摘要:

In the analysis of the power transmission through junctions between thick plate structures, it is necessary to consider not only the transverse bending motion of the plates but also the in‐plane wave motion. That is, for these types of structures both the shear and rotary inertia effects and the in‐plane wave effects must be considered simultaneously in order to obtain a complete solution to the problem. In this paper, a statistical energy analysis approach is developed to evaluate the power transmission through the junction between two plates in an L‐shaped configuration, where the solution includes the shear and rotary inertia effects (Mindlin bending) and the in‐plane waves effects. Analytical results are presented for the absorption (or transmissibility) of the junction and the ratio between the incident bending wave power and the transmitted waves (bending and in‐plane) power. Results for pure (classical) bending are also presented for comparison with the Mindlin bending results.

ISSN:0001-4966    

DOI:10.1121/1.400304

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The influence of whole‐body vibrations, noise, and a combination of the two, caused by heavy road traffic (150 events/night) on sleep, subjectively experienced sleep quality, and performance was investigated under controlled laboratory conditions for male and female subjects 20–35 years of age. A room was built above a vibrator table, with the legs of the bed mounted directly on the table through holes in the floor. Vertical vibrations were found to be attenuated by the mattress with 20–40 dB for frequencies>10 Hz, whereas horizontal vibrations were slightly amplified. It could be concluded that when traffic noise [50‐dB(A) peak level] is accompanied by vibrations with peak levels of 0.24 m/s2vertically and 0.17 m/s2horizontally as measured on the frame of the bed (stimulus duration approximately 2 s, dominant frequency approximately 12 Hz), sleep is more disturbed than is the case when noise alone occurs. The amount of REM sleep, which was significantly reduced for the vibration level mentioned above, was even more disturbed when a higher exposure level, 0.34 m/s2vertically and 0.24 m/s2horizontally, was applied. The subjectively rated sleep quality was lower for the higher than for the lower vibration level. Performance in the morning was only influenced for the higher vibration level. It could be concluded that vibration exposure levels near the recommendation made in ISO‐standard 2631 for the awake state disturb sleep in man.

ISSN:0001-4966    

DOI:10.1121/1.400305

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The use of a pulsed laser source for the generation of elastic waves in materials is investigated, taking into account optical penetration into the material. Under appropriate conditions, a significant feature of the laser‐generated elastic waveform is a precursor (sharp spike) signaling the arrival of the longitudinal wave. The shape of this precursor signal is strongly dependent on the optical and thermal properties of the material. This paper shows that the observed precursor can be understood through the use of models that account for optical penetration and thermal diffusion into the material.

ISSN:0001-4966    

DOI:10.1121/1.400306

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Results are reported from an experiment, conducted in 1987, in which an ocean bottom seismograph array of 150‐m aperture and minimally redundant design was used to record the ambient noise in deep water off the California coast. The minimum interelement spacing among the nine instruments was 8 m. The coherence lengths observed imply that the noise field in the 0.05‐ to 5‐Hz band are fundamental and higher‐mode Rayleigh/Stoneley/Scholte waves and the relative amplitudes of the modes imply that the excitation occurs within 20 km of the array. These observations imply that the noise energy is scattered into the seafloor waveguide at the boundaries of the sediment pond in which the array was sited. The implications for sub‐bottom sensors are discussed.

ISSN:0001-4966    

DOI:10.1121/1.400307

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The problem of determining the sound field due to a point harmonic source in an ocean environment with an arbitrary three‐dimensional local variation of field parameters (including irregular interfaces) is studied with the help of a variational principle. The variational principle is established by proving that the Euler–Lagrange equations of a suitable functional coincide with the field equation and the exact boundary and interface conditions of the complete elliptic boundary value problem. The method presented in the paper is a hybrid one, in the sense that the pressure field is represented by modal series expansions in the range‐independent region, and a suitable localized expansion in the range‐dependent region of the environment. These representations retain full mode coupling and, in conjunction with the variational principle, reduce the problem to a linear system of equations free of anyapriorirestrictions. It is expected that this system would be efficiently used for the numerical solution of the problem, especially in a shallow water environment and in the low‐to‐moderate frequency range.

ISSN:0001-4966    

DOI:10.1121/1.400308

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The scattering of acoustic waves in an unbounded elastic medium containing a great number of uniformly distributed fluid‐filled closed spherical pores of the same size is studied in this paper. Relations between wave slowness and porosity are derived in the lowest long‐wavelength approximation both for the compressional and shear waves. The results agree well with the experimental data. The Rayleigh’s fourth power law for the decay constants is also derived. The fluid inside the pore is assumed to be inviscid. The relations obtained are useful for geophysical exploration. As by‐products, equivalent Lamé constants of the porous elastic medium are also obtained.

ISSN:0001-4966    

DOI:10.1121/1.400309

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Reverberation intensity fluctuations are not only important because they impact the performance of active sonar systems, but also because they may provide insight into the major scattering processes. In this article, a model for the fluctuations of signals backscattered from rough surfaces is developed, and its predictions are compared with high‐resolution bottom backscatter data. For reverberation due to Bragg scattering, the model predicts a unique dependence on beamwidth, which is supported by the data. A dependence on waveform, i.e., a frequency diversity effect is also predicted. This effect is, however, not unique to the Bragg scattering process.

ISSN:0001-4966    

DOI:10.1121/1.400310

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Empirical relationships between compressional wave (sound) velocity and selected physical properties have been determined for fine‐grained abyssal sediments from the east Brazilian continental margin and Pernambuco abyssal plain of the Brazil Basin (NW South Atlantic Ocean). Porosity and wet bulk density were found to be the best sedimentological indices to sound velocity. In agreement with numerous other studies, other sediment properties were lesser, but still very good, indices. In order of decreasing statistical significance, these properties include mean grain size and percent clay. Averaged values of sediment physical properties of the Brazil Basin were compared with the published results of Hamilton and Bachman [J. Acoust. Soc. Am.72, 1891–1904 (1982)] and show good agreement. The observed differences between the two studies were insignificant and attributed to slight mineralogical differences that can be expected in light of the differing sedimentation processes of the Pacific and Atlantic Oceans. Velocities of the Brazil Basin study, converted toinsituconditions from laboratory measurements, were compared with those previously reported by Hamilton and Bachman, which were converted toinsituconditions using bottom water velocities of this study and their velocity ratios. A plot of the velocities showed good agreement, but those of the present study were about 20 m/s lower. This discrepancy is probably not mineralogical in origin, but instead results from a systematic difference in the sediment velocimeters used in the two studies. The exact cause of the discrepancy is not known, and a detailed system evaluation of the velocimeters is recommended.

ISSN:0001-4966    

DOI:10.1121/1.400311

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Extinction is sometimes a major problem in acoustic surveys of fish stocks, as it often occurs when the fish are concentrated and easiest to survey. The same may be true of certain macrozooplankton, such as krill in swarms. This study aims to describe how to correct single‐ping measurements of the vertical distribution of scatterer density for extinction. The general case is considered in which the aggregation density is variable and the mean backscattering and extinction cross sections vary with depth. By dividing the water column into a finite number of layers, with constant properties within each, a closed‐form mean‐field solution is derived. Methods of applying this to single‐ping echo records and the quality of the solution are both examined. Extinction is discussed vis‐à‐vis multiple scattering. Application of the technique in other areas, e.g., in remote probing of the atmosphere by lidar, is mentioned.

ISSN:0001-4966    

DOI:10.1121/1.400312

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Experiments on the propagation of planar acoustic waves in aqueous solutions of high‐molecular‐weight polyethylene oxide and polyacrylamide are discussed. The acoustic absorption of the solutions was determined at 2.5 and 7.5 MHz, and the growth and decay of the amplitude of the second‐harmonic component generated by the 7.5‐MHz source was measured. It was found that the absorption and second‐harmonic amplitude generally increased with both concentration and molecular weight. However, the absorption was found to be much less than in Newtonian fluids with equal zero‐shear‐rate properties (i.e., viscosity, density, and small‐signal wave speed). Additionally, the frequency dependence of the data indicate that a relaxation process occurs in both polymers at a characteristic frequency in the range 1–10 MHz. At low polymer concentration, the second‐harmonic amplitude was less than that found in water under similar experimental conditions.

ISSN:0001-4966    

DOI:10.1121/1.400313

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP