摘要:

Data and idealized curves are presented for a number of surveys on the threshold of hearing of persons who were not exposed, by and large, to intense workplace noise. From these results, and on the basis of certain assumptions, new generalized functions are presented to show pure presbycusis (aging) and sociocusis (non‐work‐noise‐induced hearing loss); typical presbycusis; typical sociocusis; and typical presbycusis‐plus‐sociocusis. Hearing level surveys, conducted in industrialized societies to reveal presbycusis, appear to reflect the joint effects of presbycusis and sociocusis, especially in males, and factory workers may suffer more sociocusis and a greater degree of presumably non‐noise‐related otological disorders (nosocusis) than are found in the general population.

ISSN:0001-4966    

DOI:10.1121/1.389580

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A dynamic approximate theory capable of predicting high‐frequency behavior of cylindrical thermoelastic bars is developed using a new theory. The cross section of the bar has an arbitrary shape and contains an arbitrary number of holes. The approximate theory is valid for all of the deformation modes such as flexural, longitudinal, torsional, etc. The use of the new method permits one to eliminate any inconsistency which may occur between lateral boundary conditions and the distributions of displacements or temperature assumed over the cross section of the bar. Accordingly, the method enables one to correctly describe the reflections of the waves propagating along the bar. This, in turn, makes the dispersive characteristics of waves in bars predicted by the approximate theory agree with those obtained from the exact theory without having to introduce any matching coefficients into the approximate theory.

ISSN:0001-4966    

DOI:10.1121/1.389581

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

In order to show and illustrate the power of the theory proposed in Part I [J. Acoust. Soc. Am.73, 1918–1922 (1983)] for the dynamic behavior of thermoelastic bars, the theory is applied to bars with circular and rectangular cross sections. In these applications, the order of the theorymis chosen to be 3. The general equations of the approximate theory governing all of the deformation modes (such as longitudinal, flexural, torsional, etc.) of circular and rectangular bars, and accomodating the thermal effects are presented. With the object of assessing the approximate theory, approximate and exact dispersion curves are compared for the flexural and longitudinal waves propagating in circular bars. A good match between these two is obtained without using any correction factors in the approximate theory. Further, experimental wave profiles are compared for longitudinal waves in rectangular bars. It is observed that the two agree quite well.

ISSN:0001-4966    

DOI:10.1121/1.389582

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

An experiment to verify the basic linearity of fisheries acoustics is described. Herring (ClupeaharengusL.) was the subject fish. Acoustic measurements consisted of the echo energy from aggregations of encaged but otherwise free‐swimming fish, and the target strength functions of similar, anesthetized specimens. Periodic photographic observation of the encaged fish allowed characterization of their behavior through associated spatial and orientation distributions. The fish biology and hydrography were also measured. Computations of the echo energy from encaged aggregations, derived by exercising the linear theory with the target strength functions of anesthetized fish and gross behavioral characteristics of encaged fish, agreed well with observation. This success was obtained for each of four independent echo sounders operating at frequencies from 38 to 120 kHz and at power levels from 35 W to nearly 1 kW. In addition to demonstrating the basic linearity of fisheries acoustics, the experiment verified both conventional acoustic measurements on anesthetized fish, at least for averaging purposes, and the echo integration method. Two simple theorems summarizing the meaning of linearity for use with the echo integration method are stated.

ISSN:0001-4966    

DOI:10.1121/1.389583

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Acoustic wave attenuation due to multiple scattering in a two‐phase medium consisting of a fluid with embedded rigid, fluid, or elastic particles of varying sizes is discussed. The formulation, involving the exciting and scattered fields of an incident acoustic plane wave, is based on theT‐matrix method. The propagation features of coherent waves in the mixture are described by the dispersion equation which is derived by applying standard statistical approximations to the discrete random medium. Special attention is focused on the pair‐correlation function between the scatterers using the self‐consistent approximation (SCA) which seems better than the Percus‐Yevick approximation (PYA) when the volume fraction becomes significant. Besides deriving low‐frequency analytical results for coherent wave speed and attenuation, the dispersion equation has been solved numerically for higher frequencies for particles with uniform and Gaussian size distributions.

ISSN:0001-4966    

DOI:10.1121/1.389584

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Weak sparks, of length 0.5–1.0 cm and energy per discharge 0.01–0.1 J, served to produce intense acoustic transients resemblingNwaves. Amplitude decay and waveform elongation were studied, for propagation distance up to 2 m, through the use of a wideband capacitor microphone with essentially uniform response from dc to 1 MHz. Within the range of propagation distances for which the first (compression) phase of theNwave was completely formed, the duration of this compression phaseTand its amplitudepswere found to agree with the theoretical relationsT=T0[1+σ0 ln(r/r0)]1/2andps=(r0 ps 0/r)[1+σ0 ln(r/r0)]−1/2, where σ0is a parameter that depends upon the values ofpsandTat a reference distance from the sourcer0. The time required for the amplitude of the head shock to increase from 5% to 95% of peak value was observed to vary from 0.45 μs (imposed by the microphone response) to greater than 2.0 μs as the wave traveled outward and as its amplitude decreased. Finally, the microphone was calibrated through use of the variation with distance of measured values ofT; this new method has led to calculation of a free‐field sensitivity that agrees within ±1 dB with the results of other calibrations.

ISSN:0001-4966    

DOI:10.1121/1.389585

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The parametric mixing of two modulated surface acoustic waves propagating in opposite directions is studied with reference to nonlinear signal processing applications by the coupled mode theory of nonlinear surface waves. In the case of propagation in opposite directions the combination frequency waves generated out of the nonlinear interaction of the two primary waves and their harmonics with one another do not satisfy the phase‐matching condition. Hence there exists no mode coupling between the primary waves. At the same time the amplitude of each combination frequency wave is proportional to the product of a harmonic amplitude of one of the interacting waves with a harmonic amplitude of the other wave. The variation of these harmonic amplitudes as functions of the slow scale variables are governed by two sets of coupled amplitude equations, each set pertaining to one primary wave and its harmonics. It is further shown that, in addition to surface wave modes, there also exist bulk wave modes in a certain range of values of the primary wave frequency ratio. The paper concludes with indicating the relevance of the present study to the acoustical implementation of nonlinear signal processing operations like convolution, correlation, etc.

ISSN:0001-4966    

DOI:10.1121/1.389586

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

An analytical description for the field of a parametric focusing source is derived. It is valid for spherically concave sources with small aperture angle and highka, under conditions of quasilinear interaction (strong shocks precluded). The solution furnishes computations on the phase and amplitude of difference frequency sound along the axis and in the focal plane, as well as on the width of the radiation lobe in the focal region. Underwater experiments conducted with anf/2 lens coupled to a dual, interleaved primary array are discussed. The results support the utility and validity of the analytical model for describing the distribution of sound along the acoustic axis and across the focal plane. The difference frequency radiation was found to be effectively focused, in that the width of the beam became quite narrow in the focal plane.

ISSN:0001-4966    

DOI:10.1121/1.389587

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A spatial impulse response model of the parametric array is developed and used to give physical insight into its behavior when operated in a transient mode. Experimental measurements of the spatial dependence of the acoustic pressure waveforms produced by the transient parametric array are compared with the result of the convolution of the spatial impulse response with the specific pressure waveforms used. Particular emphasis is given to the case where the primary field is discontinuously terminated. In such cases effects due to the finite aperture of the parametric array and effects due to its termination may be seen by a point hydrophone either as separated or superposed events in time, depending on the geometry, when the parametric array is operated in the transient mode. Although the impulse response model is restricted by various assumptions, it can contribute to an understanding of the parametric array operation under more general circumstances.

ISSN:0001-4966    

DOI:10.1121/1.389588

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Acoustic shadowing by an isolated seamount has been studied by examining the multipath propagation measurements obtained in a shot run that passed over the seamount peak. Source depths of 24 and 196 m were used in the experiment. In the acoustic shadow, the propagation loss for the shallow 24‐m shots increased by 10–15 dB over the loss expected in the absence of the seamount. Examination of the pressure‐time history for shots deployed in the shadowing region revealed that the signals consisted of two components. The first and dominant pulse was determined to be a diffracted wave which passed over the seamount by rough‐surface forward scattering and diffraction. The subsequent group of weaker pulses was attributed to the energy which had passed over the seamount by a series of surface–bottom interactions. The shadowing loss increased by 3 dB per octave for frequencies greater than 50 Hz, in agreement with theory, but is appreciably greater than the predicted values at lower frequencies. The shadowing loss for the 196‐m shots was about 5 dB less than that observed for the shallower shots.

ISSN:0001-4966    

DOI:10.1121/1.389562

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP