摘要:

Some simple analytical expressions for two important optical characteristics of focused high‐resolution circular arrays are derived, specifically those for beam patterns and depths of field. The expressions generated, in addition to providing simple estimational tools for array design purposes, also lend themselves to straightforward interpretation in terms of the Fourier (farfield) and Fresnel (nearfield) kernels. Applications to refocusing arrays are discussed, as well as similarities and differences between such (sonar oriented) systems and radar and optical systems.

ISSN:0001-4966    

DOI:10.1121/1.389871

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Theoretical performance capabilities of the optimum filter for maximization of signal‐to‐noise ratio (SNR) and a suboptimum approximation, easy to realize in practice, are compared to the conventional matched filter (designed for white noise only) in the case of low Doppler signals masked by surface reverberation. Spectrum models for surface backscattering as a function of local environmental parameters are based on recent results describing scattering from a subsurface bubble layer. Significant improvements in SNR in the low Doppler region as well as a dramatic dependence of performance on windspeed and direction, and the ratio of reverberation to uncorrelated noise power are predicted. Results serve as average performance limits to be compared to those obtained by algorithmic implementations of optimum and adaptive filters.

ISSN:0001-4966    

DOI:10.1121/1.389872

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Two approaches for noise source identification based on theory for multiple‐input systems have been investigated. The concepts of the frequency response function and the coherent residual spectral density function were used to estimate the spectra of the noise sources that were accounted for in the multiple‐input model. The key factors which determine the applicability of the techniques for noise source identification are found to be the degrees of coherence between themeasuredinputs. The experimental results showed that, for a noise system with multiple sources which were slightly coherent to each other, the techniques presented might be able to estimate the spectra of the noise sources, provided there was only modest measurement contamination.

ISSN:0001-4966    

DOI:10.1121/1.389873

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

This paper represents a method of calculating the subjective preference of sound fields in concert halls before construction. Subjective preference judgments (paired comparison tests) were systematically performed using fully independent objective parameters of acoustic information which describe the signals to the two ears. The sound fields with various combinations of listening level, delay of early multiple reflections, subsequent reverberation time, and magnitude of the interaural cross correlation were simulated with the aid of a digital computer. The optimal conditions maximizing the subjective preference could be found for each objective parameter, because the parameters had an almost independent effect on the subjective preference judgments. Based on the linear scale value, which is obtained by applying the law of comparative judgment, we can calculate a total preference value according to the ‘‘principle of superposition.’’ Examples of calculating the preference values by use of the plan and the cross section of a concert hall are described.

ISSN:0001-4966    

DOI:10.1121/1.389874

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Annoyance due to chronic exposure to blast noise and vibration was assessed in residential areas near two surface mines and a quarry. It was found possible to base useful prediction of the prevalence of high annoyance on a metric of outdoor ground vibration related to high centiles of the long term distribution of exposure levels.

ISSN:0001-4966    

DOI:10.1121/1.389875

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

This article describes the theory and a simple experiment carried out to demonstrate the phenomenon of Anderson localization in an acoustical context. This is an effect whereby the propagation of vibration in a structure which is not entirely regular is impeded by the irregularities, giving rise on the average to an exponential decay of vibration level away from the driving point, even in the absence of any dissipation. The structure used in the experiment was a stretched string with masses attached to it. This string was studied with regular spacings of the masses and after the masses had been moved in a controlled way to provide a small degree of irregularity. In both cases, the transmission of energy from end to end of the string was measured as a function of frequency, and also the mode shapes in the second and fourth passbands were measured so as to demonstrate the underlying physics of the localization phenomenon, in which the individual modes making up each passband change from being extended throughout the structure in the regular case to being localized in specific areas of the structure in the presence of irregularity. All measurements yielded satisfactory agreement with the theoretical predictions.

ISSN:0001-4966    

DOI:10.1121/1.389847

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

We use Biot’s theory to search numerically for the velocities of the various surface waves, both the true slow surface wave and the pseudosurface waves, at an interface between a fluid half‐space and a half‐space of a fluid‐saturated porous medium. We focus on the high‐frequency range where the Biot–Plona slow wave is propagatory. We find that for an open‐pore surface situation, the true surface mode exists for a limited range of material parameters and changes continuously into a slightly leaky pseudo‐Stoneley mode as the mode speed increases past the slowest bulk wave speed; for the sealed pore situation there exist simultaneously a true surface mode (for all values of material parameters) and a pseudo‐Stoneley mode. The pseudo‐Rayleigh mode has features similar to those of the pseudo‐Rayleigh mode for a fluid/nonporous solid case.

ISSN:0001-4966    

DOI:10.1121/1.389876

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A 2‐D space–time reflection Green’s function is developed using the Cagniard–de Hoop technique for a fluid/lossless porous media planar interface configuration. It is then used to study the effects of the various surface waves whose velocities were calculated in the previous article. The time‐dependent pressure response to a modeling source signal (Blackman–Harris pulse) for certain simulated experimental configurations is also presented. It is concluded that the amplitudes with which the new surface modes are generated are large enough to enable their detection.

ISSN:0001-4966    

DOI:10.1121/1.389877

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A nonlinear theory based on the notion of oriented continuum is presented with a view to describing piezoelectric powders. The generalized kinematics comprehends the macroscopic motion of grains as well as the internal deformation (rotation, stretch) of grains (via a so‐called director field) in connection with local electromechanical interactions. A complete set of coupled dynamical electroacoustic equations is deduced from first principles for this class of granular media. Thermodynamical arguments then allow one to construct general constitutive equations that exhibit both thermodynamically reversible and irreversible processes. Of necessity, the theory is first constructed in a fully nonlinear framework. In a second stage, the nonlinear equations are perturbed, by means of a Lagrangian variation, about a free finite initial state in which the directors are randomly oriented. These initial privileged directions provoke a local breaking of the ideal symmetry of the material and there results a nonhomogeneous medium. In addition, in the perturbed equations thus obtained, nonlinear terms related to ponderomotive couples and orientational electromechanical couplings are involved. Among the equations obtained, one governs the internal deformations of grains, which materialize in a microgyration of grains about their center of mass and a microstretch in the direction along which the local electromechanical interactions take place. This work collects all the essential ingredients which are required in a study of physically interesting phenomena such as memory echoes in resonant piezoelectric powders.

ISSN:0001-4966    

DOI:10.1121/1.389878

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Based on an oriented continuum model, a continuum approach for powders made of piezoelectric grains proposed in a previous work is applied to the study of electroacoustic echoes in resonant piezoelectric powders. The nonlinear equations obtained through this approach govern the piezoelectric oscillations and microgyration motion of each grain, as well as the global behavior of the powder and lead to the echo formation. The evaluations of the dynamical echo at 2τ and the memory echo atT+τ are presented. A memory phenomenon lying in the reorientation of grains due to a ponderomotive couple inside the powder sample is placed in evidence. The global response of the sample consists in summing up all the elementary electric polarizations associated with the piezoelectric oscillations of grains. Then the echoes are caused not only by the nonlinear interaction effects between the deformation of grains and the electric field, but also by a refocalization of collective oscillations which, after a complete dephasing period, gives a coherent signal through the inversion of the phase evolution. This continuum approach presents definite advantages over previous ones. Typical numerical values of parameters involved in this phenomenon as well as physical interpretations are given.

ISSN:0001-4966    

DOI:10.1121/1.389840

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP