measurements are effected by compensating for the changes in the equivalent series electrical resistance because of changes in l by adjusting a shunt resistance across the crystal so the e.m.f. is held constant, irrespective of l. Absorption in the medium is calculated from the values of shunt resistance for the path lengths of interest. Measurements were taken at three frequencies in the range 899.0 kc to 2514 kc. The quantity 2αλ2 as measured by the interferometric method is not constant but shows a systematic change with frequency.

SOUND ABSORPTION IN NON‐REACTIVE GAS MIXTURES

Reed Lawlor

J. Acoust. Soc. Am. Volume 4, Issue 4, pp. 284-287 (1933); (4 pages)

Online Publication Date: 13 Jun 2005

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Abello's results for binary gas mixtures may be derived simply if it is assumed that the gases mixed are non‐reactive. It is found that if any number of non‐reactive gases are mixed, the attenuation constant of the mixture is a linear homogeneous function of the volume percentages of the components. Values of the absorption coefficients of CO2, N2O, H2, He, A, and N2 at 612 kc are calculated.

ON MINIMUM AUDIBLE SOUND FIELDS

L. J. Sivian and S. D. White

J. Acoust. Soc. Am. Volume 4, Issue 4, pp. 288-321 (1933); (34 pages) | Cited 8 times

Online Publication Date: 13 Jun 2005

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The minimum audible field (M.A.F.) has been determined from data taken on 14 ears over the frequency range from 100 to 15,000 c.p.s. The observer is placed in a sound field which is substantially that of a plane progressive wave, facing the source and listening monaurally. The M.A.F. is expressed as the intensity of the free field, measured prior to the insertion of the observer. Similar data are presented for binaural hearing, over the range from 60 to 15,000 c.p s., obtained with 13 observers. At 1000 c.p.s. the average M.A.F. observed is 1.9 × 10−16 watts per cm2, corresponding to a pressure 71 db below 1 bar. Included are data showing how the M.A.F. varies with the observer's azimuth relative to the wave front. Another type of threshold data refers to minimum audible pressures (M.A.P.) as measured at the observer's ear drum. The differences obviously to be expected between M.A.F. and M.A.P. values are due to wave motion in the ear canal and to diffraction caused by the head. The M.A.F. data are discussed in relation to the M.A.P. determinations from several sources. Some possible causes of difference between the two, which are due to experimental procedure and may add to the causes already mentioned, are pointed out.

THE ABSORPTION BY POROUS MATERIALS AT NORMAL INCIDENCE—A COMPARISON OF THEORY AND EXPERIMENT

H. L. Penman and E. G. Richardson

J. Acoust. Soc. Am. Volume 4, Issue 4, pp. 322-330 (1933); (9 pages)

Online Publication Date: 13 Jun 2005

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COMPONENT TONES FROM A BELL

Arthur Taber Jones and George W. Alderman

J. Acoust. Soc. Am. Volume 4, Issue 4, pp. 331-343 (1933); (13 pages)

Online Publication Date: 13 Jun 2005

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The gravest sixteen partials of three bells (weights 900 kg, 675 kg, 464 kg) have been examined by the method described by Tyzzer. The numbers of nodal meridians and nodal circles found for the partials of these bells agree with those found on three smaller bells by Tyzzer, except that some additional circles are now found in the upper parts of the bells. The levels at which nodal circles are found are given. The relative frequencies of certain modes of vibration of the bells now studied are compared with those for the corresponding modes of a thin ring. Various reasons are adduced for believing that the vibrations of bells have a considerably larger compressional component than has usually been supposed. Oscillograms have been obtained from which the amplitudes of the partials have been calculated. When a bell is struck in the usual manner the most prominent partial at first is the fifth. In less than half a second—on the bells investigated—the fifth has yielded to the third, which is the most prominent until about four seconds after the stroke. After that the first partial is the most prominent. An appendix gives an approximate method of correcting for the beats with which some partials are affected.

A TUNING‐FORK AUDIOMETER AND NOISE OBSERVATIONS IN NEWPORT NEWS, VIRGINIA

E. Z. Stowell

J. Acoust. Soc. Am. Volume 4, Issue 4, pp. 344-352 (1933); (9 pages) | Cited 1 time

Online Publication Date: 13 Jun 2005

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