摘要:

The spectral theory of wave propagation in a weakly range‐dependent environment is deduced from the corresponding theory for a wedge‐shaped structure. The solution obtained is compared to that derived previously, which suggests that some modifications are necessary in the previous formulation in order to bring the solution into a symmetric form.

ISSN:0001-4966    

DOI:10.1121/1.393667

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A wider‐angle parabolic wave equation (PE) previously obtained by factorization of the Helmholtz equation and interpretation of the square‐root operator is derived by another method. The wider‐angle capabilities of this PE are compared with the standard PE, using an analytically tractable case. The problem of using a PE in the nearfield is discussed.

ISSN:0001-4966    

DOI:10.1121/1.393668

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The composite roughness model is applied to bottom backscattering in the frequency range 10–100 kHz. For angles near normal incidence, the composite roughness model is replaced by the Kirchhoff approximation which gives better results. In addition, sediment volume scattering is treated, with account taken of refraction and reflection at the randomly sloping interface. In applying the model to published data it is found that sediment volume scattering is dominant in soft sediments except at small and large grazing angles. For coarse sand bottoms, roughness scattering dominates over a wide range of grazing angles. Implications for acoustic remote sensing are discussed.

ISSN:0001-4966    

DOI:10.1121/1.393669

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The sonar equations provide a convenient way to calculate the detection range of actual and proposed sonar systems. They are simple, but approximate, relationships between parameters that describe the effects of source, target, and acoustic medium independently. The purpose of this paper is to derive an exact form of the active sonar equation by using the Helmholtz integral formulation to solve the boundary‐value problem with source and target both present in the medium. The resulting equation involves combinations of linear integral operators; however, it is suitable for solution by numerical techniques already developed for radiation from objects of arbitrary shape. Furthermore, it is shown that these integral operators reduce to multiplicative factors which represent general definitions of the source level, transmission loss, and target strength when the source‐to‐target distance is large. Thus this work establishes a basis for the sonar equations as the limiting form of an exact boundary‐value problem and presents formulas for calculating the sonar parameters.

ISSN:0001-4966    

DOI:10.1121/1.393670

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Low‐frequency ambient noise under pack ice of the central Arctic Ocean has long‐term variations (periods greater than 1 h) which correlate highly with composite measures of stress applied to the ice by wind, current, and drift. These composites are the horizontal ice stress and the stress moment, and are derived from meteorological and oceanographic data observed simultaneously with the noise. Atmospheric cooling, a known high correlate of midfrequency noise under the ice, is not important at low frequencies.

ISSN:0001-4966    

DOI:10.1121/1.393671

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Wide‐angle seismic reflections obtained in a deep ocean environment are analyzed to obtain estimates of the sound‐speed profile and density contrasts in the surficial sediment layers. The analysis begins with a new deconvolution technique based on autoregressive modeling, which is used to deconvolve the bubble‐pulse signature from the data. Sound‐speed profiles are then obtained by inverting the identified arrivals by first treating them as reflection events, and then as refraction events. The first inversion was performed twice, once using a Backus–Gilbert approach (to produce a smooth model) and once using a linear programming method (to produce a blocky model). For the refraction inversion, the Garmanyp−τ method was used. Finally, the offset‐dependent phase shifts of supercritically reflected arrivals were estimated using the complex Karhunen–Loeve transform. These phase shifts were used with Rayleigh’s equation in a new inversion scheme to obtain density and sound‐speed contrasts in the shallow sediments.

ISSN:0001-4966    

DOI:10.1121/1.393743

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The generation of acoustic waves by a modulated thermal source is examined for anisotropic materials. A wave equation is developed to include the effect of a thermoacoustic source for the anisotropic case. The dependence of the thermoacoustic source term upon the material elastic constants is identified for a thermal source varying in one dimension. This dependence is examined for several classes of crystalline anisotropy and it is found that thermoacoustic generation varies with crystallographic orientation. The directions of maximum and minimum thermoacoustic generation are not consistent for a given crystal class and are dependent upon the specific values of the elastic constants for the material.

ISSN:0001-4966    

DOI:10.1121/1.393672

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A number of high‐accuracy frequency estimators have been critically compared using simulated and real passive sonar data. Techniques considered are the standard Prony method, the extended Prony method of Marple, the Kumaresan–Prony algorithm, the Pisarenko algorithm, the maximum likelihood approach, and a phase interpolation method developed by the authors. The latter two techniques appear to offer the most promise for the analysis of sonar data.

ISSN:0001-4966    

DOI:10.1121/1.393673

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A linear, mathematical model of cochlear biomechanics is presented in this paper. In this model, active elements are essential for simulating the high sensitivity and sharp tuning characteristic of the mammalian cochlea. The active elements are intended to represent the motile action of outer hair cells; they are postulated to be mechanical force generators that are powered by electrochemical energy of the cochlear endolymph, controlled by the bending of outer hair cell stereocilia, and bidirectionally coupled to cochlear partition mechanics. The active elements are spatially distributed and function collectively as a cochlear amplifier. Excessive gain in the cochlear amplifier causes spontaneous oscillations and thereby generates spontaneous otoacoustic emissions.

ISSN:0001-4966    

DOI:10.1121/1.393674

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Acoustic emissions in the form of cubic difference tones (CDT’s), 2f1−f2, were measured in the ear canals of gerbils and cats. The state of the cochlea was manipulated by means of acute exposure to noise and was monitored with the aid of the whole‐nerve response to tone pips. The resulting shifts in the levels of emissions generated by pairs of primary tones of equal intensity were then compared to the corresponding threshold shifts of the whole‐nerve response across frequency. Data obtained from normal ears before injury indicate that the absolute thresholds of the whole‐nerve responses across frequency are not necessarily good predictors of theabsolutelevels of CDT emissions generated by 70‐ and 80‐dB SPL primaries. While high emission levels were often linked to low whole‐nerve thresholds in pre‐exposed ears, instances of animals with sensitive whole‐nerve thresholds coupled with very weak emissions were also found. Conversely, animals with poor whole‐nerve thresholds (shifted by up to 30 dB) could occasionally have high levels of emissions. After acute noise injury, however, theshiftsof emission levels as a function of the center frequency of the primary‐tone pair largely corresponded to the threshold shifts seen in the whole‐nerve response. In other words, the temporary level shift of an acoustic emission largely reflected the acute change to a specific cochlear region associated with the primary frequencies.

ISSN:0001-4966    

DOI:10.1121/1.393675

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP