摘要:

This analytical study aims to explain the experimental result of the mode coupling of elastic waves in a long circular wire by means of the surface corrugations of the wire. It is assumed herein that the wire is made of ideally homogeneous, isotropic, linearly elastic material and its free surface is subjected to weak and continuous corrugation. On the basis of the weakness of corrugation, the multiple scales are introduced to formulate the problem in a perturbation scheme. It is found, as expected, that the mode coupling does occur as the result of scattering from the corrugated surface at a certain frequency where the two modes have the same phase velocity. Coupled‐mode equations which govern the amplitude variation in the coupling are then derived systematically from the compatibility condition. The coupling coefficients depend on the modes involved, the geometrical characteristics of the surface corrugation, and the material constant of the wire. The resultant variation of amplitudes well characterizes the pulse distortion observed in the experiments.

ISSN:0001-4966    

DOI:10.1121/1.383727

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Waveform distortion in a free field of intense sinusoidal pressure waves propagating in air was measured along the axis of a 122‐mm‐diam plane piston source for four fundamental frequencies, 14.7, 24.6, 35.7, and 63.6 kHz. The acoustic pressure of the fundamental and harmonic components was measured versus source amplitude at distances from 0.15 to 2.00 m from the source at amplitudes up to 160 dBre20 μPa. It was observed (1) that the pressure waveform reaches the stable form of an iterated shock at a smaller distance from the source for larger source pressure amplitudes, (2) that waveform distortion increases with frequency at a given source amplitude and a given distance, and (3) that a limiting level exists for the fundamental amplitude at a given point in a spherically diverging sound field with increasing source level. An empirical propagation relation due to Allen was further developed to include frequency dependence and air absorption. Complete shock formation was inhibited for the highest frequency sound field, presumably because of atmospheric molecular absorption.

ISSN:0001-4966    

DOI:10.1121/1.383791

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Deformations of drops and bubbles opposed by surface tension and driven by radiation stresses at the interface are calculated using spherical harmonic expansions for the radial and tangential stresses. Superimposed acoustic waves produce stresses which oscillate at the difference frequency ω of the waves in addition to static stresses. When the effects of viscosity on the acoustic waves are omitted, the tangential radiation stress vanishes; a procedure is proposed for calculating the radial stresses from the theory for ’’Acoustic Radiation Pressure on a Compressible Sphere’’ [K. Yosioka and Y. Kawasima, Acustica 5, 167–173 (1955)]. The calculation of the response assumes incompressible second‐order flow and omits the body forces which are normally asociated with acoustic streaming. Resonance phase shifts and enhancements of the response should occur when ω is close to the natural oscillation frequency of a mode. Quadrupole resonance phase shifts and enhancements have been observed by the author [J. Acoust. Soc. Am. 67, 27–37 (1980)]. Diverse applications of the theory include the possibilities of: inference of the interfacial tension from the response; emulsification by exciting large amplitude oscillations; and deformation or splitting of bubbles by radiation stresses. The decay time of free oscillation is also calculated; a new term is found which is small but significant for drops surrounded by a liquid and supplements the theory for ’’The Oscillations of a Fluid Droplet Immersed in Another Fluid’’ [C. A. Miller and L. E. Scriven, J. Fluid Mech. 32, 417–435 (1968)].

ISSN:0001-4966    

DOI:10.1121/1.383798

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A new effect of acoustic radiation pressure was detected: the oscillating deformation of mm‐radius liquid drops (para‐xylene) acoustically levitated in a host liquid (water). A novel method is described for detecting μm amplitude deformations which utilized light scattered by the drop at the scattering angle normally associated with the rainbow. This rainbow photometry technique was used to measure the phase and relative amplitude of steady‐state, low‐frequency (?100 Hz) deformations induced by a modulated 217.5‐kHz acoustic wave. Some of the predictions in ’’Shape oscillation and static deformation of drops and bubbles driven by modulated radiation stresses—Theory’’ [J. Acoust. Soc. Am. 67, 15–26 (1980)] agreed with the measurements. Deformations driven by the radiation pressure could greatly exceed the first‐order displacement of the interface. The deformation amplitude varied as the square of the acoustic pressure, and phase had the expected dependence on the modulation frequency when data was normalized according to an experimentally determined damping parameter. The damping, however, was significantly larger than predicted for pure liquids. The interfacial tension inferred from the quadrupole resonance properties was 4% lower than static measurements. The apparatus was also used to levitate superheated drops and the technique may be useful for determining their interfacial tension.

ISSN:0001-4966    

DOI:10.1121/1.383738

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The thermal effect on the propagation velocities in deformed isotropic thermoelastic material are investigated theoretically. First, isotropic thermoelastic materials in which the strain, the temperature or the entropy density, and the temperature gradient are adopted as the independent state variables, are defined. Then, homothermal and homentropic waves, which are purely mechanical waves, are defined. The propagation velocities of the principal longitudinal and transverse waves are determined. The transverse waves of two types have the same velocities, while the longitudinal waves of two types have different velocities, and the difference is the second order of the strain or stress.

ISSN:0001-4966    

DOI:10.1121/1.383786

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A theory of diffraction is presented that is based on the line integral along the free edge of a semi‐infinite barrier. Expressions, which are not integrable analytically, are given for the cases where the source/receiver line is and is not perpendicular to the edge of the barrier. In this latter case the ’’offset’’ between source and receiver along the barrier enters into the mathematical expression in a more complicated way than hitherto supposed. When the barrier is situated on ground of finite impedance, the amplitude of the sound field is no longer expressible as a single constant that can be taken outside the integrand, but becomes a more complicated function than when the barrier is in free space and involves other parameters such as the source, receiver and barrier heights, and the impedance of the ground. These expressions are suitable for calculating (by numerical integration) the sound field behind a barrier due to the combined effects of direct, reflected, and ground waves in the practical situations where source and receiver are within a few meters of the ground and each several meters from the barrier.

ISSN:0001-4966    

DOI:10.1121/1.383787

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The sound field due to a point source behind a barrier on ground of finite impedance has been calculated from five theories that differ mainly in their theoretical approach to diffraction and the model for ground impedance. These predicted values for the sound field have been compared with results measured outdoors using plywood barriers on different combinations of hard and soft ground. Each of these theories allows for interference due to differences between several paths of propagation, determined by the geometry of the source, receiver, barrier, and ground. One of these theories that shows good agreement with measurements, has been extended to calculate the sound spectrum level behind a barrier due to an incoherent line source, and further, to calculate the overall orA‐weighted sound level for a known source spectrum. Results suggest that there is a significant effect, due to the presence of the ground, that is much greater than that due to absorptive properties of the barrier. Results also predict sound level reductions that differ from predictions using well‐known barrier theories (most noticeably a smaller insertion loss): these differences can be of the order of 10 dB(A) depending on geometry, source spectrum, and acoustical condition of the ground.

ISSN:0001-4966    

DOI:10.1121/1.383788

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

An experimental study of the acoustic pressure reflection coefficient at a sudden area expansion in a flowduct in regard to both magnitude and phase has been undertaken. The study was limited to low‐flow Mach numbers, frequencies supporting plane‐wave propagation, and an area transition ratio of 1:4. The magnitude of the coefficient is flow dependent and relatively insensitive to frequency while the phase is frequency dependent and relatively independent of flow. A semi‐empirical analytical model has been symthesized on the basis of available information that incorporates both frequency characteristics and flow affects through second order in the Mach number. The Karal discontinuity correction is important and appears to be a valid representation of the reactive acoustic fields at sufficiently low Mach numbers. The flow primarily influences the acoustic impedance level of both the upstream and downstream ducts and secondarily the acoustic pressure and velocity coupling parameters.

ISSN:0001-4966    

DOI:10.1121/1.383789

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

In order to investigate the influence of foliage on sound transmission through vegetation, we have examined four model forest situations in an anechoic chamber. In general, we found that the foliage acts in the midfrequencies as a noise amplifier, which is important for the vocalization and communication of animals. In the high frequencies the foliage of plants acts as a good noise filter. The acoustic properties of this filter are determined by the plant species, and depend mainly on the maximum size of the plant organs. A clear correlation is found between the maximum size of the plant leaves and the frequency, from which the filtering capacity of the plants starts. Also, a correlation could be detected between the total biomass of a specific plant species and the noise ’’attenuation’’ in the 1/3‐octave bands that are examined. The total noise attenuation caused by the vegetation depends on the spectrum of the noise: the sound pressure level of a noise spectrum with high levels in the high frequencies will be decreased by the plant foliage but the traffic noise spectrum will be changed only in pitch. We conclude that this filtering property of plant communities could be useful in noise abatement.

ISSN:0001-4966    

DOI:10.1121/1.383790

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Many attempts to measure source terms of jet noise by means of jet flow–jet noise cross correlations have been only partially successful. The major difficulty is suspected to be spurious ’’probe noise’’ generated by turbulence–sensor interaction. The present analysis predicts the effects of probe noise on measured cross correlations and cross‐spectral densities. The description relies on Ribner’s self and shear noise model, a development of Lighthill’s theory of jet noise. Predictions are supported by jet noise–jet flow correlations measured with a hot wire and with a nonintrusive device: a Laser Doppler Velocimeter (LDV). The hot wires are found to generate spurious but well correlated noise. This leads to major errors in jet noise diagnostics of low‐speed flows.

ISSN:0001-4966    

DOI:10.1121/1.383792

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP