摘要:

In this paper, the problem of signal detection in severe and/or changing noise environments, often encountered in underwater acoustics and communications, is considered. The detectors operate in sequential, i.e., variable sample size, mode at near optimum levels for a particular noise environment and are robust by maintaining high efficiency in other than the nominal noise environments by adapting their optimum nonlinearity using anm‐interval polynomial approximation (MIPA) of it. Furthermore, the assumption of independent samples is relaxed, allowing higher transmission rates for lower error rates. The proposed sequential detectors work well for coherent and noncoherent detection, are asymptotically optimum and, for small signal‐to‐noise ratios (SNR), increase their transmission rate up to four times as compared to their fixed‐sample size counterparts. The performance of the detectors is evaluated in typical underwater noise fields. It is demonstrated that their efficiency with respect to sequential MIPA detectors with independent sampling is improved, in some cases up to the order of the dependence of the samplesQ. It is emphasized that the MIPA detectors are easy to implement. The estimation and updating of the detector parameters may be accomplished using parallel processors operating in a recursive mode without disturbing the decision process.

ISSN:0001-4966    

DOI:10.1121/1.402053

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Active minimization of total power output and active absorption of sound power are analyzed, using a general impedance‐based approach, for an array of controllable secondary sources and an array of original primary sources. When the total power output of the two arrays is minimized, and the primary source array is all in phase, the power output of each of the secondary sources is found to be exactly zero. When the power absorption of the secondary source array is maximized, the net power output of the primary source array can be either reduced or increased, compared to that in the absence of control, depending on the properties of the transfer impedances. If the primary and secondary sources are well coupled (as is the case when they are spaced less than a quarter wavelength apart in free‐space or when they are in an enclosure excited near the natural frequency of a lightly damped mode) minimizing the total power output gives worthwhile reductions in the radiated power of the primary source. Maximizing the power absorption of the secondary source under these conditions, however, can considerably increase the power output of the primary source. If the sources are well separated, compared with the wavelength, in the free‐field, or are placed in a diffuse sound field in an enclosure, the coupling between the sources is weak. Under these conditions the secondary source can effect little reduction in the power output of the primary source. Although acoustic power can still be absorbed by the secondary source under such circumstances, the amount of absorbed power will always be small compared with the power output of the primary source.

ISSN:0001-4966    

DOI:10.1121/1.402054

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Bies and Hansen [J. Acoust. Soc. Am.88, 2743–2754 (1990)] have proposed an alternative formulation of the relationship between noise exposure and noise‐induced hearing impairment to that presented in International Standard ISO 1999, in which they assume that presbycusis and noise‐induced permanent threshold shift (NIPTS) are additive on an antilogarithm basis. Data concerning deterioration in hearing threshold levels at 4000 Hz due to aging in war veterans with NIPTS do not support the Bies and Hansen assumption but provide support for the formula for combining presbycusis and NIPTS incorporated in International Standard ISO 1999.

ISSN:0001-4966    

DOI:10.1121/1.402055

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Criteria based on Zwicker’s model of loudness and the A‐weighted sound‐pressure level are used to find the source–receiver distance at which atmospheric absorption becomes significant. The problem is studied for a wide range of temperature and relative humidity variations. It is found that both criteria are nearly equivalent. For the majority of environmental noise sources, including motor vehicles and trains, air absorption cannot be neglected at distances beyond a few hundred meters.

ISSN:0001-4966    

DOI:10.1121/1.402056

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A technique for deriving the optimal surface velocity distribution on the surface of a finite baffled beam has been developed. The optimal velocity distribution minimizes the radiation efficiency of the beam for a specified maximum permissible mode and frequency. A modal expansion of the surface velocity in terms of unknown modal amplitude coefficients, the Rayleigh integral, and a far‐field intensity integration are employed to obtain a quadratic expression for the radiation efficiency of the beam. Application of a suitable constraint to avoid trivial solutions leads to an eigenvalue problem identical in form to the Rayleigh quotient employed in dynamic mechanical systems. The eigenvector of modal amplitude coefficients corresponding to the lowest eigenvalue yields the minimum radiation efficiency, while the eigenvalue itself is the actual value of the minimum radiation efficiency. Near and below coincidence, the optimal eigenvector of modal amplitude coefficients yields a radiation efficiency significantly less than the radiation efficiency of any single modal component acting alone. Simply supported and clamped–clamped boundary conditions are considered, and numerical examples are presented for each.

ISSN:0001-4966    

DOI:10.1121/1.402057

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The distribution of deformation, temperature, and stress in a homogeneous isotropic, thermally conducting, infinitely extended, stretched elastic plate due to a flat nose cylindrical projectile has been studied in the context of the Green–Lindsay theory of thermoelasticity. As the ‘‘second sound’’ effects are short lived, the discussion is confined to small‐time approximations. The short‐time solutions have been obtained by using the Laplace transforms technique. The deformation is found to be continuous but the temperature and stress are found to be discontinuous. The jumps obtained have been computed numerically and are represented graphically for carbon‐steel material.

ISSN:0001-4966    

DOI:10.1121/1.402058

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The scattering of sound waves by an air‐filled, elastic, spherical shell in deep waters, in the frequency domain is analyzed. This exact analysis is based on the classical formulation of three‐dimensional elastodynamics, for fluid‐loaded spherical shells of arbitrary thickness. Form functions, residual responses, and the partial‐wave expansions of both, are determined. Results are displayed in relatively wide frequency bands for various increasing shell thicknesses. The resonance features in the sonar cross sections (SCS) are isolated by means of a new hybrid modal background that substantially improves the results found with the earlier (rigid/soft) backgrounds of the resonance scattering theory (RST). The resonance features in the SCSs that correspond to each mode, and also to each of the various shell waves that propagate around its periphery are isolated. There seem to be over half‐a‐dozen shell (generalized Lamb and Stoneley) waves manifesting their influence in the SCSs within the examined band. Three large‐amplitude features are most noticeable. There is a (slow) wave due to the double curvature of the shell which is responsible for a large, spiky feature at low frequencies.There is the sphericalA0wave, caused by the coincidence effect, responsible for a broad ‘‘bump’’ with superimposed spikes, which appears in the midfrequency region, nearx∼(h’)−1, whereh’is the relative shell thickness. There is also a thickness resonance feature at high frequencies, which is due to a wave that propagates through the shell thickness at the incidence point. There is a spherical Stoneley‐type wave that resides mostly in the water in the frequency region below coincidence,xc, at which theA0wave is activated. Finally, there are two resonance families caused by spherical analogs of the symmetric,Sn, and the antisymmetric,An, Lamb waves originally studied in flat plates. These spherical generalizations are denoted by the same symbolsSn,An, with the understanding that they now refer to shells. This model’s results for phase velocities, cutoff frequencies, etc..., are compared to those produced by simpler approaches in order to size the approximations introduced by those simpler models. This serves to establish the benchmark nature of the present model and of the calculations it produces, which are displayed in many instances. Dispersion plots are generated for the phase velocitiescp(x) of all the above‐mentioned shell waves. Thesecp(x) are proportional to the (real parts of the) roots of determinantal conditions that are also analyzed. Critical angles, coincidence phenomena, and reflection and transmission properties of the shell are examined and all the required physical interpretations are given.

ISSN:0001-4966    

DOI:10.1121/1.402059

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

In this paper an alternative method to first threshold crossing for acoustic emission (AE) source location is presented. For wave propagation in dispersive media, the accuracy of source location can be improved by locating corresponding phase points on the transducer outputs to determine the difference in arrival times. The phase point location was done by cross‐correlating the transducer outputs with a single frequency cosine wave modulated by a Gaussian pulse. Experiments were performed using a lead break as the AE source on the surface of an aluminum plate. Due to the plate geometry and source orientation, the wave produced was highly dispersive. Although this wave was unsuitable for first threshold crossing techniques, the time differences needed for triangulation could be determined using the cross‐correlation technique.

ISSN:0001-4966    

DOI:10.1121/1.402348

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The analytical solution for seismic wave propagation associated with a point force in a fluid‐filled porous medium is developed. The point force solution is applied to solve the boundary value problem of seismic wave propagation in a stratified poroelastic medium. The coupled Biot vector wave equations are expressed in cylindrical coordinates and expanded in Fourier series with respect to azimuth. The resulting equations are transformed to the wave‐number domain using the Hankel transform method. Following this analysis, one set of three coupled partial differential equations associated with fast compressional, slow compressional, and vertically polarized shear waves is derived. The unknowns are the radial and vertical displacements associated with the solid frame motion as well as the fluid pore pressure. A separate partial differential equation associated with waves whose particle motion is polarized in horizontal planes (SHwaves) is derived as well. The general solution of the three coupled differential equations is obtained by the Kupradze method.This solution of the Biot’s motion equations in the wave‐number domain leads directly to closed form expressions for the vector wave displacement and the pressure produced by a point force in a poroelastic unbounded medium. In order to develop the solution of a point force in the presence of a stratified porous medium, the displacement‐stress matrix, the pressure, and the vertical component of the displacement of the fluid in its relative motion versus the solid for different regions are expressed in terms of upgoing and downgoing waves and unknown wave coefficients. The wave coefficients are determined by applying boundary conditions of continuity of displacements, pressure and stresses across each layer interface, and the radiation conditions at infinity. To determine the unknown wave coefficients, a method that consists in expressing the kernels of the Hankel transform integrals in terms of factorization of upgoing and downgoing wave amplitudes in each layer is used. These factorizations are based on the generalized reflection and transmission coefficient matrices, which are formed recursively, from one layer boundary to the next, including all of the reflection/conversion/transmission properties of the layered medium.Their factorization method allows the field within each layer above or below the source to be determined once the field in the medium containing the source is known. The final equations provide a complete description of the field throughout the layered medium. The particular form of the equations makes possible the simultaneous evaluation of the response at a number of detector locations for a number of different source positions in a borehole for interwell seismic applications. Numerical model results demonstrate the validity of this theoretical development for predicting spectral responses associated with porosity and permeability effects. The seismic pressure response of a thin gas‐saturated porous layer was analyzed. The results inferred that the gas‐saturated porous layer strongly attenuates the waveforms observed by detectors within the layer. Alternatively, large multiple reflections and converted waves from the layer are observed by detectors in the water‐saturated porous formation.

ISSN:0001-4966    

DOI:10.1121/1.402060

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Propagation of ultrasound across a solid layer with equally spaced parallel interfaces is studied by using a transfer matrix method. For a layer with identical interfaces the propagation of ultrasound is governed by a dispersion relation, which displays passing and stopping bands in the frequency domain. For a layer with interfaces that have random deviations from mean interface properties, the Fürstenberg theorem and Monte Carlo simulation have been used to study propagation of ultrasound across the layer. It has been shown that at all frequencies there is an exponential decay in the amplitude of the transmitted wave. This decay, which increases with increasing frequency, defines a localization phenomenon, since wave motion will be confined to the insonified side of the layer.

ISSN:0001-4966    

DOI:10.1121/1.402061

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP