摘要:

Measurements of volume backscattering strength have recently been made to a depth of 1500 m in the Northeast Atlantic. Scattering layers with strengths of −80 dB and −70 dB have been found at depths of approximately 500 and 1000 m, respectively. The layer at 1000 m has been shown to comprise fish shoals distributed with a density of 3.2 in 108m3, each shoal having a target strength of about −6 dB and comprising about 50 fish. Each fish is thought to have a swimbladder resonant at 2 kHz at 1000 m and hence to have a radius of 1.6 cm. The shoals have been shown to vary with geographical location but not with the time of day, 1000 m being about the maximum depth of daylight penetration. The layer at 500 m does not vary much with geographical location but does exhibit diurnal variations due to the well‐known vertical migration of fish in a diurnal cycle.Subject Classification: [43]30.40.

ISSN:0001-4966    

DOI:10.1121/1.381196

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The Sommerfeld model, a constant‐density isospeed half space over a second constant‐density isospeed half space, has offered extensive insight into the role of the ocean bottom in acoustic propagation by exploiting properties of the ground or lateral wave and the specularly reflected waves. This paper presents some new results for this model. A uniform asymptotic expansion is obtained for the source and receiver on the interface, which reduces to the exact solution as either the density ratio or the sound‐speed ratio approaches unity. In addition, this result shows the ground or lateral wave decays initially—not like 1/kr2, the classical result, but like 1/r, and as the index ratio approaches unity this region increases. For the source and receiver above the interface, the reflected fields are expressed in a uniform expansion involving the parabolic cylinder functions. Away from the critical ray these expressions reduce to the usual specular and lateral fields. The expressions reduce to the interface results and are not singular as the sound‐speed index approaches unity, and confirm the above decay predictions for the lateral wave.Subject Classification: [43]30.20, [43]30.30; [43]20.15.

ISSN:0001-4966    

DOI:10.1121/1.381197

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

In this paper, a unified approach of the possible bondings between two elastic half spaces is studied. Assuming that two elastic half spaces are separated by a Newtonion viscous liquid layer of thicknessH, and of shear viscosity coefficient η, the condition for the existence of interfacial waves propagating along the layer is derived. This condition is separately examined in the three possible limits of η/HwhenH→0: (1) η is finite or tends to zero such that η/H→∞; (2) η tends to zero such that η/H→0; and (3) η tends to zero such that η/H≠0 finite. It is established that the case (1) corresponds to welded contact of solid half space studied by Stoneley and cases (2) and (3) correspond, respectively, to smooth and loosely bonded interface of solid half spaces. This theory established that η/Hcan be used under the above‐stated limits as a measure of bonding at the interface, hence its inverse as a measure of smoothness.Subject Classification: [43]35.24; [43]20.15.

ISSN:0001-4966    

DOI:10.1121/1.381198

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Image‐intensity functions have been calculated for acousto‐optic phase‐resolved schlieren in the phase grating (Raman–Nath) diffraction limit. The calculation applies to the case where a circular stop is used. Any number of diffraction orders may be stopped. The results are programmed into a laboratory GT‐44/PDP‐11 real‐time graphic system and compared to experimental observations. The latter include TV monitoring of both the diffraction orders and the image distribution and photomultiplier‐pinhole recording of the image‐intensity distribution. It appears that phase resolution will permit quantitative pressure level determinations. The results aid significantly in interpreting phase‐resolved schlieren observations from radiation and scattering mechanisms.Subject Classification: [43]35.65.

ISSN:0001-4966    

DOI:10.1121/1.381206

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The Rayleigh wave velocity on an otherwise isotropic‐free surface is changed by the application of a static stress. A technique is described for calculating these changes as a function of the third‐order elastic constants and the static stress configuration. Conversely, these velocity changes can be used to measure the third‐order elastic constants. These corrections in the velocity appear as additional terms to the isotropic Rayleigh equation. Three independent Rayleigh velocity measurements are sufficient to determine the third‐order elastic tensor for an isotropic solid. Experimental data is provided to validate the calculated results.Subject Classification: [43]35.26; [43]20.35; [43]35.54.

ISSN:0001-4966    

DOI:10.1121/1.381207

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The equations for calculating the acoustic field produced within a nonmagnetic metal by an interaction of eddy current with a static magnetic field were obtained on the assumptions that (1) an ultrasonic wave is generated by the electromagnetic force through classical and macroscopic phenomena; (2) the electric, magnetic, and elastic properties of the metal are linear, isotropic, and homogeneous throughout the metal, which occupies semi‐infinite space; (3) the whole system is axially symmetric; and (4) eddy currents and elastic waves show a steady‐state sinusoidal variation. The acoustic field produced by a specific electromagnetic ultrasonic transducer with axial symmetry was calculated numerically, and the results showed a well‐defined ultrasonic wave beam, which was narrower than had been expected from the size of the transducer.Subject Classification: [43]35.60; [43]85.48.

ISSN:0001-4966    

DOI:10.1121/1.381208

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The plane‐strain problem of the scattering of a Rayleigh wave by a thin surface strip is formulated in terms of a pair of coupled integral equations, one of which is singular. The equations are equivalent to a set of boundary conditions which represent the effect of the strip on the substrate. A technique based on Gaussian approximation of the integrals is employed to solve the equations numerically for two representative cases over a range of wavelengths. The power reflection and radiation coefficients and the phase of the reflected surface wave are evaluated for the representative cases.Subject Classification: [43]35.54; [43]20.15, [43]20.30.

ISSN:0001-4966    

DOI:10.1121/1.381209

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Predicting community noise levels requires the addition of background ambient sound levels and the source’s contribution to ambient sound levels. Baseline data are obtained, from community sound surveys, in the form of histograms and cumulative distributions. Source (for example, a construction site) sound level data are also obtained as histograms and cumulative distributions. The problem of combining the two distributions is made somewhat difficult by the logarithmic nature of the physical variable. The prediction is accomplished by the mathematical convolution of the variables representing baseline and the source. Since the data are provided as histograms rather than as continuous probability density functions, it is convenient to decompose the density functions into a sequence of Dirac delta functions. The result can be programmed for a rapid and efficient computation. Results are provided for the combination of various construction site sound level distributions (e.g., normal and bimodal) with community background ambient sound. Data are shown of the effect on community noise of a source contribution which is attenuated by distance.Subject Classification: [43]50.75.

ISSN:0001-4966    

DOI:10.1121/1.381210

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The transmission loss characteristics of flow‐reversing muffler chambers were predicted by a numerical approach based on the finite element method. The theoretical model developed is described in this paper; its validity is established experimentally with a number of different chambers. The standing wave method was used to measure transmission loss and measurements were conducted with and without steady air flow. Depending on the inlet and outlet configurations, the flow‐reversing chambers have two completely different transmission loss characteristics, one is similar to that for two sample expansion chambers in series and the other is similar to that for a side‐branch resonator. Using the theoretical model developed, transmission loss curves were calculated for a large number of different flow‐reversing chamber dimensions.Subject Classification: [43]50.30, [43]50.40.

ISSN:0001-4966    

DOI:10.1121/1.381211

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The contribution of type and sound levels of community noises to the perception of loudness, noisiness, and annoyance was studied in a laboratory situation. The psychological attributes were precisely defined to the 30 observers, the method of magnitude estimation was used for scaling the attributes, and the scales were calibrated to a common unit of measurement. It was demonstrated that observers in carefully designed laboratory experiments are able to use and produce scales of loudness, noisiness, and annoyance for community noise. The relationships between the attributes were satisfactorily described by linear functions, the parameters being specific to the type of community noise. In general, community noises are judged to be more noisy (or annoying) than loud although the importance of noisiness relative to loudness varies with type of noise as well as loudness level. A linear model is suggested that describes community noises with regard to perceptual attributes. The model states that annoyance and noisiness are proportional to loudness, thus encouraging the viewpoint that psychoacoustical research may well concentrate on the attribute of loudness.Subject Classification: [43]65.50, [43]65.52; [43]50.75, [43]50.20.

ISSN:0001-4966    

DOI:10.1121/1.381212

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP