摘要:

The theory presented in two previous papers [Part I, J. Acoust. Soc. Am.72, 1056–1061 (1982) and Part II, J. Acoust. Soc. Am.74, 1013–1020 (1983)] is further developed and generalized to the case of no axisymmetry. Emphasis is given to a study of the parametric generated sound. Asymptotic formulas are presented, showing the development of the difference frequency sound from the source (transducer) into the farfield. New numerical results for the axisymmetric case are given for the linear and the parametric field of a source which is not radiating uniformly. New experimental results are reported and compared with the theory.

ISSN:0001-4966    

DOI:10.1121/1.390586

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

When a high velocity jet is directed toward the mouth of a tube closed at the downstream end, large, nonlinear flow oscillations may occur within the tube. Shock waves propagate up and down the tube and generate strong heating of the tube walls. The device is called a Hartmann–Sprenger tube (in short, H–S tube). It has been proposed by several investigators to strengthen the shock waves and consequently the heating effects by tapering the tube. Conical and multistepped configurations have been investigated, but also tubes having a sudden area contraction (single step). This latter geometry produces remarkable pressure and thermal heating amplification compared to a constant area tube. The paper presents theoretical and experimental results obtained for the single‐step H–S tube. It is shown that several oscillation modes can exist. These are separated by intervals of instability. The influence of geometrical parameters (upstream to downstream cavity diameter and length ratios) and of jet Mach number on oscillation mode is investigated.

ISSN:0001-4966    

DOI:10.1121/1.390587

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

An experiment to measure phase (travel time) and intensity fluctuations in sound pulses transmitted at 2, 4, 8, and 13 kHz over an 18.1 km wholly refracted Fermat path is discussed. Simultaneously with the acoustic monitoring the index of refraction fluctuations were measured in space and time with sufficient resolution to determine the correlation function of the medium. The site was the Cobb Seamount in the northeast Pacific (46°46′N, 130°47′ W), and the time period was 30 days in June–July, 1977. In terms of both the quality and quantity of acoustic and oceanographic measurements, this experiment represents a significant improvement over an earlier experiment in the same location [J. Acoust. Soc. Am.60, 46–59 (1976)]. The acoustic measurements cover a wider range of acoustic frequencies and more closely represent measurements from a single Fermat path. Approximately 25% of the acoustic data are discussed here; the representations of the correlation function of the index of refraction are based on all of the oceanographic data. The physical processes responsible for the fluctuations in the index of refraction are those due to the tides, internal waves, and finestructure. The effects of internal waves are treated in detail. The moments of the observed intensity fluctuations are discussed, as are the spectral distributions of the second moments of phase and intensity. The observations are compared with theoretical predictions based on the Rytov approximation and on a multiple scatter formulation (approximate solution to the fourth moment equation).

ISSN:0001-4966    

DOI:10.1121/1.390588

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Herein some exact effective sound velocity profiles for rigorous ray tracing are developed numerically. The equations ofmotion for rigorous ray tracing are developed and it is shown that in general a ray is not normal to the wave surfaces of constant phase.

ISSN:0001-4966    

DOI:10.1121/1.390589

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

This paper presents an analysis of the effects of microroughness of the ocean bottom on a sonar signal. The results best apply to features where the roughness amplitude is less than one‐quarter of an acoustic wavelength such as with ripples, beds of rocks, and nodules. The shape of the probability density function (PDF) of the echo envelope is examined in terms of the rms roughness and a new parameter, the correlation area of the bottom. The area is equal to the product of thexandycorrelation distances along the floor. The PDF is shown to be extremely sensitive to small changes in the roughness. Furthermore by determining the rms roughness from standard coherent reflection measurements, the correlation area may be extracted directly from the PDF. Thus both vertical as well as lateral information is obtainable from sonar data. The technique can be used to discriminate between different types of bottoms that may have the same roughness but different correlation areas, for example a floor with ripples versus one with rocks or nodules. The analysis combines (1) a general statistical model employing the Rice PDF [S. O. Rice, inSelectedPapersonNoiseandStochasticProcesses, edited by N. Wax (Dover, New York, 1954), pp. 133–294], and (2) a theory originated by Eckart [C. Eckart, J. Acoust. Soc. Am.25, 566–570 (1953)]. The analysis is applied to sonar data collected from the continental shelf near Cape Hatteras, North Carolina. The results are consistent with the known characteristics of the area.

ISSN:0001-4966    

DOI:10.1121/1.390590

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Earthquakes along the mid‐Arctic ridge radiate earthborne compressional and shear waves, which in turn excite the Arctic acoustic channel. These waterborne arrivals, calledTwaves, have been observed about 300 km from the source and apparently enter the acoustic channel by scattering of vertical rays into nearly horizontal ones by the Arctic ice canopy above the source. TheTwaves are acoustically energetic (up to 400 kJ), are of surprisingly long duration (up to 72 s at its 8‐dB down levels), have low‐frequency content (peaks in the 5–15‐Hz region), and have haystack spectra (4th power positive and negative dependence below and above the peak frequency, respectively). Characteristics of these sporadic noises are displayed for one event in a sonogram, in short time frequency spectra, in a time series, and in directional spectra. Directional analysis was accomplished with use of a large (∼1 km) two‐dimensional horizontal array. Speculations on duration of the observed events suggest that source depth, water‐column reverberation above the source, and source magnitude, including ‘‘rapid‐fire’’ multiple source excitations, could be the primary causes.

ISSN:0001-4966    

DOI:10.1121/1.390591

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Ambient noise vertical directionality measurements at low frequencies have shown that noise arrives from long distances at near horizontal angles, from moderate distances at larger angles, and from local sources at high angles. Therefore levels of noise from different sources can be measured by examining vertical linear array beam powers at the appropriate angles. However, when measurements are made, it is important to be able to distinguish between acoustic signals, ambient noise, and data contamination. Some experimental results are examined, using conventional and adaptive beamforming and eigenvalue–eigenvector decomposition, and used to establish criteria useful in deciding whether the measurements are ambient noise or otherwise.

ISSN:0001-4966    

DOI:10.1121/1.390592

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

This paper presents a method for solving wave propagation problems of an infinite bar of arbitrary cross section with a circular cylindrical cavity. In the analysis the exact solution of the wave equations based on the three‐dimensional theory of elasticity which satisfies the boundary conditions of the inner circular cylindrical cavity has been utilized. The boundary conditions of the outer arbitrarily shaped surface of the bar are satisfied by use of the Fourier expansion collocation method. The analysis derives the frequency equation for determining the phase velocities of the bar of arbitrary cross section with the cavity. As an example numerical calculations have been carried out for an elliptical cross‐section bar with a circular cylindrical cavity.

ISSN:0001-4966    

DOI:10.1121/1.390593

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

An analysis is presented for the vibration and stability of an orthotropic circular cylindrical shell subjected to an axial static load by use of the transfer matrix approach. The applicability of the thin‐shell theory is assumed, and the governing equations of free vibration of the shell are written in a matrix differential equation by using the transfer matrix of the shell. Once the matrix has been determined by a solution to the equation, the natural frequencies and the critical loads are calculated numerically in terms of the elements of the matrix for a given set of boundary conditions at the edges. This method is applied to orthotropic circular cylindrical shells simply supported at the edges, and the effects of the length ratio, orthotropy, and axial load on the vibration and stability are studied.

ISSN:0001-4966    

DOI:10.1121/1.390594

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The thresholds of perception of vibration by recumbent men are a significant index for the governmental agency concerned with regulation of vibration exposure. The thresholds of perception of continuous sinusoidal vibrations and single and multiple bursts of sinusoidal vibrations in the horizontal and vertical directions were investigated in recumbent men. We found that the threshold curves, as a function of the frequency, had different characteristics for vertical vibrations than for horizontal vibrations in the recumbent position. The threshold values for pulsed vibrations became smaller (easier to perceive) as the bursts were elongated until saturation occurred at 4 s duration. Threshold values for pulse durations longer than 4 s were the same as those for continuous vibration. This suggests the existence of temporal integration of sensation of vibrations as in psychoacoustics.

ISSN:0001-4966    

DOI:10.1121/1.390568

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP