摘要:

Nonlinear sound waves from a uniformly moving source with dimensions smaller than the wavelength of the emitted sound are investigated. They are described by spherical Burgers’ equations with parameters depending on the source velocityVand the direction angle θ from the source to the point of observation. It is seen that for certainVand θ values, both forVless than and greater than the sound velocity in the medium, shock waves occur, which do not occur in nonlinear waves from a fixed sound source.

ISSN:0001-4966    

DOI:10.1121/1.391779

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

We derive effective equations for wave propagation in bubbly liquids which include bubbles interactions effect. We homogenize both the linear and the nonlinear versions of the microscopic equations by two methods. The interaction between the pressure fields is found to increase the speed of sound relative to the Foldy approximation.

ISSN:0001-4966    

DOI:10.1121/1.391780

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

An experiment was conducted in a shallow water region of the Mediterranean Sea to study wind‐generated noise. In addition to measuring the noise field, propagation‐loss data were collected and used in a detailed modeling of the environment. The environmental information was then used as input to a noise model based on wave theory that computes the noise field in the water column for a given (unknown) source strength. By comparing model predictions with data, the influence of the environment on recorded noise levels could be removed and a measure of the noise source spectrum levels obtained as a function of wind speed. It was found that noise levels correlate better with wind speed than with wave height. In addition it was found that the nearfield contribution dominates the noise level, with the result of producing virtually constant noise intensity over depth from moderate to high wind speeds and frequencies above 200 Hz.

ISSN:0001-4966    

DOI:10.1121/1.391781

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Perturbation theory and boss models for rough surface scattering are compared for the case of a surface bossed with oblate hemispheroids (heighta≤radiusb). In particular, the surface consists of identical, hard, hemispheroidal bosses sparsely and independently distributed on a hard plane by means of a uniform probability law. To apply perturbation theory we compute the surface correlation function, operate on that function, and compute an effective boundary admittance. Finally, we compare that admittance with (farfield) near‐exact results for hemispherical bosses and for oblate hemispheroidal bosses. Calculations of the magnitude of the reflection coefficient ‖R‖ are presented showing that for low frequencies, i.e.,kb=0.1, the discrepancy is approximately 70% fora=bbut less than 10% fora/b=0.1. In general, the error decreases as grazing angle increases and decreases asa→0. Thus, at low frequencies perturbation theory is shown to give excellent results for ‖R‖ whena/b≪1, despite the discontinuous and infinite slope in the surface where the bosses meet the base plane. We also examine the effects of increasing frequency for hemispherically bossed surfaces, and in particular, we see errors as large as 3 dB for sparse, hemispherical bosses witha=b=5 m and frequencies less than 60 Hz. We conclude that perturbation theory is excellent in the case of oblate hemispheroids, shows significant percentage errors for hemispheres and should not be used for prolate hemispheroids.

ISSN:0001-4966    

DOI:10.1121/1.391731

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

We study the elastodynamic spectral response of thick and thin shells in water when they undergo resonance scattering caused by the incidence of sound waves that impinge upon them at selected aspects. All the shells considered are elastic, air‐filled, and of cylindrical and prolate spheroidal shapes. Their thickness is intentionally varied three orders of magnitude from very thick (i.e.,h≡1−b/a=90%) to very thin (i.e.,h=0.1%), so that the effects of shell stiffness on their scattering behavior can be quantitatively analyzed and understood. The quantitiesaandbare the outer and inner shell radii, respectively. Material composition effects are studied by comparing various metals such as steel and aluminum. The shell motions are described by the exact equations of elastodynamics in all cases, so that comparisons may be drawn concerning the regions of applicability of pertinent shell theories. For the spheroidal shells no exact solution is possible, thus, we have presented results based on an extendedT‐matrix method that still makes use of the exact 3‐D equations of elastodynamics to describe the shell vibrations.For a wide range of shell thicknesses and compositions, we have computed the positions of the poles of the scattering amplitudes, in the complex frequency planex1. Here,x1isk1a, wherek1=ω/c1, ω is the angular frequency of the incident wave,c1is the sound speed in the outer medium, andais the shell’s outer radius. These poles group themselves in certain families and give rise to surface waves that circumnavigate the shell, either in the water side (Franz or SEM‐type poles or waves) or inside the shell metal (Rayleigh/whispering gallery waves for solid bodies, or Lamb momentless and flexural waves for shells). This latter set of waves is the least attenuated and the most dominant. We have shown how the pole families affect the spectral shape of the form function, and that of it’s partial waves, with their interacting modal backgrounds and resonances. As the shells become thinner, only one‐pole family is shown to remain within the displayed boundary of the complexx1plane. This single‐pole family becomes responsible for the generation of the first‐order set of symmetric/antisymmetric Lamb waves in the shell. All these concepts are illustrated with many examples which show the crucial importance of the pole‐position diagrams to understand the spectral behavior of the form function, and the way it reveals information about the shell and the types of waves it supports on its surfaces. A transition from the rigid to the soft background is observed as the shells become very thin, and it is used to properly isolate the resonances of very thin shells. Extensions of these spectral results to the time domain are forthcoming.

ISSN:0001-4966    

DOI:10.1121/1.391732

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Sound transmission losses for a single layer wall are measured in a reverberation room facility using conventional methods (ASTM E90) and the more recently introduced sound intensity method. The specimen is placed in five positions in the tunnel between the two reverberation rooms and measurements are made for four different absorptive conditions in the smaller room. Significant differences between the two measurement techniques are found at low frequencies and at high frequencies. Low‐frequency differences are attributed partly to the increase in sound energy close to the surfaces of the receiving room (the Waterhouse effect) and the lack of any consideration of this in standard test methods. Inclusion of the Waterhouse term in the conventional transmission loss formula improves the agreement between the conventional and the sound intensity measurement technique at low frequencies. High‐frequency differences are not explained.

ISSN:0001-4966    

DOI:10.1121/1.391733

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The mass conservation law is combined as one of the utilizable physical constraints with differential time‐of‐flight data to improve the tomographic estimation of flow‐velocity distribution by means of inverse operation of the matrix. The formulation and computer simulation are described.

ISSN:0001-4966    

DOI:10.1121/1.391734

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A model of cochlear macromechanics, involving a viscous cochlear fluid, is analyzed asymptotically for slowly varying membranes. The results exhibit the effect of viscosity on the amplitude, wavelength, and damping rate, and confirm the conclusion that viscosity is not important. However the method can be used on other models, such as those of cochlear micromechanics, where viscosity is more important.

ISSN:0001-4966    

DOI:10.1121/1.391735

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Rabbits were conditioned to respond behaviorally to auditory stimuli by pairing a white‐noise conditioned stimulus (CS) with a corneal airpuff unconditioned stimulus (US). The conditioned response (CR) was movement of the nictitating membrane (NM). After the subjects were responding at better than the 90% correct level, the intensity of the auditory stimulus was reduced to behavioral threshold using a staircase procedure. Simultaneous measurements of neural unit activity and behavioral NM responses were then made in rabbits performing at behavioral threshold. After the experiment was completed neural unit responses during behavioral detection trials were compared to neural responses made during nondetection trials. Neural unit responses to a constant intensity, white‐noise stimulus at behavioral threshold were well defined and essentially identical on behavioral detection and nondetection trials in the ventral cochlear nucleus, the ventrolateral division of the central nucleus of the inferior colliculus, and the ventral division of the medial geniculate body. This suggests that an auditory stimulus can be neuronally ‘‘detected’’ without being behaviorally detected, and that the neural ‘‘decision’’ to respond behaviorally is not made in these nuclei. Responses recorded from the dorsomedial division of the central nucleus of the inferior colliculus, the pericentral nucleus of the inferior colliculus, and less commonly in the medial division of the medial geniculate body were also clearly present and nearly identical during the onset of the auditory stimulus, but were sometimes consistently different for detection and nondetection conditions during the latter part of the auditory stimulus. These brain regions appear to receive both auditory and nonauditory inputs, and show responses which are more highly correlated with detection behavior.

ISSN:0001-4966    

DOI:10.1121/1.391736

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

In a previous paper [R. Lutfi, J. Acoust. Soc. Am.73, 262–267 (1983)], the following rule was proposed for predicting masking by pairs of simultaneous maskers;Xab=[XPa+XPb]1/P, where in units of power,XaandXbare the individual masking effects of the maskers,Xabis the combined effect, and 0.20≤p≤0.33. In this paper, the rule is used to predict the results of studies in the literature that have measured masking by sounds with various other complex spectra. In most of these studies, the individual maskers comprising the complex have nominally nonoverlapping power spectra. A single value ofp=0.33 yields predictions in good agreement with the data of these studies. For a study in which the component maskers overlap more appreciably, a larger value ofp=0.50 produces equally accurate predictions. The rule also predicts some general features of the results of studies in which the individual effects of the maskers in the complex are not known but can be estimated. It is suggested that the general applicability of the rule reflects a conjoint analysis by the auditory system of two or more waveform statistics.

ISSN:0001-4966    

DOI:10.1121/1.391737

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP