摘要:

The resonance formalism of nuclear‐reaction theory is applied to the problem of sound scattering from submerged elastic bodies (illustrated here by circular cylinders and spheres). It is demonstrated that the strongly fluctuating behavior of, e.g., the backscattering cross section is caused by a superposition of generally narrow resonances in the individual normal modes (partial waves), which move up in frequency from one partial wave to the next, corresponding to a series of creeping waves (’’Regge poles’’), and which are superimposed on a background of rigid‐body (potential) scattering. This fact, together with a resonance representation of the elastic field in the interior, indicates that the elastic body is relatively impenetrable to the incident wave except in the vicinity of the resonances, which occur at the eigenfrequencies of the elastic vibrations of the body. Various types of interference between resonance and background are analyzed, and the phase of the partial wave is shown to undergo a jump of π at each resonance. Decay times (ringing) of the excited resonances are found to depend inversely on their width, and the appearance of nulls in the scattering angular distribution at certain resonances is related to the cross section of the rigid body.

ISSN:0001-4966    

DOI:10.1121/1.381780

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Distant shipping noise modeled by multiple phase‐random line components has a probability density and statistics easy to state in generality but difficult to compute in particular. To obtain numerical results with reasonable effort, previous computations have been based on Gaussian approximations to the densities. Here we show that an Edgeworth‐series approximation to the density greatly improves the quality of the result, without adding much to the computational effort. Also, to justify adoption of the Edgeworth series, and to further develop the theoretical framework for distant shipping noise, exact analytical results are presented for several new cases of multiple line components.

ISSN:0001-4966    

DOI:10.1121/1.381781

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

For an ocean sound channel whose environmental parameters depend not only on depth, but in a gradual fashion also on range, the wave equation may be separated by the adiabatic range variation method of Pierce [J. Acoust. Soc. Am. 37, 19 (1965)]. This method is used here to calculate underwater sound propagation in a channel with arbitrary (but gradual) range dependence, and also with arbitrary depth dependence of the sound velocity profile, by employing Airy function solutions of segmentwise linearized problems. Our results are illustrated for a realistic deep‐water propagation case with profile data collected in the western North Atlantic, as well as a shallow‐water example from the Norwegian Sea, and compared against the experimental transmission loss data, and the results of calculations using other methods for the same cases.

ISSN:0001-4966    

DOI:10.1121/1.381782

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The sensitivity of bottom reflection loss to a density gradient is examined within the context of a physically meaningful model consisting of a fluid sediment layer having depth‐dependent density and sound speed overlying a substrate having solid properties. The bottom grazing‐angle range is divided into two regions, the low‐angle and the high‐angle regions. In the low‐angle region sediment sound‐speed gradients refract incident energy upward before it encounters the substrate. In the high‐grazing‐angle region, energy impinges on the substrate. Through the use of a numerical plane‐wave reflection coefficient model it is shown that the effect of a density gradient on low‐angle bottom loss is small. An expression indicating this effect is derived and is shown to agree with numerical bottom‐loss calculations. The high‐angle bottom loss is more influenced by a density gradient because of the impedance changes at the sediment–substrate interface which accompany a density gradient. It is shown that introduction of density gradient of 0.002 g/cm3/m can cause the high‐angle bottom loss to change on the average by 12.5%, 7%, and 3% in 100‐m clay, silt, and sand layers, respectively.

ISSN:0001-4966    

DOI:10.1121/1.381783

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A series of measurements of the scattering strength of wind‐driven water surfaces at short wavelengths (circa1 mm) have been performed. Bistatic azimuthal configurations ranging from 0° (backscatter) to 180° (specular direction) were used at three grazing angles and two conditions of roughness. Orientations with respect to the wind over a range of 180° were also studied. The experiments were conducted with short, relatively narrow‐band probing signals under computer control. The data were converted to scattering strength using a flat‐surface reference and a computed compensation for beam patterns and source and receiver time gating. The results of these measurements indicate that wind direction is an important factor in determining reverberation from the surface. Wind speed, hence roughness was confirmed to be an equally important factor, but Rayleigh parameter, an often used scalar index of effective roughness, was shown to be insufficient to predict reverberation. At short wavelengths, scattering strength was found to be insensitive to small changes in wavelength as expected. Backscatter strengths measured at 8.3 m/s at crosswind were found to agree closely with a relation proposed by Marsh [J. Acoust. Soc. Am. 35, 240–244 (1963)] relating backscattering strength to grazing angle. No satisfactory predictive theory, however, seems to be available for the general case of bistatic scattering.

ISSN:0001-4966    

DOI:10.1121/1.381784

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Schlieren visualization of ultrasonic waves is usually carried out at megahertz frequencies since it is generally realized that lower‐frequency waves are much more difficult to observe. This paper considers the factors involved in the frequency dependence of the sensitivity of an ideal diffraction‐limited schlieren system. The results indicate that relatively low‐amplitude sound waves can be visualized even at low‐kilohertz frequencies. A techique is suggested which would make the sensitivity at these low frequencies comparable to that at the higher frequencies.

ISSN:0001-4966    

DOI:10.1121/1.381785

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

It is shown that some well‐known formulas connecting viscous drag and thermal transfer in the continuum to the free molecular flow regime can be used to extend the Temkin–Dobbins formulas for sound dispersion in a dusty gas to very small particle sizes.

ISSN:0001-4966    

DOI:10.1121/1.381786

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A standard optical Schlieren method has been adapted to the problem of measuring the speed of sound in moderately supercooled and in superheated liquids under atmospheric pressure. The estimated accuracy is ±3 m/s. Results in supercooled water down to −16.75 °C and in superheated water up to 176.5 °C are reported.

ISSN:0001-4966    

DOI:10.1121/1.381787

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Harmonic wave propagation is considered in a two‐layered cylinder of dissimilar but transversely isotropic materials such as bone. The model includes the approximate piezoelectric behavior of long bone and the frequency equation is shown to constitute a 16th‐order determinant. A frequency equation is also derived for the nonpiezoelectric case. Though the frequency equations are much too complicated to be treated analytically, they are shown to lead to simpler forms for the special cases of axial symmetry, very long and very short wavelengths. In the absence of established values for the piezoelectric moduli of bone and of the elastic constants of the inner spongy layer, a numerical solution to the model does not seem worthwhile at present.

ISSN:0001-4966    

DOI:10.1121/1.381788

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

It has been suggested that the frequency spectrum of an acoustic emission burst may be a partial ’’signature,’’ identifying the source. To explore this possibility we have studied in some detail, theoretically and experimentally, two qualitatively different simulated acoustic emissions. The first is the fracture of small (20–40 μm) silicon carbide grains on a steel slab. The second is the impact of a small elastic sphere of various materials on the same slab. Our primary objective has been to test our understanding of the medium response, and, consequently, the extent to which we can determine the source spectrum from the observed spectrum.

ISSN:0001-4966    

DOI:10.1121/1.381789

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP