摘要:

Lecture demonstrations are being used less and less in modern courses in acoustics. It is suggested that acoustics courses can be enriched and vitalized by demonstrations and that they should be used more often. Descriptions of some useful demonstrations are given and references to background material are listed.

ISSN:0001-4966    

DOI:10.1121/1.1919467

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

To treat guided wave propagation in curved nonuniform waveguides, such as atmospheric, oceanic, and solid earth ducts, two methods of analysis are presented. The first is the ray method of Keller and Rubinow, which has been applied to waveguides by Maurer and Felsen. This method is illustrated by applying it to a straight uniform guide, for which it yields the exact result. It is also applied to a guide with concentric circular walls, and yields the asymptotic form of the exact result. Then it is applied to a gradually changing guide and yields a result which includes that of Rayleigh for narrow guides. The second method, which involves stretching of the transverse coordinate, has been applied to water waves by Keller to narrow guides by Matkowsky and to various guided wave problems by Rulf. It is applied to gradually changing guides and yields results which are more accurate than the ray method results for such guides.

ISSN:0001-4966    

DOI:10.1121/1.1919468

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The analytic solution for a Rayleigh‐type surface wave on a linear viscoelastic half‐space is obtained. The general viscoelastic solution predicts several properties of the surface wave distinct from those predicted by elasticity. The velocity, absorption coefficient, and displacement field are described theoretically.

ISSN:0001-4966    

DOI:10.1121/1.1919469

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Standing or traveling waves which vary algebraically with the axial distance in uniform ducts with sheared mean velocity profiles are investigated. The results show that such waves are not possible for ducts with uniform cross sections and fully developed mean flows.

ISSN:0001-4966    

DOI:10.1121/1.1919470

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

We discuss a slight modification of the classical problem of diffraction of acoustic waves off a semi‐infinite, thin, rigid surface. The modification is: the waves incident on the surface are evanescent rather than freely propagating. The solution for the diffracted sound wave in the farfield, and some variations on this solution, are considered with reference to the following problems: (1) sound diffracted by a turbulent boundary layer flow off the sharp edge of a large hard surface; (2) for a large plate undergoing flexural vibration, radiation efficiencies associated with unbaffled plate edges of different boundary conditions.

ISSN:0001-4966    

DOI:10.1121/1.1919471

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

An experimental study has been conducted to determine the strength of the acoustic radiation generated in the combustion and turbine stages of a JT8D core engine and transmitted out through the primary jet exhaust duct. The acoustic field within the tail pipe duct extension was measured by an array of microphones flush mounted on the extension wall. The interpretation of the cross‐power spectral densities of the microphone signals by means of a theoretical model of the sound propagation within the duct permitted the acoustic energy transmission out of the nozzle exit to be determined. The results show that the internally generated noise is a dominant noise component from the core engine at larger angles from the jet axis for the case of low engine power settings. Although the internally generated core engine noise component lies well below the JT8D engine jet noise and fan noise components at the higher engine power settings, it represents an additional noise parameter that must be considered in the application of fan noise and jet noise suppression techniques.

ISSN:0001-4966    

DOI:10.1121/1.1919472

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

In this paper, following a brief review of previous contributions to the field of Parametric Acoustics, plane and spherical wave solutions of Burgers' equation are derived by the method of successive approximations for the difference‐frequency signal which results from nonlinear interaction in the medium of two different‐frequency primary waves radiated simultaneously by a finite‐amplitude source. These solutions are then matched asymptotically obtain a farfield solution which includes the influence of nonlinear interaction within the Fresnel and Fraunhofer zones of the primary waves, from which an expression for the “equivalent difference‐frequency source level” is derived. The analysis assumes that the primary‐wave amplitudes, which induce the nonlinear interaction are below their respective shock thresholds as defined by a criterion which relates the steepest slope of the waveform to a predetermined upper bound approximation of an ideal discontinuity. In order to determine the predictive reliability of the solutions, a parameter which characterizes the “equivalent difference‐frequency source level” is compared with empirical results obtained over a wide range of experimental conditions. Finally, a quasiplane three‐dimensional equation (which in one dimension assumes the form of Burgers' equation) is also analyzed in order to obtain a second‐order farfield directivity function for the difference‐frequency signal.

ISSN:0001-4966    

DOI:10.1121/1.1919473

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Explosive sound sources were detonated at shallow depths in two long‐range deep ocean experiments. Signals were recorded from a shallow hydrophone in the first experiment (May 1967) and from a SOFAR channel hydrophone in the second (May 1969). The propagation was strongly influenced by subsurface sound channels. The shot arrival patterns were analyzed by a graphical method to determine the convergence zone locations and the average sound speed in the subsurface channel. In the first experiment the bottom was flat and the medium could be characterized by a single velocity profile having a weak subsurface channel. Numerous bottom reflections were observed. In the second experiment the bottom was rough. Two profiles were required to characterize the medium, one of which exhibited a strong subsurface channel. Bottom reflections were not in evidence. Energy was detected which traveled more than 800 km entirely within the subsurface channel.

ISSN:0001-4966    

DOI:10.1121/1.1919474

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The acoustic transmitting capacity of a medium is limited by nonlinear propagation effects. As the source amplitude is increased to very high levels, acoustic saturation sets in, a state in which the amplitude at a field point approaches a limiting value, independent of the source amplitude. A theoretical and experimental study of saturation of spherical waves is presented in this paper. Predicted saturation amplitudes for the fundamental component in an originally sinusoidal wave are obtained by matching the well known weak‐shock solution to a linear‐theory solution at the old‐age distancermax. The saturation formulas are verified by experiments done in fresh water with a 3‐in.‐diam piston projector operating at 454 kHz at source levels up to 135 dBre1 μbar at 1 yd. Amplitude response curves were measured on the acoustic axis at six distances ranging from 0.76 yd to 111 yd. At the longer ranges the curves become isotonic, thereby demonstrating saturation. High‐amplitude beam patterns show the effect of finite‐amplitude attenuation in that the major lobe is broadened and minor lobe suppression is reduced. Finally, the saturation formulas are used to develop curves of maximum SPL as a function of range, with frequency a parameter, for both fresh and sea water.

ISSN:0001-4966    

DOI:10.1121/1.1919475

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The solution of the reduced scalar wave equation for an almost stratified medium is written in the form of an asymptotic power series. The vertical structure of the solution is expressed as a linear combination of the normal‐mode eigenfunctions whose coefficients satisfy two‐dimensional eikonal and transport equations. Although smooth caustics may be treated for the two‐dimensional scalar wave equation, the problem of a uniform approximation near a point source has not yet been resolved. Finally, the theory is applied to acoustic propagation in a realistic model ocean and the results are compared with measurements.

ISSN:0001-4966    

DOI:10.1121/1.1919476

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP