Index of content:
Volume 60, Issue 1, July 1976
60(1976); http://dx.doi.org/10.1121/1.381045View Description Hide Description
In this investigation, Biot’s theory was employed in the study of wave motions in fluid‐saturated porous rocks. Consistent with the described experimental arrangement, Biot’s equations were solved using a Laplace transformation. The theory predicts two dilatational waves: a slightly dispersed fast wave propagating ahead of a heavily dispersed and attenuated slow wave. By comparing these results with experimental results, it becomes evident that the measured waves are in fact the fast waves.
Subject Classification: 20.15, 20.40.
60(1976); http://dx.doi.org/10.1121/1.381055View Description Hide Description
The paper deals with diffraction of a transient elasticPwave, generated by stress boundary conditions (SBC), as it impinges on a stress‐free surface at grazing incidence. The relevant boundary‐value problem concerns an impulsive load applied uniformly to the cylindrical surface of an infinite cavity drilled into a half‐space. The process thus consists of a quarter plane deformed by a Pwave, its diffraction, and reflections from the boundaries. In an earlier work the author gave a general analytical–numerical method for transient multidimensional boundary‐value problems and its employment to the present problem. Here, the resultant wavefronts and the response of the quarter plane are described and interpreted.
Subject Classification: 20.15, 20.30.
60(1976); http://dx.doi.org/10.1121/1.381058View Description Hide Description
The behavior of the solution to acoustic backscattering by elastic spheres in water expressed as a form function vs k a, relative to the normal modes of vibration of solid homogeneous elastic spheres is examined. Specific free modes of vibration are identified with significant changes in form function. The solution for spheres of tungstencarbide,aluminum, and brass are considered for values of k a as high as 28. Minima in the form function usually occur at or near frequencies at which sphere resonances occur. Form functions are examined which are computed from mixing rigid and elastic terms in the waveharmonic series. In this way it is possible to see the effect of an isolated mode of vibration on the form function that describes the backscattering. Calculated resonance widths as well as observed pulse decay times allow determination of analytical and experimental Q’s of isolated resonances.
Subject Classification: 20.30, 20.60, 20.50; 40.20.
60(1976); http://dx.doi.org/10.1121/1.381069View Description Hide Description
The analysis of waves in bends in acoustical ducting of rectangular cross section is extended to the study of motion near discontinuities. This includes determination of the characteristics of the tangential and radial components of the nonpropagating modes. It is established that attenuation of the nonpropagating modes strongly depends on frequency and that, in general, the sharper the bend, the less attenuation may be expected. Evaluation of a bend’s impedance and of impedance generated reflections is also presented in detail.
Subject Classification: 20.45, 20.40.
60(1976); http://dx.doi.org/10.1121/1.381044View Description Hide Description
The propagation of low‐frequency acoustic waves in the lower atmosphere is studied using observations of explosive sound sources at ranges of 4.3 and 9.3 km. By observing groups of 30 firings spaced closely in time, the effects of atmospheric turbulence on the propagation are determined; the differences in arrival time between microphone pairs in an approximately 1‐km array being the experimental parameter measured. The time difference distributions are found to be Gaussian with standard deviations which show a strong diurnal variation, the increased effect of atmospheric turbulence being evident in mid‐day observations. Propagation over nearby paths is shown to be spatially correlated out to separations of several hundred meters and this correlation is shown to be in agreement with the theoretical predictions of Chernov [W a v e P r o p a g a t i o n i n a R a n d o m M e d i u m, L. A. Chernov (McGraw–Hill, New York, 1960)]. Using these theoretical relations, atmospheric scale sizes in the range from 200 to 500 m (mean 320 m) are obtained together with acoustic velocity fluctuations with standard deviations in the range from 0.5 to 1.2 m/sec (mean 0.8 m/sec).
Subject Classification: 28.40, 28.60.
60(1976); http://dx.doi.org/10.1121/1.381046View Description Hide Description
An analysis, using complex angles of incidence as the variable in a steepest descents integration, is made of the propagation of sound from a point source over a locally reacting complex impedance boundary. The integration process contains more terms than are normally used in the analysis, and the solution is valid over a large range of horizontal distances. Conditions for the existence of surface waves are discussed in some detail. A comparison is made of measured and calculated values of pressure on the boundary.
Subject Classification: 28.40; 20.15, 20.30.
60(1976); http://dx.doi.org/10.1121/1.381047View Description Hide Description
A collimated ’’monochromatic’’ acoustic wave (sinusoidal wave of 3–90 kHz) was directed across a two‐dimensional turbulent jet perpendicular to its plane of symmetry. Extensive study of the statistical properties both of the turbulent medium and of the received signal (random amplitude and phase modulation) offers physical insight into the general phenomenon of a periodic wave (pressure) scattered by random inhomogeneities (solenoidal velocity field).
Subject Classification: 28.60.
60(1976); http://dx.doi.org/10.1121/1.381048View Description Hide Description
Amplitude and phase (transit time) fluctuations in pulses sent between a fixed transmitter and a fixed receiver over a wholly refracted 17.2‐km path were recorded for 144.5 h. The sites were the southwest flank of Cobb Seamount and a lesser peak 17.2 km away, both at 1000 m depth. Eight‐cycle pulses at 4166 Hz and 16‐cycle pulses at 8333 Hz were sent alternately every 15.7 sec and received at three receivers located at 0, 5, and 15 m along a horizontal arm located perpendicular to the transmission path. Power spectra have been computed from the time series of phase and amplitude at a single receiver and the phase difference between two receivers. Extreme care was used in the analysis of the data to ensure that the time series obtained represented a single path. The power spectra of the phase data exhibit dominant tidal peaks at 24, 12.4, and 6.2 h; the power spectra of the amplitudes show less evidence of the tides. Between the inertial frequency and the Väisälä frequency, the power spectra of the phase, amplitude and phase difference fall off at approximately ω−3, ω−1, and ω−1/2, respectively. The power spectra of the phase differences for the 15‐, 10‐, and 5‐m spacings scale according to a plane wave arrival. The phase‐difference variance is below theoretical predictions, and the amplitude variance is above theoretical predictions. The high‐amplitude variance is tentatively identified as ’’micro’’ multipath interference. Evidence is presented to show that the oceanographic regime of the Cobb Seamount area is typical of the open ocean environment.
Subject Classification: 30.20, 30.25.
60(1976); http://dx.doi.org/10.1121/1.381049View Description Hide Description
A synthetic aperture experiment was conducted using a ship‐towed source and midwater receiving hydrophones buoyed upward 4700 and 8000 ft from bottom moorings in a depth of 17 200 ft. The tows were conducted along straight lines perpendicular to the direction to the hydrophones at ranges of 58, 93, 128, 198, and 268 NM. A cw signal at 400 Hz and a swept FM signal between 350 and 450 Hz were transmitted continuously during the tows. Although major difficulties were encountered in maintaining straight tows in the presence of surface swells, it was possible to construct a number of synthetic apertures for the 400‐Hz transmissions on an intermittent basis. The lengths of these apertures extended to 1/2 NM and more, corresponding to coherence time intervals of up to 7 1/2 min in the ocean.
Subject Classification: 30.20; 60.30.
60(1976); http://dx.doi.org/10.1121/1.381050View Description Hide Description
Experimental measurements of sound absorption in air, measurements of ultrasonicabsorption in H2O/O2 mixtures at 500 °K reported here, and experimental measurements in mixtures of H2O with other impurities have been analyzed to determine the energy transfer rates in mixtures of O2 and H2O as a function of temperature. A set of energy transfer rate coefficients was found that give results consistent with all the experimental data examined and with vibrational energy transfer theory. The rotational relaxation time for H2O/O2 mixtures was determined from the 500 °K ultrasonicmeasurements.
Subject Classification: 35.20, 35.35.
60(1976); http://dx.doi.org/10.1121/1.381051View Description Hide Description
Ultrasonic absorption has been measured in mixtures of H2O with 15%, 35%, and 55% Ar and N2 at 500 °K. The classical absorption was subtracted from the measured values and the remainder was analyzed in terms of parallel vibrational and rotational relaxation. By extrapolating to 100% H2O, the rate of self deactivation was found to be (2.0 ± 0.4) × 108 sec−1 atm−1 for vibration and (8.3 ± 0.5) × 108 sec−1 atm−1 for rotation. Extrapolation to 0% H2O yields a rate of (4.0 ± 2.0) × 106 sec−1 atm−1 for H2O vibrational deexcitation by N2 or Ar. The isothermal–isometric rotational relaxation time for H2O in one atm of Ar was found to be (1.2 ± 0.2) × 10−9 sec. These results at 500 °K are compared with other work at higher and lower temperatures in order to form a basis for extrapolating to room temperature.
Subject Classification: 35.20, 35.35.
60(1976); http://dx.doi.org/10.1121/1.381052View Description Hide Description
Brillouin spectra of molten NaNO3 have been investigated at two different scattering angles, 40° and 140°, in a temperature interval ranging from 310° to 450 °C. The measured frequency shifts and linewidths of the Brillouin components were used to determine the hypersonic velocity and attenuation of thermal waves in the frequency range 3.4–10.2 GHz. The determination of the Brillouinlinewidths has been performed using a convolution technique, necessary when narrow linewidths are to be determined. The hypersonic velocity measurements show no appreciable dispersion compared either to ultrasonic data or to each other. In the line broadening measurements, however, the attenuation shows some dispersion especially at the higher frequencies, thus indicating that the measurements are performed at frequencies just below a relaxation frequency. A calculation of this frequency using a theory with a single relaxation time confirms this assumption.
Subject Classification: 35.24; 35.30
60(1976); http://dx.doi.org/10.1121/1.381053View Description Hide Description
A new method is presented for calculating the transmission and reflection coefficients of Rayleigh waves by a high impedance obstacle of finite length. The problem is formulated in terms of simultaneous integral equations which can be solved by a kind of iterative procedure. As a result, the transmission and reflection coefficients can be expressed by complex contour integrals along branch cuts associated with a function in the integrands. The transmission and reflection coefficients are computed numerically by assuming a fused‐silica substrate and are compared with those obtained by using Jones’s method of approximation. It is inferred that the present approach, though it takes more computing time when compared to Jones’s method, can predict the transmission and reflection coefficients with reasonable accuracy even if the obstacle is small.
Subject Classification: 35.54; 20.15, 20.30.
60(1976); http://dx.doi.org/10.1121/1.381054View Description Hide Description
A simplified expression is derived for the optical irradiance in the nearfield of a sinusoidal phase grating. Application of this expression to an ultrasonically produced optical phase grating permits the interpretation of experimental data for the magnitude of retardation and relative position of the grating. From these parameters, it is possible to evaluate the acoustic pressure distribution, total acoustic power, and soundvelocity of a progressive ultrasonic beam.
Subject Classification: 35.65.
60(1976); http://dx.doi.org/10.1121/1.381076View Description Hide Description
The sonoluminescence from 17 different gases dissolved in water was measured. The results show an inverse relation with the thermal conductivity of the gas. The thermal conduction losses of the gas in the cavitation bubble during its rapid collapse are considered, and an expression found for the final temperature of the gas. The sonoluminescence results for the five rare gases studied agreed with this theory.
Subject Classification: 35.68, 35.32.
60(1976); http://dx.doi.org/10.1121/1.381077View Description Hide Description
A fourth‐order equation has been derived that is suitable for analyzing symmetrically constructed, isotropic three‐layer plates. The formulation includes the effects of shear deformation in each layer, as well as the rotatory inertia of the composite while enforcing continuity of displacements and shear stresses at layer interfaces. From virtual work considerations, the natural boundary conditions are also derived in an invariant form. Since the equations of motion and boundary conditions are tensor equations, an arbitrarily shaped plate can be analyzed using the most convenient coordinate system. Natural frequency comparisons have shown that the current theory is in excellent agreement with an exact elasticitysolution, at least in the range where such data exists. By using the complex modulus concept or the correspondence principle, the governing equation can be used in analyzing composite plates that consist of (a) a viscoelastic core sandwiched between similar elastic faces and (b) an elastic plate with two symmetric viscoelastic coatings. Several examples have been analyzed to illustrate the damping that can be obtained by the use of a viscoelastic inner layer.
Subject Classification: 40.20, 40.24.
Frequency spectra of extensional vibration in Pb(Zr⋅Ti)O3 disks with Poisson’s ratio larger than 1/360(1976); http://dx.doi.org/10.1121/1.381078View Description Hide Description
The frequency spectra of axially symmetric extensional vibration in piezoelectric Pb(Zr⋅Ti)O3ceramic disk plates having Poisson’s ratio (σ) of 0.37 have been investigated theoretically and experimentally. The theoretical frequency spectra were calculated with approximation of elastically isotropic disk plates for r (diameter/thickness) of 1 to 30. It was found that the terrace‐type spectra which appear in a frequency band between the cutoff frequency of the symmetric thickness‐shear mode and the frequency minimum Ω* do not construct a constant frequency bandwidth, in contrast to the case of σ<1/3. Experimental spectra were observed with the Pb(Zr⋅Ti)O3 disk plates for r of 4.8 to 7.2. The experimental spectral agreed well with the theoretical ones, by fitting the cutoff frequency of the fundamental thickness‐extensional mode of the elastically isotropic plate to that of the piezoelectric Pb(Zr⋅Ti)O3ceramic plate. From the results of these analyses, it has been found that the Pb(Zr⋅Ti)O3ceramic disk plates have resonant vibration responses at or near the cutoff frequency of the symmetric thickness‐shear mode, although these vibrations cannot be induced piezoelectrically.
Subject Classification: 40.24; 85.52.
60(1976); http://dx.doi.org/10.1121/1.381079View Description Hide Description
The equations of motion for a three‐layered cylindrical shell are derived. The thickness and material properties may be different for each layer. Dissipative materials may also be considered. The solution for a simply supported cylindrical shell excited by a time‐harmonic load is obtained in vacuum and in a fluid‐acoustic medium. The response of the shell and the corresponding acoustic pressure field is found and illustrated for a specific configuration.
Subject Classification: 40.26; 20.60.
60(1976); http://dx.doi.org/10.1121/1.381080View Description Hide Description
The coherent or conventional gain of a linear array in an uncorrelated noise field is presented for various models of signal coherence decay with increasing sensor separation. For the case of linear decay, the coherent array gain is shown to approach a maximum of D L /s as the array length increases; where D L , the linear decay length, is defined as the vanishing point of the signal coherence and s is the adjacent sensor separation having some minimum value commensurate with the assumption of uncorrelated noise. For exponential decay of signal coherence a good approximation to the maximum coherent array gain is shown to be 2 D E /s for D E 〉〉s, where D E is the exponential decay length. However, for both linear and exponential decay it is shown that additional gain is achievable beyond the coherent gain limit using incoherent processing. Also a closed form expression is derived for the coherent gain of an array for Gaussian phase fluctuations which are correlated in a linear fashion.
Subject Classification: 60.20, 60.30.
60(1976); http://dx.doi.org/10.1121/1.381056View Description Hide Description
C57BL/6 inbred mice were subjected to simulated sonic booms of different rise time, intensity, and number of sonic booms. The most important pathological change was found to be bleeding in the basal turn of the scala tympani of the inner ear. Even a single boom having a rise time of 0.1 msec, peak overpressure of 3.3 psf, and duration of 120 msec was found to cause bleeding in at least one inner ear of each exposed mouse. Similar results occurred for a single ’’superboom’’ having a 10−psf peak overpressure but a longer rise time of 5 msec. The frequency of the appearance of blood clots in the scala tympani was found to increase with the number of booms administered. This cumulative effect was observed even when the booms were administered at the rate of one every 24 h. The traces of bleeding usually disappeared within a period of eight weeks. The extent of the bleeding in the scala tympani did not increase when mice were exposed to a rapid succession of simulated sonic booms (18 000 booms in one hour). However, three out of twenty mice suffered a rupture of the basilar membrane and destruction of the Corti organ in the basal turn of the cochlea.
Subject Classification: 65.20, 65.40; 50.70; 80.40.