摘要:

A numerical method is developed that could predict the pressure distribution of a ducted source from farfield pressure inputs. Using an initial value formulation, the two‐dimensional homogeneous Helmholtz wave equation (no steady flow) is solved using explicit marching techniques. The Von Neumann method is used to develop relationships which describe how sound frequency and grid spacing effect numerical stability. Stability considerations limit the approach to high‐frequency sound. Sample calculations for both hard and soft wall ducts compare favorably to known boundary value solutions. In addition, assuming that reflections in the duct are small, this initial value approach is successfully used to determine the attenuation of a straight soft wall duct. Compared to conventional finite difference or finite element boundary value approaches, the numerical marching technique is orders of magnitude shorter in computation time and required computer storage and can be easily employed in problems involving high‐frequency sound.

ISSN:0001-4966    

DOI:10.1121/1.382353

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

We present a new method, identical in its predictions to the Luneburg–Kline method, for the calculation of differential cross sections of sound geometrically reflected from impedance cylinders in inverse integer powers of ka. The method employs Watson transformation and saddle‐point integration techniques and allows easier evaluation of higher‐order terms than the Luneburg–Kline method, as demonstrated. This is a crucial improvement, as it enables us to determine exactly the number of terms to be retained for the best value for cross section.

ISSN:0001-4966    

DOI:10.1121/1.382354

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Measurements were made at various ranges between 1 and 131 m of the fundamental and second harmonic components of a finite‐amplitude sound wave produced by a 10.2‐cm diam projector driven at 450 kHz in fresh water. Various levels were used, so that nonsaturated, as well as saturated, conditions were obtained. The nonsaturated nearfield levels are in good agreement with the theory of Ingenito and Williams [J. Acoust. Soc. Am. 49, 319–328 (1971)], while the nonsaturated farfield levels agree with the theory of Safar [J. Sound Vib. 13, 1–7 (1970)]. Because the second harmonic component becomes locally generated at ranges of the order of 1/α (α=fundamental absorption coefficient), the Safar theory can be modified to fit the saturated levels at such ranges by correction of the fundamental levels for finite‐amplitude losses. The second harmonic beam patterns at such ranges closely approximate the square of the measured fundamental beam patterns.

ISSN:0001-4966    

DOI:10.1121/1.382355

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Experiments have been carried out on a cylindrical cavity containing hydrogen, the unsteady heat input being supplied by a modulated electric arc. A nonlinear acoustic theory due to Chester [J. Fluid Mech. 18,44–64 (1964)] has been extended to deal with unsteady heat input. the experimental resonance curves are found to be highly distorted, and this distortion is not predicted by the nonlinear theory. The damping appears to be about double the theoretical damping, and the peak amplitude is less than half the theoretical prediction. An approximate theoretical criterion for the appearance of shock waves is derived. No shock waves were observed in the experiments, and this is shown to be consistent with the theoretical criterion.

ISSN:0001-4966    

DOI:10.1121/1.382356

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

An experiment has been conducted in NASA Langley’s free‐jet anechoic facility to assess the effect of a free‐jet shear layer on sound radiation from an acoustic source placed inside the jet: The nozzle diameter was 1.22 m and operated at an exit velocity of 35 m/s. Two different types of sources were used; one was a compact source with a nearly uniform radiation pattern, the other was a noncompact source with highly directional radiation pattern. The acoustic source was driven by by pure tone and 1/3‐octave band white noise. Acoustic measurements were made both inside the jet flow in the potential core region and outside the jet in the acoustic farfield. The effects of mean flow convection and mean shear refraction on sound radiation were determined. The experimental results compared favorably with analytical predictions. The effect of free‐jet turbulence on sound scattering was found to be insignificant.

ISSN:0001-4966    

DOI:10.1121/1.382357

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

We have extended the work of Munk and Zachariasen by deriving an improved apex approximation for the mean‐square phase and phase‐rate fluctuations. Our results are compared with Munk and Zachariasens’s results and with those obtained by numerically evaluating the exact expressions.

ISSN:0001-4966    

DOI:10.1121/1.382358

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Measurements were made of the reverberation resulting from surface backscattering of incident monochromatic acoustic waves at operating frequencies of 175 and 455 kHz. These measurements were used to determine the frequency shift and frequency spreading of the backscattered acoustic field. The frequency shift was found to be in qualitative agreement with simple models of the surface scattering mechanism except that the magnitude of frequency shift was proportional to wind speed only for the lower range of wind speeds. The frequency spreading was observed to increase withdecreasinggrazing angle in contrast with simple model predictions. Asymmetry in the spectrum envelopes, inferred from the measurements made, is attributed to the wide beamwidth in the aximuthal plane of the sonar used.

ISSN:0001-4966    

DOI:10.1121/1.382378

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

This paper considers the acoustical effects of variations of sediment absorption with depth for several sediment types. A simple measure of absorption effects is given by the total absorption along the sub‐bottom ray paths. This total absorption is considered for a variety of absorption and sound‐speed gradients for both semi‐infinite and finite layers. The plane wave reflection loss (bottom loss) is then considered for several treatments of attenuation, layering, and sediment type. It is found that, for clay sediments, plausible variations in absorption gradients can result in variations in bottom loss of several decibels. These conclusions are reinforced by consideration of the absorption gradient effects on normal mode attenuation coefficients.

ISSN:0001-4966    

DOI:10.1121/1.382359

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

We investigate the process of resonance excitation of a spherical, fluid (or gas) filled elastic shell imbedded in another fluid, in the course of scattering of a plane incident sound wave from the shell, using the recently developed formalism of resonance scattering theory. Numerical studies are performed for air‐filled aluminum shells immersed in water. We isolate the resonant component in the scattering amplitude by subtracting a nonresonant ’’intermediate’’ background, thus exhibiting the pure resonance peaks in the frequency region 0

ISSN:0001-4966    

DOI:10.1121/1.382360

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

The empirical relationships linking fish length and target strength for geometric region scattering, which have been developed by Love [J. Acoust. Soc. Am. 62, 1397–1403 (1977)], are shown to apply in the case of peak dorsal values, to some other marine organisms. Peak dorsal target strengths for squid, crab and penaeid prawns lie within 6 dB of the appropriate predictions due to Love. Some zooplankton values are within similar limits provided insonifying frequencies are sufficiently high to ensure that geometric region interactions occur. The target strength data reviewed show little overall frequency dependence in the geometric region, as would be expected if the scatterers were modeled as finite cylinders.

ISSN:0001-4966    

DOI:10.1121/1.382361

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP