摘要:

The finite difference calculations of Neely [E.D. thesis, California Institute of Technology, Pasadena, CA (1977)] and the WKB solution of Steele and Taber [J. Acoust. Soc. Am. 65, 1001–1006 (1979)]generally agree, except in the ’’phase plateau’’ region, where both are irregular. In the present work, an improvement in the accuracy of the WKB procedure is gained by using a closed‐form integral which gives the phase and damping functions in the short wavelength region. The present results agree qualitatively with the preceding but show a smooth variation with frequency and distance along the cochlea.

ISSN:0001-4966    

DOI:10.1121/1.384640

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Starting from first principles, the optimum receiver for the detection of a harmonic set embedded in white Gaussian noise is derived. The receiver obtained also provides the maximum likelihood estimate of the fundamental frequency of the harmonic set. Three signal conditions are studied: (1) the harmonic amplitudes are known exactly, (2) the harmonic amplitudes are unknown, but have known, independent probability distributions, and (3) the harmonic amplitudes are unknown and have unknown, independent probability distributions. For cases (1) and (2) the optimum receivers are derived. For case (3) the receiver obtained cannot be claimed to be optimum; however, it does have intuitive appeal. In each case the receiver cross correlates the (transformed) spectral estimates with a periodic impulse train whose period corresponds to each possible fundamental frequency of the harmonic set. The receiver for case (3) has an additional interesting property in that it discards all spectral estimates less than a certain value. Some extensions to these results are discussed for situations where (a) the spectral estimates are obtained from data lengths much shorter than the observation interval, and (b) multiple similar harmonic sets may be present.

ISSN:0001-4966    

DOI:10.1121/1.384641

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A theoretical study is presented on the response of a vertical array of hydrophones to shallow‐water signal and noise fields. A normal‐mode propagation model was used to provide the incident signal field. The field due to wind‐generated noise was obtained from a recently developed model based on wave theory. Two methods of combining hydrophone outputs are investigated: broadside beamforming and a mode‐matching approach. In the first case, limitations to signal gain are found due to the propagation properties of the shallow water environment. The highest gains are achieved with an array of length 15λ to 30λ. The second method involves applying amplitude and phase weightings in accordance with the shape of the largest signal mode. This enhances the signal gain for arrays longer than 8λ. The values of signal gain are primarily dependent on range and bottom type. The noise gain is found to be very dependent on sound speed profile; noise is rejected by a broadside array to a greater extent in summer conditions than in winter. Overall results for shallow‐water array gains show degradation from deep‐water gains of between 8 and 20 dB for a 20λ array for the cases studied.

ISSN:0001-4966    

DOI:10.1121/1.384642

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Any Hermitian matrix, such as the spatial correlation matrix of measured data from an array of sensors at some frequency, can be represented as the sum of the dyads formed from its eigenvectors, weighted by the corresponding eigenvalues (the spectral decomposition of the matrix). Each term of this sum is of the form of the spatial correlation matrix due to a single source, received at the array with random amplitude, but fixed wave front (not necessarily planar). The spectral decomposition can thus be interpreted as analysis of the actual correlation matrix into components due to a number of (perhaps hypothetical) uncorrelated sources, with the appropriate complex delay factors and average powers displayed explicitly. In this note, we discuss this briefly, and illustrate with an example.

ISSN:0001-4966    

DOI:10.1121/1.384643

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The absolute minimum attainable values of incremental mean‐square error in bearing and range estimation caused by random perturbations of sensor locations are determined and compared with the performance of conventional processors. At low signal‐to‐noise ratios the optimum estimator realizes no improvement over the conventional systems; at high signal‐to‐noise ratios it realizes only slight improvement.

ISSN:0001-4966    

DOI:10.1121/1.384616

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Maximumaposteriori(MAP) techniques are applied to the problem of estimating the unknown parameters of a frequency modulated narrow‐band signal in additive white Gaussian noise. Signals are assumed to have unknown amplitude, initial phase, and frequency versus time history, althoughaprioriinformation concerning the frequency trajectory may be available. In addition, it is assumed that the signal frequency trajectory can be adequately modeled by a continuous piecewise linear function of time. It is shown that the MAP estimator maximizes a linear combination of (1) coherent match of signal induced outputs to received observations, and (2) a term dependent only uponaprioriknowledge. This second term reduces to a measure of trajectory smoothness in the special case of a Gauss–Markovapriorifrequency model. Lastly, a MAP detection scheme is suggested for cases in which the presence of such signals is an issue.

ISSN:0001-4966    

DOI:10.1121/1.384617

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A new method is proposed to acquire impulse responses in concert halls with large signal‐to‐noise ratios and with a high resolution. In our proposal an omnidirectional loudspeaker is used which is driven by an amplified sweep: a signal containing all frequencies of interest smeared out in time. By using a deconvolution technique, an almost perfect pulse is obtained with a high peak pressure and a short effective duration. Measurements were made in two different concert halls to illustrate the practical implications of the new technique.

ISSN:0001-4966    

DOI:10.1121/1.384618

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Results for the interference pattern in a reverberant sound field have been derived by means of an eigenmode analysis and compared with results obtained from the free‐wave model. It is found that the incoherent or random nature of the sound field assumed in the free‐wave model can only be achieved if the room is excited by random noise. For single frequency excitation an equivalent incoherent field can be approached by additional averages of results over many different source locations. The analytical solutions are supported by a computer study based on the eigenmode model. A detailed discussion about the correction factor due to boundary interferences in relation to sound power measurement is also given.

ISSN:0001-4966    

DOI:10.1121/1.384619

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

This paper presents the development of a finite element for the analysis of spherical shells. The derivation of stiffness and mass matrices is based on the improved shell theory which takes into account the effects of rotary inertia and shear deformation. This finite element is then used to determine the natural frequencies and mode shapes for the free axisymmetric vibration case for free, hinged, and fixed boundary conditions. The transient response in the radial direction at the apex of the shell is calculated for shells subjected to time‐dependent internal radial pressures.

ISSN:0001-4966    

DOI:10.1121/1.384621

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Reflection characteristics of torsional waves in a semi‐infinite rod connected to an elastic half‐space are analyzed under two specific incident pulse loads, applying Laplace transformations with respect to time and numerical inverse Laplace transformations. The time histories of the torsional stress and rotation of the semi‐infinite rod at an arbitrary point are shown. The advantage of this method, i.e., applying the numerical inverse Laplace transformations, is that it is simpler than using Fourier analysis and synthesis, and applicable to any incident pulse load except where the rise time interval of the incident pulse load is too short. In the case of a step pulse load, the stress curves approach that of the fixed boundary condition and the rotation curves approach asymptotically the static rotation value of each given shear modulus ratioGR/Gand velocity ratioK. In the case of a half‐sine pulse load, the reflected stress peak values decrease and those of the rotation increase with an increase inGR/GandK. A cross−over effect occurs whenGR/G≳1 andK≳1; it becomes pronounced asGR/GandKincrease.

ISSN:0001-4966    

DOI:10.1121/1.384622

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP