摘要:

Free wave motion on a fluid‐loaded semi‐infinite membrane whose plane is extended by a rigid baffle is discussed. We consider in particular the membrane displacement field and the associated acoustic radiation when a wave on the membrane is normally incident on the membrane boundary. Typical values for the reflection coefficient for the waves, and for the radiated acoustic power are given for a wide range of membranein vacuowavespeeds and fluid loading parameters. It is shown that the commonly used predictions for edge‐mode radiation obtained by neglecting the influence of the fluid on the membrane displacement are valid except when the fluid loading is very high.

ISSN:0001-4966    

DOI:10.1121/1.1914491

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

For much of the World's oceans, the stability of the water column beneath the thermocline is quite well represented byN2(z) = gρ−1 ∂zρ′ = NO2e−2z/B, where ∂zρ′ is the vertical gradient in potential density,NO≈ 3 cycles/h is the surface‐extrapolated “Brunt‐Väisälä” frequency, andB≈ 1.3 km is the stratification scale. This leads to an idealized sound channelC(z) = C1 [1 + ε(η + e−η − 1)] = C[1 + ε(1/2)η2 − (1/6)η3 + …]with a minimum velocity at the axisz = z1, η = (z − z1)/12Bbeing dimensionless depth relative toz1. The parameters ε,z1are explicitly expressed in terms of the five coefficients α, β, γ,a, b(temperature, salinity, pressure coefficients ofC; temperature and salinity coefficients of ρ′), given only the form ofN(z) and a representativeT, Srelation. The up‐down asymmetry of the channel, a consequence of the fundamental structure of the oceans, plays a first‐order role in the propagation characteristics. As an application, the ray arrivals for an axial source and receiver are computed.

ISSN:0001-4966    

DOI:10.1121/1.1914492

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

An experiment performed in deep water near San Clemente Island, California, showed that discrete scatterers were the primary source of acoustic volume reverberation at the test site. Scatterers reflected the incident waveform without apparent distortion. Estimates of the scatterer density and scatterer reflectivity distribution were obtained from experimental data, but the estimates were distorted by the transducer beam pattern and by overlapping echoes. Analysis of the reverberation envelope showed that reverberation was homogeneous in the horizontal plane and that the reverberation envelope amplitude was not Rayleigh distributed. A computer was programmed to simulate volume reverberation according to a discrete scattering model developed by Faure, Ol'shevskii, and Middleton. The scatterer density and reflectivity estimated from experimental data were changed until simulated and measured reverberation envelopes were statistically alike. When simulated and measured envelopes were alike, improved estimates of scatterer density and scatterer reflectivity were obtained from the simulation. The scatterer density which gave best agreement between simulated and measured envelopes was 0.0010 scatterers per cubic foot. The scatterer reflectivity was approximately exponentially distributed.

ISSN:0001-4966    

DOI:10.1121/1.1914493

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The effects of a fluid‐loaded spherical shell on the propagation to the farfield of an internally generated, transient acoustic pulse is obtained. The pulse is generated by a spherical transducer that is located concentrically within the shell and has a specified spatial and temporal velocity distribution. A shell theory that incorporates the effects of transverse shear and rotary inertia is used. The fluid between the transducer and the shell, and the fluid external to the shell, is described by the linear acoustic wave equation. The spatial dependence of the transmitted pulse is expanded in a series of spherical harmonics. The temporal dependence is determined by numerical inversion of the Fourier transformed solution. Computations are performed for axisymmetric transducer velocity pulses of realistically short duration. It is found that a steady‐state analysis is incapable of predicting the nature of the farfield pulse, since the applied waveform and the farfield waveform were dissimilar. The shell membrane breathing mode, shifted up in frequency from itsin vacuovalue because of the stiffness of the confined volume of fluid between the transducer and the shell, is found to dominate the farfield response.

ISSN:0001-4966    

DOI:10.1121/1.1914494

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

In this paper the spectral and covariance properties of surface‐scattered acoustic fields are determined for slightly rough and very rough sea surfaces that are irradiated with plane wave single‐frequency sources. The results obtained are general in the sense that they depend only on the frequency‐wavenumber spectrum of the stochastic sea surface and not on any particular surface spectrum. Exact results are obtained for sinusoidal seas and these results are compared with various approximate solutions. The approximations clearly indicate the connection between sinusoidal seas and arbitrary seas.

ISSN:0001-4966    

DOI:10.1121/1.1914495

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Reported instances of stable nonflow in He II Josephson effect experiments have in most cases been achieved by a flow of liquid helium from one reservoir to another, a flow driven by ultrasonic pumping. We have examined some of the characteristics of this pumping. We confirm that a threshold transducer power is necessary to initiate pumping and find that the threshold power has a temperature dependence of ρsTα, 0<α<1, in the range 1.5K

ISSN:0001-4966    

DOI:10.1121/1.1914496

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

This paper examines an analytical model of the dynamic response, vibration isolation, and sound radiation of a resiliently supported track slab. Railroad rails are viewed as beams fastened elastically to a slab which is resiliently mounted to a substructure, and the slab is assumed to be a plate of finite width and infinite length. The analysis accounts for several different mechanisms of vibration transmission from the rail to the slab, including structural nearfield, farfield, and coincidence effects. The results show that the slab is a very effective vibration isolator but is a sounding board at low frequencies. Proper choice of rail and slab parameters can make this low‐frequency radiation comparable to the low‐frequency sound ordinarily generated by an electrically powered rail vehicle.

ISSN:0001-4966    

DOI:10.1121/1.1914497

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

An approximation method is presented for solving the equations of motion that describe the vibration of an elastic structure immersed in an infinite acoustic fluid medium. The mathematical model that is developed uses the finite element method to calculate the vibrational characteristics of the elastic body and the acoustic pressure field of that portion of the fluid which closely surrounds the vibrating body. Analytical methods are used to obtain the boundary conditions for this mathematical model. This technique can be used to predict the response of an elastic structure over a wide range of frequencies and the acoustic pressure at a large number of field points in both the nearfield and farfields. It avoids the difficulties encountered at eigenvalues of the interior problem by determining exactly the one degree of freedom needed to overdetermine the surface equations. Experimental validation of theoretical predictions is given for a piezoelectric cylinder, and a computer‐generated contour plot of a predicted nearfield pressure distribution is shown.

ISSN:0001-4966    

DOI:10.1121/1.1914498

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

A general procedure is presented for the nonlinear analysis of the forced response of structural elements to harmonic excitations. Internal resonances (i.e., modal interactions) are taken into account. All excitations are considered, with special consideration given to resonant excitations. The general procedure is applied to clamped‐hinged beams. The results reveal that exciting a higher mode may lead to a larger response in a lower interacting mode, contrary to the results of linear analyses.

ISSN:0001-4966    

DOI:10.1121/1.1914499

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Pitch can be signalled by interaural differences in timing, but it is unknown how the intercochlear timing mediates pitch. Pitch of complex tones can be well understood in terms of a central pitch mechanism that reads across the frequencies of a centrally presented spectrum. We consider whether pitch phenomena signalled by binaural timing are also compatible with auditory processing through tonotopic organization. Known binaural pitch phenomena produced by stimuli comprising two wideband Gaussian noise sources, each with different interaural delay were examined. The major findings are that (i) a single noise source with interaural delay τ is adequate to produce a pitch corresponding to 1/τ, (ii) the medially lateralized pitch image is little affected by the addition of a second independent noise source presented in antiphase, (iii) the dichotic pitch is similar to monaural repetition pitch, for which the stimulus to the ear comprises a noise source added to its delay, and (iv) the image of dichotic repetition pitch is much more faint and compact than that of monaural repetition pitch. Durlach's [J. Acoust. Soc. Am.34, 1096–1099 (1962)] equalization and cancellation model of binaural signal detection is adapted to describe the compatibility between the binaural pitch phenomena and auditory mediation of pitch by central spectral patterns.

ISSN:0001-4966    

DOI:10.1121/1.1914500

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP