摘要:

Many underwater‐acoustics propagation problems are solved using geometrical acoustics. This approach uses an approximate formulation of the wave equation which is exact in the limit of infinite frequency. The computation of the ray paths and the associated intensities involve the use of four first‐order nonlinear ordinary differential equations. In order to have smooth, continuous solutions, the velocity field (i.e., the speed of sound field) and its derivatives, which appear in the coefficients of the set of differential equations, must be twice continuously differentiable (C2) throughout the region. The use of splines—cubic polynomials which are continuous through the second derivative—insures compliance to these requirements. This paper gives a technique for calculating a spline field in two dimensions, given data points which are available at specific ranges, but at different depths. The technique is easily extendable to include arbitrary location of data points. This use of splines, in order to provide a smooth function for use in differential equation‐solvers found so often in computer algorithms, should be of utility in various fields.

ISSN:0001-4966    

DOI:10.1121/1.1903455

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Commonly occurring infrasonic waves recorded during the winter months are believed to originate by means of aerodynamic source mechanisms. Acoustic power spectra of these commonly observed signals are compared to the power spectra one would observe from theoretical source mechanisms. From this comparison, the theoretical source mechanism believed responsible for production of the observed signals is thought to be isotropic turbulence in the lee of mountain peaks. Experimentally recorded signals and their power spectra are shown. Source regions of three signals are identified.

ISSN:0001-4966    

DOI:10.1121/1.1903456

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The analysis studies the broadening of the frequency spectrum of an acoustic plane‐wave pulse backscattered into a finite‐angle receiver by the action of a transverse mean wind which moves turbulent velocity and temperature fluctuations through the beam. The results include the frequency spectrumI(k) of the acoustic pressure as a function of wavenumber and the spectral equivalent widthb, as a function of mean transverse wind speed and receiver aperture. For broad beams, both temperature and velocity fluctuations contribute to the apparent 180°‐backscatter intensity, and the contributing eddy scales vary from λ/2 to λ/√2. Again for broad beams and for typical experimental parameters for atmospheric echosondes, the broadening effects of mean transverse wind in the scattering volume outweigh the broadening effects of turbulence trransported by a mean transverse wind during propagation to the scattering volume. However, as the receiver beam angle decreases, the spectral broadening drops to zero, regardless of mean wind. Figures are included to show the effects of wind broadening for the case of intermediate receiver beam angles.

ISSN:0001-4966    

DOI:10.1121/1.1903457

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The paper deals with the problem of estimating the sound field generated by a limited region of turbulence containing foreign bodies of arbitrary shape and arbitrary composition. According to the Lighthill‐Curle acoustic analogy, the aerodynamically‐generated sound is the same as that due to a distribution of quadrupole and dipole sources within the fluid and over the surfaces of the bodies. It is known that the presence of foreign bodies will usually convert some of the intense nearfield energy of Lighthill's quadrupoles into the form of sound attributal to dipole and monopole sound whose amplitude is much greater than that of the free‐flow noise. Current trends towards more sophisticated theories have again focused attention on this problem with drastically different results. Calculations are here presented to determine the extent whereby this multipole‐conversion process actually is taking place. It is shown that a method involving matching of inner and outer expansions yields intensity laws of different orders of magnitude comparing favourably with observed results. Curle and Powell's attempts to solve the problem have been reexamined to show inconsistencies their analysis. In particular, Curle's retarded‐wave solutions are recasted by use of Fourier synthesis. It is suggested that much of the discrepancies between the results in the present paper and the predictions of Curle's theory can be explained by a more carefully conducted analysis, throwing emphasis on various possibilities by which pseudosound tuned to the presence of foreign bodies may give way to sound fields being many orders of magnitude stronger than those emerging from Curle's analysis.

ISSN:0001-4966    

DOI:10.1121/1.1903458

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

In many acoustic systems the aperture dimensions of the projecting and receiving transducers are comparable. The variation of received signal level with separation distance along the acoustic axis in the transition region between near‐ and farfields has been studied by several workers, but apparently little has been published on the directivity patterns for this case. This paper presents the results of an experimental investigation of the directivity patterns for a series of projector‐hydrophone pairs of various ratios of apertures as a function of range from the nearfield to the farfield.

ISSN:0001-4966    

DOI:10.1121/1.1903459

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Experimental investigations were made of the covariance structures of the envelope functions of several nonstationary reverberation processes. It is shown that, when the process is stationary, the covariance of the envelope is approximately equal to the square of the envelope of the covariance of the narrow‐band process. However, when the nonstationarity of the reverberation process becomes noticeable, then this simple relationship is no longer valid.

ISSN:0001-4966    

DOI:10.1121/1.1903460

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Fluctuations in the amplitude and phase of low‐frequency sound propagated to long range in the ocean are predicted. Phase fluctuations are attributed to the passage of acoustic radiation through the internal gravity‐wave field; predictions are based on measured and modeled internal wave spectra. Ray theory is used to determine phase and amplitude variations as a function of time, space, and acoustic frequency. It is shown and experimentally verified that mean‐square phase fluctuations are depth dependent.

ISSN:0001-4966    

DOI:10.1121/1.1903461

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

A time harmonic pressure is applied at the inner boundary of an acoustic fluid body, composed of two parallel, nonconcentric circular cylinders, which is itself embedded in another infinite acoustic fluid. The resulting radiation is determined as an expansion in terms of the nonconcentricity. For an applied internal pressure in ajth spatial mode, each order of this expansion introduces a “nearest neighbor” mode, e.g., the first order requires a (j+i)th and a (j−i)th mode. The solution procedure is based on an expansion of the Helmholtz integral equation formulation.

ISSN:0001-4966    

DOI:10.1121/1.1903462

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

As motivated by the recent discovery of heat pulses propagating in dielectric crystals at low temperature, a continuum theory of thermoelasticity, which is modified to include the effect of thermal phonon relaxation, is applied to investigate the propagation of plane harmonic waves in unbounded anisotropic solids. Four characteristic wave speeds are found, three being analogous to those of isothermal or adiabatic elastic waves. The fourth wave, which is predominantly a temperature disturbance, corresponds to the heat pulses known also as the second sound. Velocity, slowness, and wave surfaces of the thermoelastic waves are analyzed and are illustrated with numerical and graphical results for NaF and solid helium crystals. A new definition of the group velocity for waves in a dissipative and dispersive anisotropic medium is proposed and is calculated and compared with the energy transport speed of thermoelastic waves.

ISSN:0001-4966    

DOI:10.1121/1.1903463

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Vibrational relaxation in water vapor has been observed using both ultrasonic velocity dispersion and absorption. The raw data were corrected for real‐gas effects and for classical absorption. Results are compared with those of other investigators. Observed relaxation times appear much too short to be explained by a simple V‐T process, and are interpreted in terms of a V‐R transfer mechanism. Values ofkVRrange from about 1×108sec−1atm−1at 374 K to 6×108sec−1atm−1at 946 K.

ISSN:0001-4966    

DOI:10.1121/1.1903464

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP