摘要:

A theoretical model of ambient noise in an isovelocity, shallow water channel with independent, randomly distributed surface sources is presented. Bottom losses in the channel are assumed to be sufficiently small for the modal energy from distant sources to predominate over the continuous radiation from nearfield sources. For frequencies at which the channel can support about ten or more modes the predicted noise field is essentially homogeneous over a large proportion of the water column away from the boundaries. This allows the vertical spatial coherence of the noise to be expressed in terms of a plane‐wave directional density function. The properties of this function are shown to be characteristic of the modal structure of the noise field.

ISSN:0001-4966    

DOI:10.1121/1.384161

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A simplied approach is described for determination of phase perturbations produced by variations in sound speed and current in the ocean. It is shown that corresponding perturbations of the ray geometry may be ignored in determining the phase perturbations, when the former are regular in a specific sense. Principal advantages in the procedure include its efficiency in calculation of phase variations and its indication of situations when ray‐geometric perturbations may significantly influence phase. The method is demonstrated for both shallow‐ and deep‐ocean examples, when the phase perturbation arises from weak horizontal deviations from a horizontally uniform sound‐speed structure. It is also illustrated for current in the deep ocean, considering cases with and without horizontal variations in current speed. Examples for both types of horizontal variations are shown in which they make significant contributions to ray phase. Finally, the procedure is applied to some single‐path acoustic observations in which the time series of travel time is dominated by tidal variations. Ranges of possible environmental changes in sound speed and current, leading to the observed travel‐time variations, are indicated.

ISSN:0001-4966    

DOI:10.1121/1.384162

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A consistent environmental‐acoustic model for a deep moving ocean is formulated. The acoustic model for regularly perturbed SOFAR rays is approximately solved using a type of WKB  (J) expansion. Interfacing conditions between the hydrodynamics and acoustics are developed which lead to constraints on acoustic frequency and transmission range. As an application, transmissions are considered through stochastic internal‐wave fields, which have been modeled in a previously published paper by the authors. Formulas for ray phase variances are derived. These formulas are asymptotically evaluated for rays with relatively significant depth variation, using the stationary phase method. New results are obtained for the dependence of the variances on internal‐wave primitives, such as energy spectra. Expected multipath intensity is calculated for transmission through an ocean with static state modeled by a bilinear sound‐speed profile. The effects of the internal‐wave field and of varying internal‐wave parameters on the expected intensity are shown to be significant.

ISSN:0001-4966    

DOI:10.1121/1.384163

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Expressions are stated for the transmissibility and for the driving‐point impedance of an internally damped circular plate of radiusawith a clamped boundary that is driven by a vibratory point force at an arbitrary distance μafrom the plate center. Expressions are also stated for the plate transmissibility and driving‐point impedance when the plate is loaded at the arbitrary driving point either by a lumped mass, by a dynamic vibration absorber, or simultaneously by a lumped mass and a dynamic absorber. In all cases, representative calculations of transmissibility and impedance are plotted versus the square root of frequency. These curves clearly show the dependence of transmissibility and impedance on the plate damping factor, the value of the parameter μ, and the extent of the mass loading. They also show the effectiveness of the dynamic absorber, which varies with the value assigned to μ.

ISSN:0001-4966    

DOI:10.1121/1.384164

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A locally optimum detector correlates the data spectrogram with a reference spectrogram in order to detect (i) a known signal with unknown delay and Doppler parameters, (ii) a random signal with known covariance function, or (iii) the output of a random, time‐varying channel with known scattering function. Spectrogram correlation can also be used for maximum likelihood parameter estimation, e.g., estimation of delay or center frequency of a signal. To estimate an analog input signal from its spectrogram, a modified deconvolution operation can be used together with a predictive noise canceler. If no noise is added to the spectrogram, the mean‐square error of this signal estimate is independent of the window function that is used to construct the spectrogram. When estimates of specific signal parameters are obtained directly from the spectrogram, these estimates have mean‐square errors that depend upon both signal and window waveforms. Spectrogram correlation can be used for classification as well as for estimation and detection. Parameter estimators and detectors are, in fact, specialized kinds of classifiers.

ISSN:0001-4966    

DOI:10.1121/1.384165

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Adaptive beamforming algorithms are often designed under the assumption that the sources illuminating the array are mutually uncorrelated. When the assumption is not valid, the performance of the adaptive beamforming algorithm in certain applications is severly limited. A simple example is included which illustrates how suppression of a look direction signal occurs when a power minimizing adaptive algorithm is used in the presence of correlated sources. This paper establishes certain properties of the eigenvectors of the correlation matrix of an array illuminated by a field of correlated discrete sources. On the basis of these properties an adaptive algorithm is proposed for resolving the direction of all discrete sources, even if they are mutually correlated. Simulation results are presented showing the performance of the algorithm proposed in this paper.

ISSN:0001-4966    

DOI:10.1121/1.384166

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

This paper presents a mathematical analysis of the signal‐to‐noise power ratio degradation incurred when Gaussian signals in Gaussian noise are level‐quantized, time‐sampled, transmitted over a noisy digital channel, and then reconstructed by a low‐pass filter. Application of this analysis occurs when very weak acoustic or seismic signals in strong white noise are collected and transmitted over a digital channel to an analysis location remote from the collection point, and where detectability of the information‐bearing signals at the analysis location depends on the original signal‐to‐noise power ratio in small frequency bands. Therefore, it is important to understand the degradation of the original signal‐to‐noise ratio occurring during transmission. Curves are presented which show bounds on the degradation as a function of the quantizer step size and the channel bit error probability. Also considered is a scheme for transmitting a low‐rate digital sequence by periodically interrupting the level‐and‐time‐quantized analog signal. Three schemes are proposed for doing this, and one scheme is mathematically analyzed. It is shown that this scheme increases the degradation of the reconstructed analog signal by about 0.15 dB over that of the uninterrupted case.

ISSN:0001-4966    

DOI:10.1121/1.384167

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

If a two‐dimensional acoustic reflectivity function is defined within the interior of a circle, reflectivity data may be acquired by transmitting acoustic pulses from isotropic elements distributed around the circumference of the circle and recording the resulting backscattered sound as a function of time. If the reflectivity function fulfills the conditions of an ’’idealized’’ weakly reflecting medium, the resulting pulse–echo data may be regarded as the line integrals of this function defined over circular arcs centered at points lying on the circumference of the enclosing circle. In this paper we show that on the basis of these line integrals the unknown reflectivity in the interior of the circle can be expressed explicitly in terms of its line integrals defined over the set of paths consisting of all circular arcs whose centers lie on the circumference of the enclosing circle. We propose that the resulting reconstruction equations could provide the foundation for a new approach to reflectivity tomography. A numerical example is also given.

ISSN:0001-4966    

DOI:10.1121/1.384168

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The power spectrum of a white noise waveform modified by an arbitrary, real data window is derived. It is shown that the weight of a component at frequencyfrelative to the cumulative weights of all other components in estimates of the power density at this frequency is greatest when a rectangular data window is used. On this basis it is concluded that the detectability of a small cw signal in white noise is greatest if a rectangular data window is used. Some examples are worked out to show the estimated loss in detectability when various other windows are used.

ISSN:0001-4966    

DOI:10.1121/1.384179

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Statistical properties of the beampatterns of three‐dimensional random arrays of arbitrary geometry are derived. The class of random arrays studied differs from those previously analyzed in that it includes partially random arrays. A partially random array is defined, for purposes of this paper, as a random array in which the various element positions may have different probability density functions and may be interdependent. Results presented include the expected value and variance of the pattern function, expected array power and the associated directivity index, and the variance of the array power. The results are applied to random and partially random disk arrays. The partially random disk array is assumed to have elements arranged in vertical strings. It is found that an array withM‐element strings is characterized by a region corresponding in elevation to the main lobe and extending 360° in azimuth in which the side‐lobe variance is a factor ofMgreater than that in the rest of the side‐lobe region.

ISSN:0001-4966    

DOI:10.1121/1.384180

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP