Index of content:
Volume 60, Issue 5, November 1976
60(1976); http://dx.doi.org/10.1121/1.381192View Description Hide Description
Guidelines are presented for the preparation of illustrated talks which are audience oriented and which are aimed at the efficient transfer of technical information. Early decisions concerning the required number of slides are helpful in initial planning for a good‐quality talk. Detailed considerations are the establishment of limited objectives, selection of appropriate slide material, development of a text which is well coordinated with the slides, and accurate timing.
Subject Classification: 10.60.
Spherical‐wave reflection from a boundary of reactive impedance using a modification of Cagniard’s method60(1976); http://dx.doi.org/10.1121/1.381193View Description Hide Description
The solution using Cagniard’s method for the reflection of a spherical wave from a locally reacting plane boundary is compared to that using the steepest descents method. The contribution from the presence of a pole is less arbitrary using this method and the transition from the pole to the no pole case is shown to be gradual. A further solution in the time domain for an incident time impluse function is also derived.
Subject Classification: 20.15, 20.30; 28.40.
60(1976); http://dx.doi.org/10.1121/1.381199View Description Hide Description
A finite‐element approach is developed to simulate acoustic transmission systems of arbitrary profile with circular cross section. Sound‐transmissioncharacteristics are considered in which axisymmetric radial modes are taken into account. Some simple examples of acoustic filters with various profiles are calculated for which the numerical values are compared with the measured ones, as well as those of the one‐dimensional models based on an electrical transmission line analogy. The procedure is a valid and powerful means to cope with filters of any profile and may pave the way to solutions of problems of this kind.
Subject Classification: 20.40; 85.32..
60(1976); http://dx.doi.org/10.1121/1.381200View Description Hide Description
Analysis of reflection of plane waves from a flat boundary in a micropolar half space poses tedious calculations. In this paper, utilizing the concept of slowness surfaces, a simple and efficient method is proposed as an alternative to the previous analytical methods. Critical angles of incidence, the nature of the reflected body, and boundary waves corresponding to the incident longitudinal displacement and coupled transverse displacement microtational waves are studied.
Subject Classification: 20.15, 20.30.
60(1976); http://dx.doi.org/10.1121/1.381201View Description Hide Description
The value of a diagram in which the angle of an acoustic ray is plotted versus its depth is explored. The concept of source line in angle‐depth space is developed. Characteristics of different sound channels, reflection, multipath, caustics, and the analysis of obstacles such as seamounts are described within the context of the diagram. The application of Liouville’s theorem is discussed.
Subject Classification: 30.20; 20.20.
60(1976); http://dx.doi.org/10.1121/1.381202View Description Hide Description
Explosive data from a carefully controlled transmission‐loss measurement in the Northeast Atlantic Ocean are used to evaluate, in detail, the capability of the AESD parabolic equationmodel. The analysis is performed on a path‐by‐path basis by comparison of computed (using a ray‐tracing program) and measured travel‐time differences. Sensitivtiy studies show that residual discrepancies could be attributed to uncertainties in source detonation depth and the assumption of piecewise‐linear bathymetry. The data show an arrival with no ray‐tracing counterpart; evidence, based upon its mean speed of propagation and the frequency dependence of its energy content, is presented to argue that it is an arrival ducted in the main sound channel. The behavior of this arrival with range is determined by the range dependence of the sound‐speed environment.
Subject Classification: 30.20; 20.20.
60(1976); http://dx.doi.org/10.1121/1.381203View Description Hide Description
Estimates of sound (and infrasound) spectral levels are given using a theoretical model of acoustic production based on quadratic interactions of oppositely traveling ocean waves, and incorporating present day surface wave spectral models. It is shown that reasonable agreement with measurements exists for frequenices less than 10 Hz but predictions are 10–15 dB low and have the wrong spectral shape at higher frequencies.
Subject Classification: 30.40; 20.15; 30.70; 28.65.
60(1976); http://dx.doi.org/10.1121/1.381204View Description Hide Description
In a recent experiment employing a number of hydrophone systems deployed at various positions in the Northeast Pacific Ocean, several operational periods were dedicated to an ambient sea‐noise study. Signals received at hydrophones distributed at various depths throughout the water column were analyzed to characterize the noise field at each site. The results obtained revealed a noise‐level dependence on frequency, hydrophone depth, and sound velocity profile. The general features of the noise field could be interpreted in terms of distant shipping and local wind effects, the two major directional noise sources believed to be dominant in the deep ocean. This paper discusses some specific features of the noise field and examines the implication of these to the anomalously high attenuations reported so often at frequencies below 100 Hz.
Subject Classification: 30.20, 30.70.
60(1976); http://dx.doi.org/10.1121/1.381205View Description Hide Description
An experimental investigation to determine the acoustic transmission from a source in air to a hydrophone in a wind roughened ocean at shallow depression angles is described and the results presented. This experiment was conducted at the NUC Oceanographic Tower in 18‐m‐deep water. Data were obtained for acoustic frequencies of 500–4000 Hz and wind speeds of 3–15 knots. Short pulses of sound were used in order to distinguish the direct path from the bounce paths. The mean energy at the hydrophone was obtained by averaging the direct path energy over many independent pulses. The data show that more energy reaches the hydrophone in the rough ocean than predicted by the smooth surface theory, with the excess increasing for the shallower depression angles. The data also show that the transmission increases with increasing source frequency and ocean roughness and is independent of source altitude for the conditions considered. The experimental results are compared with a geometric rough surface model and show good agreement.
Subject Classification: 30.20, 30.25; 20.20.
60(1976); http://dx.doi.org/10.1121/1.381195View Description Hide Description
Low‐frequency propagation measurements in sound channels often show a frequency‐independent excess loss that is evidently caused by internal scatter from large‐scale inhomogeneities. Using the Garrett–Munk internal wave spectrum (GM 75) as the scatteringmodel, we have obtained an approximate solution of the ray‐diffusion equation on the deep sound‐channel axis. The estimated attenuation coefficient, ∠5×10−4 dB/km, is consistent with the smaller experimental values reported.
Subject Classification: 30.20; 20.20; 30.40.
60(1976); http://dx.doi.org/10.1121/1.381196View Description Hide Description
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 108 m3, 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: 30.40.
60(1976); http://dx.doi.org/10.1121/1.381197View Description Hide Description
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/k r 2, 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: 30.20, 30.30; 20.15.
60(1976); http://dx.doi.org/10.1121/1.381198View Description Hide Description
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 thickness H, 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/H when H→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/H can 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: 35.24; 20.15.
60(1976); http://dx.doi.org/10.1121/1.381206View Description Hide Description
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: 35.65.
60(1976); http://dx.doi.org/10.1121/1.381207View Description Hide Description
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 elastictensor for an isotropic solid. Experimental data is provided to validate the calculated results.
Subject Classification: 35.26; 20.35; 35.54.
Theory and numerical calculation of the acoustic field produced in metal by an electromagnetic ultrasonic transducer60(1976); http://dx.doi.org/10.1121/1.381208View Description Hide Description
The equations for calculating the acoustic field produced within a nonmagnetic metal by an interaction of eddycurrent with a static magnetic field were obtained on the assumptions that (1) an ultrasonicwave 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) eddycurrents and elastic waves show a steady‐state sinusoidal variation. The acoustic field produced by a specific electromagneticultrasonic transducer with axial symmetry was calculated numerically, and the results showed a well‐defined ultrasonicwave beam, which was narrower than had been expected from the size of the transducer.
Subject Classification: 35.60; 85.48.
60(1976); http://dx.doi.org/10.1121/1.381209View Description Hide Description
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: 35.54; 20.15, 20.30.
60(1976); http://dx.doi.org/10.1121/1.381210View Description Hide Description
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: 50.75.
60(1976); http://dx.doi.org/10.1121/1.381211View Description Hide Description
The transmission loss characteristics of flow‐reversing muffler chambers were predicted by a numerical approach based on the finite element method. The theoreticalmodel 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 theoreticalmodel developed, transmission loss curves were calculated for a large number of different flow‐reversing chamber dimensions.
Subject Classification: 50.30, 50.40.
60(1976); http://dx.doi.org/10.1121/1.381212View Description Hide Description
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: 65.50, 65.52; 50.75, 50.20.