摘要:

An expression for the wave number of the fundamental mode in a uniform duct of arbitrary cross section is obtained correct to first order in V/cand inc1/c0, where V is the velocity of the fluid in the pipe andc=c0+c1is the local sound speed withc0=constant. Correction terms are also obtained. The calculation applies to a fluid with the equation of state of an ideal gas or to a liquid and is valid for frequencies well below cutoff. The speedcand velocity V can be arbitrary functions of position across the cross section given only ∇⋅V=0. For the gas, the wave number, which is a constant, can be expressed in terms of the average density and the total mass flow rate.

ISSN:0001-4966    

DOI:10.1121/1.381596

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A new type of receiving array has been designed and constructed for the purpose of visualizing an object by using the ultrasonic wave at low frequency. The array consists of 24 receiving elements around three coaxial circles on a spherical surface. The multiplication of three additive outputs, each of which is the sum of the eight element outputs on a circle, is performed for the signal processing of the array. After theoretical and experimental studies, it has been proven that the array proposed is appropriate for the high‐directional receiver in the ultrasonic visualization system. A preliminary experiment on imaging the object in the air has been carried out.

ISSN:0001-4966    

DOI:10.1121/1.381602

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The purpose of this paper is to show that the essential properties of the propagation of noise over real soil surfaces can be modeled using the properties of fibrous absorbent materials and the appropriate theory of sound propagation along a finite impedance boundary. This work is an expansion of the original proposals of Delany and Bazley [J. Sound Vib. 16, 315–322 (1971)]. The model predictions are shown to be in good agreement with a variety of experimental ground absorption measurements. The application of the model to the measurement of aircraft reverse thrust noise is described.

ISSN:0001-4966    

DOI:10.1121/1.381603

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The reflexion of sound at an interface between two fluid half‐space in relative motion (one in motion and the other at rest), but with different density and different sound speed, is considered. The reflexion coefficient is found in an explicitly closed form. A brief discussion of the effect of mean velocity, i.e., corresponding Mach number on different noise generating quadrupole directivities concludes the presentation of the paper. The graphs provided illustrate this aspect in a rather more convincing way.

ISSN:0001-4966    

DOI:10.1121/1.381604

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

In February 1975, a low‐frequency acoustic propagation experiment, Project TASMAN TWO, was conducted in the South Tasman Sea to the west of the South Island of New Zealand. The aim was to study low‐frequency acoustic propagation in a region having considerable variations in oceanic environment. Sound signals, set to detonate at the SOFAR axis, were air dropped out to a range of 3000 km along paths chosen to sample three distinct water masses (Subtropical Water, Australasian Subantarctic Water, and Circumpolar Subantarctic Water) and to traverse a large bathymetric feature (the South Tasman Rise). In addition, convergence zone (RSR) propagation in subtropical water was investigated with the sound signals set to detonate at 18 m. The results show that the three individual water masses have significantly different attenuations at frequencies below 500 Hz. It was established that Circumpolar Subantarctic Water has an attenuation nearly five times greater than Subtropical Water. For SOFAR propagation conditions, major acoustic transmission discontinuities were not always observed at the boundaries between the water masses, but strong shadowing effects with level changes of as much as 15 dB were clearly associated with the bathymetric feature. The experimental data for both SOFAR and RSR propagation are compared with predictions made using the parabolic equation model. Although the predictions match most general trends observed in the data, poor‐detailed agreement was achieved where rapid horizontal gradients in either sound speed or water depth were experienced.

ISSN:0001-4966    

DOI:10.1121/1.381605

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

An analytical approach is used to obtain an approximate solution for deep‐ocean mesoscale eddies, including depth‐dependent effects. The solution is used in the development of an environmental‐acoustics model which relates acoustically relevant quantities, such as sound‐speed and current distributions, to eddy parameters. Parameters of the model are depth of influence, radius, rotational direction and maximum speed, and translational velocity. An application to a particular Gulf Stream ring is made, and the resulting current and sound‐speed structures are shown to be in qualitative agreement with observations. Then, general results are presented for rotational current structure, maximum horizontal sound‐speed change, and maximum SOFAR‐axis elevation as functions of eddy radius and peak current speed. It is shown explicitly how these quantities change significantly with eddy size and strength. This model provides a basis for subsequent analytical studies of sound transmission through an arbitrary eddy or eddy field.

ISSN:0001-4966    

DOI:10.1121/1.381606

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A general scheme is outlined to explain excess propagation loss in sound channels in which the stochastic variability of the environment is combined with the usual deterministic effects on propagation loss. As a result, the excess loss in a shallow‐water sound channel is a function of both the bottom loss and the stochastic variability of the environment. A method, previously reported by the author [Acustica 35, 18 (1976)], to ’’handle’’ stochastic sound‐speed variations in ray‐tracing routines is applied to model the excess propagation loss for the Hudson Bay data. The model results are in excellent agreement with the experimental data without fitting the parameters and exhibit the dependence predicted above. Additional results show that difficulties usually inherent in deterministic ray tracing as, for example, the focusing of energy at the source depth and sharp caustics may easily be overcome by using a stochastic ray‐tracing approach.

ISSN:0001-4966    

DOI:10.1121/1.381607

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The sound from a steady distant sinusoidal source in the sea fluctuates in amplitude because of multipath interferences, surface motion, internal waves, microstructure, and other propagation effects. Such fluctuations are found to follow a Rician or modified‐Rayleigh distribution having as a parameter the fraction of random power in the received signal. At the output of conventional sonar processor—namely, a narrow‐band filter, a squarer, and an integrator—the fluctuation statistics are determined by the propagation processes occurring in the sea between source and receiver. On the other hand, ambient sea noise is found, from analyses of field recordings, to have fluctuation statistics determined by the processor itself; ambient noise samples at the processor output obey a chi‐square distribution having a number of degrees of freedom equal to twice the bandwidth‐time product of the processor, as would be expected from a Gaussian input. The two distributions—Rician power for signals and chi‐square for noise—while formally different, have remarkable similarities in the limits. In short, the output fluctuation statistics of narrow‐band signals and Gaussian ambient noise can apparently be predicted from estimates of the degree of randomness introduced by the prevailing propagation conditions, and from a knowledge of the processor, respectively, provided these statistics remain stationary during the analysis period.

ISSN:0001-4966    

DOI:10.1121/1.381608

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A model study using coherent ray‐tracing methods was based on a cold‐water eddy measured in the Altantic. The eddy, whose diameter is about 320 km was passed through the propagation path at a rate of 4 km/day between a fixed source and fixed receiver separated 445 km. A brief description of the range‐dependent model required for this study is presented. The major effects of the eddy were in changing the ray type (number of cycles, RR–RSR, etc.) and in the travel time which had a major effect on the multipath resultant. Time series of amplitude and phase are presented with a qualitative comparison to previously measured data.

ISSN:0001-4966    

DOI:10.1121/1.381609

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The simultaneous transmissions of acoustic pulses in opposite directions between two point in midocean has been used to separate current effects on acoustic propagation from the effects of sound‐speed structure. Two 2250‐Hz sources were suspended at about 1‐km depth from ships 25 km apart. Differences in travel times of the oppositely traveling pulses are interpreted in terms of ray‐averaged currents. The measured values contain an unknown contribution from ship drift, but differences between upper and lower ray paths present in this experiment are insensitive to drift. Further, the nonreciprocity in pulse shape and amplitude of opposing transmissions is found to be of the same order as the fluctuations in time of one‐way transmissions. This demonstrates that currents contribute to acoustic scattering.

ISSN:0001-4966    

DOI:10.1121/1.381619

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP