摘要:

Generally believed lost, 12 of Wallace Clement Sabine’s notebooks have been offered by the Riverbank Laboratories, IITRI, of Geneva, Illinois, to Harvard’s rare books library for safekeeping. The Author has indexed the notebooks against Sabine’s published papers and has sought an answer to Sabine’s Silence in the 1900–1904 period following the opening of Boston Symphony Hall. The investigation reveals that all significant data recorded in the notebooks were either submitted for publication priuor to 1915 or were taken in 1915 just before Sabine became involved in World War I scientific service abroad. He died in January 1919. The excising of over 200 pages from his notebooks just after 1900 indicates that he may have been disappointed in his predictions of the reverberation times of Symphony Hall. This paper summarizes the contents of the notebooks and explains the reason for the discrepancy between his predictions and more recent measured reverberation times for Symphony Hall. It protrays the depth of Sabine’s experimentation, his influence on the growth of applied physics at Harvard, his extensive consultation with architects, and his service in the war effort.

ISSN:0001-4966    

DOI:10.1121/1.381348

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A solution is presented for the problem of the diffraction of a constant‐frequency spherical wave by a plane surface with constant normal specific acoustic admittance. By the use of a bidimensional Fourier transform, the transformed solution, obtained in the classical manner, is decomposed into a sum of products of functions, each of which is the Fourier image of a known function. The recognition of this enables one to readily invert the transform and, in this manner, a new representation of the exact solution is obtained. The particular case of grazing waves (source and receiver on the plane) is examined and asymptotic farfield expressions are derived. Comparison with experiments indicates that, if the plane‐wave admittance for normal incidence is used in the theory, the farfield sound‐pressure level can be predicted to within ±1.5 dB or better.

ISSN:0001-4966    

DOI:10.1121/1.381349

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The theory of acoustic‐ray bending by aircraft‐generated vortices is developed in a form convenient for application to a practical vortex tracking system. The maximum scattering angle ϑmis proportional to the vortex circulation divided by the average core radius. Since the circulation for commercial jet transports is roughly proportional to the wingspan, the value of ϑmdepends little on aircraft size but strongly on the ratio of the core size to the wingspan. For landing aircraft ϑmvaries from 0.5 to 1.4 rad depending upon the engine placement. A variety of vortex core models are considered which lead to values of ϑmbetween 0.2 and 0.9 rad. The capabilities of a pulsed acoustic vortex tracking system depend strongly upon ϑmand therefore upon aircraft type. The effects of timing errors, both vortex induced and random, on tracking accuracy are derived. The fundamental differences between refractive scattering and turbulence scattering techniques are described.

ISSN:0001-4966    

DOI:10.1121/1.381350

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A pair of sonic‐boom focusing conditions is obtained which holds for accelerated circular or linear flight in a uniform atmosphere. A geometric interpretation follows, the one focusing condition being represented by a cone while the other is represented by a plane. Points of intersection of these two surfaces are points of focus. Details are worked out for steady circular flight and it is seen that focal points at lower levels travel in horizontal circular paths whose size keeps increasing as lower and lower levels are considered. For accelerated linear motion the focusing conditions lead to a surface of focusing points which can be expressed in a form general enough to represent all values of acceleration, and whose shape is a surface of revolution generated by revolving a semicubical parabola about the flight axis. Cases of extreme focusing are also considered, one for accelerated linear flight and one for curved flight at constant speed.

ISSN:0001-4966    

DOI:10.1121/1.381351

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

This study treats the solution of the wave field produced by a point source in the presence of a layered medium on an acoustically hard backing. It is shown that if the refraction index of the layer is sufficiently large, the solution may be brought back to the problem of propagation above a boundary of local reaction. The point impedance of the surface is expressed in terms of known functions by assuming that the layer is porous, which also makes it possible to obtain an approximate solution of high accuracy. Comparisons are made to similar approximate solutions by Ingard and Lawhead‐Rudnick. A new method of measuring the ground impedance as a function of frequency by estimating four constants is suggested. Finally, experiments are added, supporting the theory that the acoustical properties of ground could be physically described by some kind of porous layer and acoustically described by a point impedance. ’’Surface waves’’ are experimentally verified in terms of the suggested theory.

ISSN:0001-4966    

DOI:10.1121/1.381352

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A fundamental question exists regarding how well information about ocean processes can be extracted from a time series of cw acoustic phase and amplitude when the propagation channel consists of multiple acoustic paths. In this paper, it is shown that there are at least two multipath propagation conditions under which an acoustic cw propagation experiment can be used to measure properties of the sound‐speed field and the ocean processes along the propagation path: narrow‐band (frequency‐modulation) path‐length modulation of all paths, and wide‐band (frequency‐modulation) path‐length modulation of a dominant path. A dominant path is one with power which exceeds the power on all other paths plus noise by at least 6 dB. Signal processing techniques for measuring the fluctuation spectra in both multipath situations are described and performance estimates are derived. The factors which limit how well information can be extracted are discussed.

ISSN:0001-4966    

DOI:10.1121/1.381353

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

For observed sound‐speed profiles, the range to a noise source in the first convergence zoneis calculated numerically by applying Gaussian quadrature techniques of various orders to the ray‐tracing range integral. The observed profile is tabulated for standardoceanographic depths. The tabulated profile is mapped into the sample set for Gaussian quadrature bycubic splines. We compare various strategies that divide the ray‐tracing range integral into convenient intervals for evaluation by the appropriate Gaussian quadrature consistentwith the singular points of the integrand.

ISSN:0001-4966    

DOI:10.1121/1.381354

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Relationships are developed between internal‐wave vertical velocity and acoustic phase. For source–receiver separations of many internal‐wave wavelengths, it is shown that the mapping of internal‐wave energy onto the phase of a single acoustic path is highly directional. Internal waves propagating perpendicular to the acoustic path produce the greatest phase modulation, and modulation of acoustic phase is reduced in all other geometries. For the omnidirectional internal‐wave spectrum proposed by Garrett and Munk, the acoustic phase‐modulation power spectrum is proportional to ω−3andj−3, where ω is the internal‐wave radian frequency andjis the internal‐wave vertical‐mode number. The first‐mode internal wave contributes more than half of the acoustic phase‐modulation power, and in this sense, the first‐mode internal wave dominates the acoustic phase spectrum.

ISSN:0001-4966    

DOI:10.1121/1.381355

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The noise generated by hydrodynamic flow over an axisymmetric body with a blunt noise is described quantitatively. Flush‐mounted piezoceramic hydrophones were used to measure that part of the turbulent boundary‐layer pressure fluctuations that propagates as true sound. Power spectra of the sound pressure were measured in a 3–50‐kHz frequency range over a wide range of Reynolds numbers (U∞D/ν?3.03×106, whereDis the diameter of the body) for the model operating in the Garfield Thomas water tunnel. The use of flush‐mounted hot‐film probes to locate turbulence transition is also described. The power spectra of the noise measured in the laminar flow regions correspond closely to those measured in the transition and fully developed turbulent regions of the flow. The exceptions were those spectra measured on the flat part of the nose, but correction for diffraction‐loss effects suggests that the noise measured there is due to the noise generated by the turbulent part of the flow. Nondimensionalization of the noise spectra measured at various arc lengths with theoretical expressions for the noise expected, that include a critical boundary‐layer thickness at the beginning of turbulence, show a general collapse of data to within 6 dB. When applicable, comparisons of the radiated noise measured on buoyant bodies are made and agreement was found to be excellent.

ISSN:0001-4966    

DOI:10.1121/1.381356

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Experimental measurements of the excess sound absorption in sea water below 1 kHz show a marked regional dependence. For example, the excess in the North Pacific Ocean is found to be only half that in the North Atlantic Ocean. Recent laboratory measurements have demonstrated that the absorption mechanism is a boric acid–borate relaxation. Since this reaction is also a buffer in sea water, the equilibrium and, consequently, the absorption coefficient should depend strongly onpH. ApH correction factor for Thorp’s formula is proposed. Over the range ofpH values encountered in the oceans (7.5–8.2), the predicted absorption is, approximately, an exponentially increasing function ofpH with a range of values of about 5 to 1. Comparison of experimental absorption coefficients with predictions based on reported localpH values shows reasonably good agreement.

ISSN:0001-4966    

DOI:10.1121/1.381357

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP