摘要:

Apparently contradictory reports in the literature concerning the acoustic streaming around a right‐circular cylinder immersed in a viscous, incompressible fluid can be reconciled by reference to a universal curve giving (δDC/a) as a function of (a/δAC), where δDCis theDCboundary‐layer thickness, δACis theACboundary‐layer thickness, andais the radius of the cylinder. The existence of such a curve has been verified experimentally for the case of a right‐circular cylinder vibrating to and fro in solutions of water and glycerin with kinematic viscosities ranging up to 50 times that of water, over an octave of frequencies. This experimental work also confirms theoretical predictions that for small oscillation amplitudes the streaming patterns will be the same whether the cylinder oscillates in a quiescent fluid or is fixed in an oscillating fluid. An approximate equation for the form of the universal curve has been obtained theoretically for the case of a thinACboundary layer, when the oscillation amplitude is small enough to insure the validity of a first‐order perturbation solution. Preliminary results of experimental work at higher amplitudes are reported, and the effect of the compressibility of the fluid on the streaming is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1907438

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP

摘要:

The effects of viscosity and heat conduction (in the reflecting medium) in producing dispersive reflection of a plane wave at the plane interface separating two media are investigated. If the reflecting medium is treated as a condensed fluid, heat conduction is found to have no effect, while, in first approximation, viscosity is found to produce no change in amplitude but a phase shift proportional to frequency (and therefore no phase distortion) at angles of incidence above critical and to produce no phase shift but amplitude distortion at angles below critical. This amplitude distortion is found to be important only in the neighborhood of a sharp wave front.

ISSN:0001-4966    

DOI:10.1121/1.1907439

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP

摘要:

Exact solutions are derived for the propagation of compressional elastic waves along an anisotropic circular cylinder having axial elastic symmetry (hexagonal elastic symmetry with the crystallographic axis coincident with the cylinder axis). Formal solutions are obtained for the displacement components, and a transcendental equation relating the phase velocity and the circumference‐to‐wavelength ratio is derived. The solutions are shown to reduce to the Pochhammer solution for the isotropic case. In the long wavelength extreme the velocity is dispersionless and is given in terms of the axial Young's modulus. In the short wave‐length limit the velocity again becomes constant, similar to the Rayleigh surface wave limit in the isotropic case.

ISSN:0001-4966    

DOI:10.1121/1.1907440

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP

摘要:

The attenuation of sound in air, in the vertical direction, has been measured on six occasions using an aircraft flying at various heights over a microphone at ground level. It was found that there was always negligible attenuation at frequencies below 1000 cps even though the air was turbulent; at higher frequencies the attenuation was found to be of the same order as the Knudsen‐Kneser results for attenuation due to molecular absorption.

ISSN:0001-4966    

DOI:10.1121/1.1907441

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP

摘要:

When predicting and comparing the acoustical properties of horns it is customary practice to formulate the propagation as a one‐parameter plane wave front problem. However, when particular attention is paid to the rapid flare near the mouth of a horn structure such as the tractrix, it also seems plausible to formulate the propagation on the basis of a one‐parameter spherical wave front theory. By visualizing the surfaces of constant phase as spheres of constant radiiaand the flow lines as tractrixes having a generating arm of lengtha, a one‐parameter wave equation and Ricatti impedance equation may be derived. Solutions to these equations have been obtained by wave perturbation and by analog computer techniques.Axial response and throat impedance measurements are compared with theoretical calculations postulating first a hemispherical and then a plane piston radiation pattern. It appears that the most satisfactory explanation lies somewhere in between these two limiting cases.

ISSN:0001-4966    

DOI:10.1121/1.1907442

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP

摘要:

Experimental investigations have been carried out on the tractrix horn structure to determine its “free‐field” radiation characteristics. Axial, off‐axis, and polar response characteristics, as well as throat impedance data on a single cell horn, are presented for both small and large baffle mounting. Pertinent data on a two cell structure are also presented. These data show the tractrix performance to be comparable with that of the well‐known exponential horn. A multicellular structure, while showing definite improvement in uniformity of angular distribution at high frequencies, exhibits undesirable band rejection characteristics within the useful frequency range of the horn.

ISSN:0001-4966    

DOI:10.1121/1.1907443

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP

摘要:

The maximum directivity index of a symmetrical, linear point array has been calculated as a function of the number and spacing of the elements in the array. The excitation required to produce a maximum directivity index is not uniform, except for integral‐half‐wavelength element spacings, and in general the minor lobes of the directional response patterns produced are not of equal, nor of small, amplitude. For element spacings exceeding a half‐wavelength, a conventional type of pattern and of excitation is found to produce the maximum directivity index. On the other hand, as the element spacing is reduced below a half‐wavelength, the directivity patterns corresponding to the maximum directivity index become super‐directive, and the directivity index may be improved relative to the value obtainable with uniform excitation. However, this improvement is obtained only at the expense of requiring large, reversed‐phase excitation. Numerical results are presented for 3‐, 5‐, and 7‐element arrays.

ISSN:0001-4966    

DOI:10.1121/1.1907444

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP

摘要:

A study has been made of wedge‐shaped horns used in conjunction with line sources. The horns are constructed from aluminum plates covered with cellular rubber. Measurements were made over the frequency range of 60 kc to 150 kc using horn lengths from 3 inches to 15 inches and horn angles up to 90 degrees. Results are given graphically in the form of directivity patterns as well as relative gain in sensitivity and beam widths as a function of the several parameters. At 80 kc beam widths as low as 10 degrees are obtained with at least 40 db suppression of the secondary lobes. The horns are easily fabricated and the beam width and sensitivity of the transducers are easily controlled by varying the horn angle.

ISSN:0001-4966    

DOI:10.1121/1.1907445

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP

摘要:

Cumulative effects of a two‐second burst of thermal noise on the threshold shift for 35‐millisecond tone pips were examined by tracing the threshold shift over the course of 100 successive noise bursts. When bursts were presented at a rate of 6 per minute, a systematic increase in the threshold shift for the 35‐millisecond tone pip was observed over the 100 burst series. Changing the burst‐pip interval from 90 to 445 to 1000 milliseconds had little effect on the degree of this accumulation, but changing the SPL of the burst had a marked effect. At a burst level of 110 db (SPL re 0.0002 microbar) the mean threshold shift increased by about 6 db between the 15th and the 100th burst. Decreasing the burst level to 100 db reduced the mean accumulation to about 4 db, and at a burst level of 90 db to about 1 db. Changing the burst presentation rate from 6 per minute to 3 per minute reduced, but did not eliminate, the cumulative effect. The degree of this cumulative phenomenon showed wide individual differences among subjects.It is concluded that, unless proper allowances are made, the repetitive stimulation required by burst‐pip audiometry may be a source of serious error. On the other hand, the reaction of an ear to repeated stimuli may prove to be a useful index of the ear's susceptibility to irreversible acoustic trauma.

ISSN:0001-4966    

DOI:10.1121/1.1907446

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP

摘要:

The masking of speech by a periodically interrupted white noise was examined over a wide range of noise levels, rates of interruption, and noise‐time fractions. To a first approximation, the masking produced by an interrupted noise is a constant fraction of that produced by a continuous uninterrupted noise. The masking produced by an interrupted noise is primarily a function of the interval between successive noise bursts and is relatively independent of the noise‐time fraction and the rate of interruption of the noise, except insofar as these determine the interval between noise bursts. The effect of interrupted noise on the loudness of speech was also examined.

ISSN:0001-4966    

DOI:10.1121/1.1907447

出版商:Acoustical Society of America    

年代:1954

数据来源: AIP