摘要:

A recent theoretical formulation by Westervelt and Larson predicts the development of highly directive sound beams in water through the thermalization of modulated laser light. The present paper reports the experimental validation of this theory with measurements carried out in a fresh water lake. Additional theory pertinent to practical implementation is also presented and verified. The experimental apparatus includes an optically pumped laser system operating in the conventional mode at wavelengths in either the red or infrared regions of the optical spectrum. Modulation of the stochastic light burst is provided by a half‐wave Pockels cell. The acoustic measurements began with an existence test which confirmed that an acoustic signal is produced by thermo‐optic demodulation of laser light in the water. The sound pulse was generated in a beam with a width and source level that are in reasonable agreement with theory. Acoustic diffraction effects associated with laser beams of finite size were also examined, as were the nearfield effects of this type of array. The relation of laser technology to further work in the blue–green spectrum is analyzed.Subject Classification: [43]20.55; [43]85.40; [43]35.65.

ISSN:0001-4966    

DOI:10.1121/1.380937

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

A description of the acoustic pressure versus time is obtained for a finite cylindrical source whose initial overpressure is constant and uniform. Small signal analysis is used and the medium is considered dissipationless. The results show a short leading condensation followed immediately by a rarefactive impulse with a long tail which ends in a second rarefactive pulse of smaller amplitude. The analytical results are amenable to numerical methods for obtaining pressure disturbances from cylindrical sources of given lengths and radii.Subject Classifacation: [43]20.55, [43]20.15.

ISSN:0001-4966    

DOI:10.1121/1.380938

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The acoustic nearfield of a circular piston is investigated using the impulse response technique. Although the harmonic field can be expressed as either a convolution integral or Fourier transform, the integrals cannot be simply evaluated. An asymptotic evaluation of the Fourier transform of the spatially dependent impulse response for the circular piston is, however, readily performed. The resulting expressions very simply illustrate the nearfield behavior of the piston. Furthermore, the expressions are shown to be in agreement with the asymptotic properties of known nearfield results and are consistent with the usual farfield expressions.Subject Classification: [43]20.55, [43]20.15.

ISSN:0001-4966    

DOI:10.1121/1.380939

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

An approach suggested by Westervelt [3rd Int. Congr. Acoust., Stuttgart, 1959] has previously been used to predict with remarkable accuracy the excess absorption due to nonlinear effects in a plane, traveling sound wave. The same approach is applied here to spherically diverging sound waves. A number of cases of practical importance in underwater acoustics are evaluated and compared with other theoretical results.Subject Classification: [43]25.22; [43]30.75.

ISSN:0001-4966    

DOI:10.1121/1.380940

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Earlier studies [J. Acoust. Soc. Am. 29, 934 (1957)] of the mutual nonlinear interaction of two plane waves of sound with each other exhibited a singularity when the waves became collinear. The singularity is removed in this study, which leads to a method for calculating the pressure absorption coefficient α of a wave with wave vector k1resulting from interaction with isotropic acoustic waves having the energy density spectrumu(k). Thus, α= (4ρ0c20)−1(1+1/2Λ)2πFk10ku(k)dk+k1F∞d12u(k) idk+k21Fk11k−1u(k)dk], in which ρ0andc0represent the density and velocity of sound of the fluid which is assumed to be dispersionless. The constant 1/2Λ =ρ0cd0−1(dc/dρ)ρ0.Subject Classification: [43]25.22, [43]25.35.

ISSN:0001-4966    

DOI:10.1121/1.380941

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

We model mathematically the spectral features of infrasound observed in the ionosphere and believed to be radiated by severe thunderstorms. We explain the dominant 2–5‐min wave period as an effect of atmospheric filtering; shorter periods are excessively attenuated by absorption in transit to the ionosphere, and longer periods are attenuated in portions of the atmosphere where the waves are evanescent because their frequencies are below the acoustic cutoff. An observed spectral ’’fine structure’’ within the 2–5‐min band is explained in terms of resonant interactions between the waves and the atmospheric temperature structure. Accurate quantitative modeling of all these details of the storm‐to‐ionosphere transmission coefficient requires numerical integration of the acoustic‐gravity wave equation, including the effects of ground reflection, absorption, and partial reflections in the atmosphere.Subject Classification: [43]28.30.

ISSN:0001-4966    

DOI:10.1121/1.380942

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The problem of finding a solution of the scalar wavefield produced by a point source in the presence of an infinite ’’impedance’’ boundary is treated. The analysis is made with a method used by Ingard, which has been corrected. It is shown that it is possible to rewrite the exact solution in terms of a single integral along a steepest‐descent contour and a Hankel function. Asymptotical expansions of the solution is in consistency with expansions by Wenzel, who found that in a special case expansions of his and Ingard’s solution differ by a ’’surface wave’’ term. The asymptotical expansions are given as examples. The main result, the single integral and the Hankel function, could easily be integrated numerically.Subject Classification: [43]28.40; [43]20.15, [43]20.30, [43]20.55.

ISSN:0001-4966    

DOI:10.1121/1.380943

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The specific dependence of the properties of a cw Doppler ultrasonic scattered signal, and of its autocorrelation and spectral density functions, on various scatterer flow parameters and diagnostic signal beam parameters is shown. These factors include the scatterer average velocity profile and the extent of any turbulence together with the signal beam angle and beam width.Subject Classification: [43]30.40; [43]35.65; [43]20.15, [43]20.30.

ISSN:0001-4966    

DOI:10.1121/1.380944

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Underwater ambient noise spectra are described for data recorded during late spring conditions under the ice sheet in McMurdo Sound. The ambient noise was at all times marked by intense biological activity, but a diurnal component became apparent during a three‐day recording period. This noise was found to be similar to that described in Arctic work and undoubtedly arose as the result of surface cracking under the influence of thermal stressing. The characteristics of this noise component, however, were found to differ in certain respects from those reported from comparable Arctic measurements.Subject Classification: [43]30.20, [43]30.70; [43]28.65.

ISSN:0001-4966    

DOI:10.1121/1.380930

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

A general asymptotic theory of scattering by a moving rough surface is presented. The theory is valid for a slowly varying refractive index, and for ocean‐surface wave heights that are small compared with the acoustic wavelength and the ocean‐surface correlation length. In contrast with the Kirchhoff or physical optics approximation, this theory is valid when the acoustic wavelength and ocean‐surface correlation length are of the same order. The ocean surface is described as a sum of gravity waves of varying frequency progressing in different directions. It is demonstrated that, to a given order of approximation in the asymptotic theory, the average energy associated with signal sideband rays is precisely accounted for by a reduction of intensity of the reflected carrier. Wave number diagrams are introduced which are extremely useful for depicting the directions of the incident and scatteres rays at the surface, demonstrating the effect of varying surface‐wave and acoustic frequency, and demonstrating the frequency cutoff of the sideband power spectrum. The phenomenon of Bragg resonance at glancing incidence of the carrier is also easily depicted. The theory can accomodate the effects of ray focusing, both in the carrier and sidebands, through the use of uniformly valid asymptotic expansions which account for the presence of caustics.Subject Classification: [43]30.40, [43]30.20, [43]30.25, [43]30.30.

ISSN:0001-4966    

DOI:10.1121/1.380931

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP