摘要:

The effects of external hydrostatic pressure fields and fluid loading on the structural and acoustic response of a point‐driven infinitely long circular cylindrical shell were examined over a range of frequencies. The external pressure field was modeled using static prestress terms in the shell equations of motion, and the structural response was characterized by the driving point admittance and the circumferential resonant frequencies. The acoustic response was quantified through calculation of the radiated sound power, both in an overall sense and on an individual modal basis. The analysis was performed for the in‐air case as well as for the in‐water case. The structural response was found to be strongly affected by fluid and pressure, resulting in significant resonant frequency shifts. However, the overall acoustic response was shown to be nearly independent of the external pressure field, both in air and in water. In addition, it was shown that relatively fewer modes contribute significantly to the sound radiation for the submerged shell as compared to the shell in air. In both cases, the sound generation was controlled by the low‐order nonresonant modes.

ISSN:0001-4966    

DOI:10.1121/1.393755

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The properties of a dual‐temperature resonant chamber to be used for acoustical levitation and positioning have been theoretically and experimentally studied. The predictions of a first‐order dissipationless treatment of the generalized wave equation for an inhomogeneous medium are in close agreement with experimental results for the temperature dependence of the resonant mode spectrum and the acoustic pressure distribution, although the measured magnitude of the pressure variations does not correlate well with the calculated one. Ground‐based levitation of low‐density samples has been demonstrated at 800 °C, where steady‐state forces up to 700 dyn were generated.

ISSN:0001-4966    

DOI:10.1121/1.393450

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Sound‐pressure levels of pure tones between 1 and 16 kHz, generated by a point source on hard ground, were measured as a function of height at horizontal distances up to 21 m. In another experiment, measurements were made above hard ground and ground of finite impedance at horizontal distances up to 250 m using frequencies between 250 and 4000 Hz. Five frequencies were generated simultaneously and measured at four (and sometimes five) heights. The sound‐pressure levels below the refractive shadow boundary are compared with a simple theory that assumes a linear change of sound speed with height. This vertical gradient was obtained by approximating the temperature and wind velocity profiles measured during the acoustical experiment. Good agreement between theory and experiment was obtained at all distances and frequencies. However, at the largest distances and highest frequencies, there was evidence of additional energy penetrating the refractive shadow from scattering by atmospheric turbulence. Therefore, in addition to their vertical gradients, the temperature and wind velocity fluctuations were also recorded. These were used to obtain the strength and scale of turbulence, and the analysis of these results is summarized. The amplitude and phase fluctuations predicted from a simple theory that approximates the source region of the turbulence by a Gaussian spectrum were compared with measurements. In particular, transverse coherence length decreases deep within the refractive shadow compared with its predicted values and with values measured above the shadow boundary.

ISSN:0001-4966    

DOI:10.1121/1.393451

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

In an earlier paper, the present authors’ work in adapting the fast field program (FFP) formulation to atmospheric propagation above a complex impedance boundary was described. It was found that numerical overflow problems for high frequencies and multiple layers limited the utility of the FFP in solving atmospheric problems. In this paper is a description of a new formulation which eliminates the overflow problems inherent in the earlier formulation. The results of these two formulations are compared for a test case and the superiority of the new formulation is demonstrated. Results of the FFP2 for a simple atmospheric profile are compared with field measurements and the applicability discussed.

ISSN:0001-4966    

DOI:10.1121/1.393452

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

In this work, the method of propagating the mutual coherence function through a deterministic environment using geometric acoustic Green’s functions is shown. It is also demonstrated how WKB Green’s functions for the acoustic field result in the locally quadratic approximation for the coherence function. Next, it is shown that an exact channel function can be constructed which smooths, or filters, the locally quadratic approximation and results in the exact mutual coherence function in a range‐independent environment. Calculating the coherence function with WKB Green’s functions reveals multipath interference which is not properly accounted for by the locally quadratic radiative transport equation. When multipaths coalesce to form a smooth caustic, extensions of the locally quadratic transport equation give correct results only because interfering rays sample only a limited part of space. In the case of smooth caustics, it is shown that the coherence function as calculated from geometric acoustics agrees in detail with recently derived results for the coherence function which are based on the extended transport equation.

ISSN:0001-4966    

DOI:10.1121/1.393453

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The sensitivity of a passive horizontal‐tracking algorithm to variations in input measurements is investigated. The algorithm determines estimates for depth, range, bearing, horizontal speed, course, and frequency for a cw acoustic source moving with constant velocity at fixed depth. The receiver is a horizontal linear array towed at a constant depth. Both source and receiver move in the upper portion of a deep ocean and are separated by a relatively short range. Dominant acoustic signals are presumed to arrive along two upper‐ocean ray paths. The algorithm uses a new combination of input quantities, including multipath information, Doppler frequency shifts, and array directional measurements. Procedures are developed for analyzing effects of input‐measurement errors on source localization. The robustness of the algorithm to small variations in acoustic measurements and environmental parameters is demonstrated for a variety of source–receiver configurations. Variance estimates of position and motion are obtained in terms of input‐measurement variances. Bounds on tracker performance are developed for measurements that are affected by noise. Results from the several types of analyses corroborate the sensitivity characteristics of the algorithm.

ISSN:0001-4966    

DOI:10.1121/1.393454

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

When a low‐frequency point source radiates sound close to a low‐roughness, steep‐sloped, rigid surface, multiple coherent forward scatter creates a boundary wave which propagates with cylindrical divergence and dispersion in the fluid region near the surface. Measurements using laboratory models of surfaces with two‐ or three‐dimensional roughness elements, and a generalization of periodic roughness theory, have been used to develop empirical formulas which describe the amplitude and phase velocity of the coherent boundary wave over natural, randomly rough, rigid surfaces. The formulas are stated in terms of the average height and the rms slope of the steep‐sloped roughness elements.

ISSN:0001-4966    

DOI:10.1121/1.393455

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The interaction between a thin plate (kH≲1,kbeing the wavenumber andHthe plate thickness) and a compressible fluid half‐space through a slightly rough boundary may be represented by the standard thin plate equations plus a smoothed boundary condition for the displacement of the fluid normal to the plate. This replaces the rough boundary by a fictitious smooth plane at which the relation ∂φs/∂z=ηφ1must be obeyed, where φsis the perturbation of the smooth boundary solution due to roughness, φ1is the total acoustic potential in the fluid at the smooth planez=0, and η is a constant which, for isotropic roughness or for corrugations, is defined by two scatter parameters. This condition is isomorphic to those used for other types of rough interfaces [I. Tolstoy, J. Acoust. Soc. Am.75, 1–22 (1984)]—the main difference residing in the numerical values of the parameters. The most important effects are connected with theflexuralmodes of the plate. Here, the roughness causes energy losses into incoherent acoustic scatter and introduces an important level of attenuation. Harmonic point source solutions are obtained for several types of interface roughness, showing that, for moderate and large rangesr(kr≳102), this effect introduces an effective cutoff frequency for κh≲1, κ being the horizontal wavenumber of the flexural waves andhthe spacing between the bosses (or corrugations of the roughness model). The effect of the roughness on the flexural wave dispersion is of the order of several percent, and should be measurable for plates of relatively low flexural rigidity.

ISSN:0001-4966    

DOI:10.1121/1.393456

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A simple geometric‐acoustic model (used previously to predict time spreads of reflecting signals) is slightly modified and extended to predict frequency and angle spreads, which can be used to predict coherence losses to sonar systems. In that model, the expected energy received from a boundary facet (via reflection) is computed as a function of thexandycoordinates of the facet. A ray arriving from any facet has a well‐defined travel time, relative‐frequency shift (for a given source and receiver motion), polar arrival angle, and azimuthal arrival angle. The frequency‐spread function is defined as the histogram of expected received energies as a function ofQ=( f−f0)/ f0, wheref0andfare, respectively, sent and received frequencies of a transmitted cw signal. The angle‐spread function is similarly defined for the polar and azimuthal angles. The angle‐spread function shows the well‐known ‘‘shifting toward the horizon’’ of the reflected image of a source as the reflecting surface gets rougher. An analogous effect occurs for frequency spread. Because the horizon limits Doppler and angle spread, increased roughness sometimes narrows the spread functions instead of broadening them. Coherence‐loss computations are performed using a plane‐wave approximation local to the receiver.

ISSN:0001-4966    

DOI:10.1121/1.393457

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Time‐domain first‐Born approximations to backscattering of observed incident elastodynamic pulses by isolated cylindrical voids or inclusions are produced with rapid and computationally inexpensive procedures using polynomial splines represented as a series of basic splines. The approximations are in good qualitative agreement, and fair quantitative agreement, with observed backscatter.

ISSN:0001-4966    

DOI:10.1121/1.393458

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP