摘要:

The radiation coupling or coupling by propagating waves is solved for a disk in an infinite baffle to a plane and back or equivalently a disk to a disk both in infinite baffles. The radiation coupling is defined as a linear filter operating between lumped mechanical components which may be incorporated into transducer models. The impulse response of the radiation‐coupling filter and the Fourier transfer function for the radiation‐coupling filter are solved in closed form. The radiation‐coupling gain (loss) is applicable to the correction of experimental data and to the absolute calibration of circular transducers by self‐reciprocity measurements.

ISSN:0001-4966    

DOI:10.1121/1.381325

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A theoretical model for the difference‐frequency radiation from a parametric acoustic array is developed. The (two‐frequency) primary wave is assumed to be radiated by a piston source and may be effectively limited in either the nearfield or the farfield of the piston. Either small‐signal absorption or saturation of the primary beam may serve as the limiting mechanism. The parametric gain is defined as a complex number whose (i) magnitude is the ratio of secondary and primary source pressures and (ii) phase is an indication of how much generation occurs within the nearfield and how much within the farfield of the primary beam. The parametric gain is evaluated as a function of the primary‐to‐secondary frequency (downshift) ratio, the amount of small‐signal primary absorption within the nearfield, and a ’’scaled’’ primary source level. Then, the parametric gain may be used to determine the beam pattern characteristics via a simple closed‐form expression. The theoretical results are found to be in good agreement with other theoretical treatments (within their realms of applicability) and in fair agreement with experimental results from various types of parametric sources. The discrepancies that do exist appear to stem from the approximation that the primary nearfield can be treated as a plane wave.

ISSN:0001-4966    

DOI:10.1121/1.381310

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

This paper presents the theory of the spherical, compliant‐tube Luneburg lens. Since a mixture of compliant tubes and water is dispersive, the compliant‐tube lens is a Luneburg lens only at a single frequency. The wave equation for the pressure field is solved for plane wave incidence. This solution corresponds to measurements made with an omnidirectional hydrophone on the lens. The wave theory is further developed to include solutions corresponding to measurements made by a dipole and cardioid hydrophone. The intensity, which is not proportional to the pressure squared, is also calculated. Numerical evaluation of these solutions is performed for a 10‐ft‐diam lens over the frequency range 500–5000 Hz (D/λ=1 toD/λ=10). This particular lens was chosen for study since it corresponds to a lens built by the Autonetics Division of Rockwell International. This lens is a Luneburg lens at 5000 Hz and is not perfect focusing for frequencies below 5000 Hz. The on‐axis gain, beam patterns, and directivity indices are calculated for the three sensors. In general, the on‐axis gain and directivity index are different. A comparison is made between measured results and theoretical results. Finally, the compliant‐tube lens is compared with a Luneburg lens (at all frequencies) and a liquid lens.

ISSN:0001-4966    

DOI:10.1121/1.381311

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The ray angle diagram presents loci as a function of depth of the angles which selected rays make with the horizontal as sound propagates in the ocean. It provides useful qualitative information about deep ocean propagation and the vertical distribution of signal and ambient noise power. The loci are based on Snell’s law. It is shown that an approximate ray angle diagram (ARAD) can be easily constructed directly from the sound‐velocity profile since on an appropriate scale the loci of one minus the cosine of the ray angle versus depth are approximately mirror images of the sound‐velocity profile. The approximations are shown to involve negligible errors for cases of interest in underwater acoustics. The use and construction of the ARAD are illustrated with examples. A simple graphical technique is presented for annotating the ARAD with range information from which rays may be plotted without the use of a computer.

ISSN:0001-4966    

DOI:10.1121/1.381312

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

We study the acoustic scattering which occurs when a plane wave is incident on an infinite hollow elastic cylinder covered with an ideally bonded layer of viscoelastic and sound‐absorbing material. The structure is immersed in water and it contains air in the cavity. The normal‐mode solution is found in all four media and programmed for numerical evaluation in the exterior fluid. Series expressions for the differential and the sonar scattering cross sections are constructed and added by computer. Two size cylinders are investigated in a parametric study, and oscillations in the cross‐sectional values due to resonances in the shell and coating materials are found and plotted for the basically low‐frequency rangek1c<20. The effect of a variable viscosity in the coating is analyzed, and as one would expect, progressively higher viscosity values damp out the rapid cross‐sectional oscillations and reduce the amplitudes to smaller values, down to an optimum point, beyond which the reverse effect begins to take place. The way the oscillating cross‐section peaks are shifted in frequency or damped out in amplitude by changing shell or coating sizes or materials, can be quantitatively used at one’s convenience for various design purposes.

ISSN:0001-4966    

DOI:10.1121/1.381313

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Two measurements (for the Hudson Bay and Baffin Bay) performed by the Naval Underwater Systems Center to determine volume attenuation at low frequencies have been reanalyzed. The reanalysis gives substantial evidence that the attenuation coefficients previously inferred for these areas are dominated by the influence of range dependence in the respective environments. The previous analysis assumed cylindrical spreading for the long‐range propagation loss, with departures ascribed to volume attenuation. The work here shows that such departures may be caused by range dependence of the environment for the Hudson and Baffin Bays. The conclusion is that noaprioriassumption about the dependence of loss on range is defensible for the long ranges involved in these experiments and that careful concurrent environmental measurements are required for use in the data analysis.

ISSN:0001-4966    

DOI:10.1121/1.381314

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The backscattering frequency responses for euphausiids and copepods are predicted using a fluid‐sphere model and measured physical properties for the zooplankters. The fluid‐sphere model is also compared with the resonant gas bubble equation.

ISSN:0001-4966    

DOI:10.1121/1.381326

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A normal‐mode theory, for scattering of sound by the ocean surface in the presence of refraction, is reviewed. The theory is based on a perturbation expansion for small surface‐wave heights. New results, based on this theory, are presented for the Doppler‐shifted signal sidebands. These sidebands are the solution of the first‐order perturbation problem and are represented exactly by a double normal‐mode expansion with coefficients given by certain integral representations. The new results are derived by a saddle‐point analysis of the integral representations, due account being taken of the presence of singularities. Specific numerical results for the case of upward refraction are shown to display considerably more structure than in the case of an isovelocity ocean. The increased structure is explained in terms of the effects of backscatter and focusing through the use of wave‐number diagrams and ray‐trajectory constructions.

ISSN:0001-4966    

DOI:10.1121/1.381327

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Infrasonic and low‐frequency flow‐noise measurements were made during laboratory tests of an H‐58 omnidirectional hydrophone. The hydrophone was tested in three configurations: bare, framed, and faired. The incident water speeds varied from 0.25 to 0.45 knot, which correspond to a Reynolds number range from 8800 to 16 000. The faired and framed configurations developed flow‐noise levels lower than the bare hydrophone, and the latter configuration developed the highest noise levels at the lowest frequencies of the 1–50−Hz band. Hydrodynamic factors which influence the acoustic measurements made during this test are discussed, and the importance of hydrophone configuration during very low‐frequency measurements is demonstrated.

ISSN:0001-4966    

DOI:10.1121/1.381328

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The vertical side‐lobe structure of cylindrical arrays driven in phase is examined. Experimental results demonstrate that the vertical side‐lobe levels are significant when the angular aperture of the array is large. The physical cause of this phenomenon is identified, and a method of eliminating excessive side‐lobe levels is explained. Experimental results showing that this method is successful are presented.

ISSN:0001-4966    

DOI:10.1121/1.381329

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP