摘要:

The physical‐optics (Kirchhoff) method is used to determine the differential scattering cross section and the sonar cross section of a sphere partially insonified by a beam intersecting the surface of the sphere at a circular spot. The sphere is first assumed bare and then partially covered with a raised, centered cap which is smaller than the beam spot. A procedure to make the Kirchhoff method yield directly thebistaticcross section is extended to account for partial insonification and for the cap’s presence. We show that partial insonifcation alone can enhance the normalized sonar cross section up to four times the geometrical acoustics value, and with the additional introduction of a moderately thin and small cap, the enhancement goes beyond an order of magnitude. Quantitative plots of the cross sections are displayed and the general results are seen to reduce properly to, and in one instance, correct, particular cases available in the literature.

ISSN:0001-4966    

DOI:10.1121/1.381411

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The theory of electroacousticNports, together with the theory of electroelastic normal modes, is applied to the calculation of the electroacoustic sensitivity of radially polarized, thin, ferroelectric shells. Electroelastic, short‐circuit, normal modes are computed by solving the simplified one‐dimensional approximate equations of thin‐shell theory, and the results are used to compute the admittance‐matrix elements of anN‐port representation of the shell. Once the admittance‐matrix elements have been computed, the calculation of the open‐circuit sensitivity follows from a straightforward application ofN‐ port theory. Computations of the open‐circuit sensitivity of capped, uniformly polarized and bipoled shells are made as a function of frequency. The result is that the bipoled shell has 6 dB greater sensitivity than does the uniformly poled shell. In addition, our calculations indicate the frequency range over which one can reasonably use classical, low‐frequency results. The paper concludes with the calculation and discussion of electroelastic, open‐circuit, normal modes of the uniformly poled shell.

ISSN:0001-4966    

DOI:10.1121/1.381412

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A curved face hydrophone array has been designed, constructed, and tested for 20 000‐ft‐depth operations. A basic description of the hydrophone design, in which the hydrophone consists of a beamforming array of staves mounted on the circumference of a 20‐in‐diam. cylindrical housing, is presented. Since conventional pressure‐release materials cannot be used in the construction, solutions to problems encountered, such as improper backing impedances for the individual elements and flexural waves genernated in the housing wall, will be briefly discussed. A presentation of the hydrophone frequency response and beam patterns shows that the transducer properties are insensitive to hydrostatic pressures as great as 1000 psi.

ISSN:0001-4966    

DOI:10.1121/1.381413

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Analytical methods are employed to obtain general results for the effect of cw source motion on the total acoustic field at a fixed receiver in the deep ocean. A bilinear sound‐speed profile is used, a long range is assumed, and SOFAR rays are considered. Equations are developed which give amplitude and phase as functions of receiver time. Then, an investigation is made of stability regions, defined as range intervals for which all rays for given numbers of SOFAR‐axis crossings arrive at the receiver. Three types of such regions are identified and examined. Amplitude and phase are linearized for a short source run, and novel numerical results for these quantities are presented. Approximate formulas for cumulative phase and Doppler‐shift frequency are derived and discussed. Finally, an analytical description is presented for two mechanisms responsible for amplitude fades.

ISSN:0001-4966    

DOI:10.1121/1.381414

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Acoustic propagation is simulated in the presence of a first‐mode internal wave with a 90‐km wavelength. For a shallow source, deep receiver, and a 450‐km source‐receiver separation, phase modulation of the multipath sum increased linearly with increasing acoustic frequency and with increasing internal‐wave amplitude. Modulation was highly dependent upon internal‐wave direction with the greatest modulation occurring when the internal‐wave propagation direction was perpendicular to the acoustic path.

ISSN:0001-4966    

DOI:10.1121/1.381415

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A number of experiments based in the South Pacific Ocean have now been reported in which the attenuation of underwater sound at very‐low‐frequencies displays an apparent regional dependence, providing a possible explanation for the considerable spread in at sea attenuation measurements below 1kHz. This regional dependence has been interpreted in terms of acoustic scattering from oceanographic inhomogeneities of various forms, and at least two recent theoretical variants based on such a mechanism have been proposed. This paper presents evidence of a similar regional dependency in the North Pacific Ocean and examines the significance of the data to the two theoretical treatments currently most successful in accounting for the effects observed.

ISSN:0001-4966    

DOI:10.1121/1.381416

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Attenuation of the coherent component of sound propagating in shallow water with rough boundaries is studied. Normal‐mode theory is used in conjunction with a boundary perturbation method. The inclusion of surface and bottom scattering cause the eigenvalues to become complex and explicit mathematical expressions for the imaginary parts of these eigenvalues are derived. These expressions correspond to the individual normal‐mode attenuation coefficients due to scattering. Numerical results are presented for sample shallow‐water environments with depth‐dependent sound‐speed profiles and lossy bottom sediments. The profile structure and the acoustic properties of the bottom sediment have a significant effect on the total attenuation due to scattering.

ISSN:0001-4966    

DOI:10.1121/1.381417

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The correlation of ambient noise fluctuations in each of seven frequency bands at frequenciesf1with those in other bands (f2) was investigated for a 4 1/2−d period in February 1976 with a hydrophone laid on the bottom in deep water. The bandwidth was generally 6 Hz; band center frequencies ranged from 5 to l50 Hz. A total of 572 consecutive samples, 640 sec each, was taken concurrently in each frequency band. The average level for each sample was used in the computation of correlation coefficient. Tonal line components were excluded. The correlation coefficients betweenf1andf2generally fell off with increasing frequency spacing betweenf1andf2, but the patterns differed with different values off1. Day‐to‐day processing showed very high variability, due perhaps to changing weather.

ISSN:0001-4966    

DOI:10.1121/1.381418

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

This paper discusses a free‐ and a forced‐vibration problem of elastically cross‐supported rectangular viscoelastic plate with various edge conditions. A three‐elements viscoelastic model is adopted in the analysis. The result for the viscoelastic plate is obtained from the correspondence principle by applying the Laplace transform to the constitutive equation for the viscoelastic materials and to the equation of motion in terms of unknown forces which are equivalent to reaction forces and resisting moments of the supports. Some numerical results are shown for steady‐state and transient response problems.

ISSN:0001-4966    

DOI:10.1121/1.381419

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

The behavior of dynamic vibration absorbers of conventional and novel design has been investigated experimentally and found to compare closely with prediction. The dynamic absorbers were employed to suppress the transmissibility at resonance across a 1‐df primary system. Initially considered was a dynamic absorber with a conventional mass‐spring‐dashpot configuration; the primary system was undamped. Subsequently considered were (1) so‐called dual dynamic absorbers, and (2) a single, nominally undamped absorber, or two such absorbers, attached to the primary system after it had been damped heavily. The dual absorbers—a conventional viscously damped absorber used in parallel with a less massive undamped absorber—introduced a pronounced transmissibility trough without the appearance of unwanted ’’compensating’’ peaks at lower and higher frequencies. The attachment of a nominally undamped absorber to the heavily damped primary system also introduced a pronounced trough without giving rise to compensating peaks. Further, the attachment of two such absorbers introduced pronounced troughs at two frequencies that could be varied independently of one another. Thus, the novel absorber systems considered here behaved as mechanical ’’notch’’ filters, providing at specific ’’low’’ frequencies a high degree of isolation that other passive systems cannot duplicate without exhibiting a marked loss in isolation at neighboring frequencies.

ISSN:0001-4966    

DOI:10.1121/1.381420

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP