Volume 60, Issue 3, September 1976
Index of content:
60(1976); http://dx.doi.org/10.1121/1.381128View Description Hide Description
Time‐averaged holography has been used to quantitatively investigate sound radiation from an edge‐clamped circular flat plate mounted in an infinite rigid baffle. For a particular mode of vibration, the plate response is measured using holography and the sound power radiated is measured in a reverberant room with the plate mounted in one of the room walls. From these measurements a radiation efficiency is determined. The theoretical plate responce is calculated using both classical and Mindlin–Timoshneko plate theory and is shown to agree well with experimental measurements.Radiated sound power is calculated for each mode of interest by solving the wave equation in oblate spheroidal coordinates at the plate surface. These calculations are verified by direct evaluation of the Rayleigh integral in the farfield. Good agreement is obtained between experimentally measured radiation efficiencies and theoretical predictions. Small discrepancies between theory and experiment are discussed.
Subject Classification: 20.55, 40.24, 35.65.
60(1976); http://dx.doi.org/10.1121/1.381129View Description Hide Description
A matrix theory is developed for investigating the scattering of elastic waves in solids by an obstacle of arbitrary shape. The scattering matrix which depends only on the shape and nature of the obstacle relates the scattered field to any type of harmonic incident field. Expressions are obtained for the elements of the scattering matrix in the form of surface integrals around the boundary of the obstacle, which can be evaluated numerically. Using the principle of reciprocity and the conservation of energy, the scattering matrix is shown to be symmetric and unitary. These properties are essential to assure the accuracy of numerical calculations. Both two‐ and three‐dimensional problems are discussed, and the obstacle may be an elastic inclusion, a fluid inclusion, a cavity, or a rigid inclusion of arbitrary shape.
Subject Classification: 20.15, 20.30.
60(1976); http://dx.doi.org/10.1121/1.381130View Description Hide Description
Upon invoking Huygen’s principle, matrix equations are obtained describing the scattering of waves by an obstacle of arbitrary shape immersed in an elastic medium. New relations are found connecting surface tractions with the divergence and curl of the displacement, and conservation laws are discussed. When mode conversion effects are arbitrarily suppressed by resetting appropriate matrix elements to zero, the equations reduce to a simultaneous description of acoustic and electromagneticscattering by the obstacle at hand. Unification with acoustic/electromagnetics should provide useful guidelines in elasticity. Approximate numerical equality is shown to exist between certain of the scattering coefficients for hard and soft spheres. For penetrable spheres, explicit analytical results are found for the first time.
Subject Classification: 20.15, 20.30.
60(1976); http://dx.doi.org/10.1121/1.381131View Description Hide Description
It is well known that the beam pattern of a parametric acoustic source whose difference frequency is generated largely within the nearfield of the primary beam includes a multiplicative aperture factor that can be important at large angles when ka≳1 (k is the difference frequency wave number, a is the aperture radius). Not so well known, however, is the fact that the same aperture factor arises in the case of a spherically spreading, conical primary beam of finite initial aperture. The importance of the aperture size in determining the off‐axis behavior of parametric sources is discussed.
Subject Classification: 25.35; 20.30; 30.75.
60(1976); http://dx.doi.org/10.1121/1.381132View Description Hide Description
Three sound recordings of tornadoes have been spectrally analyzed over the frequency interval between 100 and 2000 Hz. The low‐frequency analysis was limited by the response of the microphones used for the recordings; the upper frequency limit was imposed both the microphone response and the low‐signal level. One recording was made by James Cramer of a tornado which passed through Clay Center, Kansas on 25 September 1973. A second recording was made by Richard Allen Lindley of a tornado that passed through Guin, Alabama on 3 April 1974, and a third recording was made by Tom Bittman of a tornado which damaged his home in Tulsa, Oklahoma on 8 June 1974. All of the recordings are of low quality and the audio information required major spectral corrections, but the data analysis does indicate that the audio emissions from the tornadoes decrease in intensity as a function of increasing frequency. An attempt has been made to correlate the data with real (and conjectured) physical characteristics of the tornadoes and attendent atmospheric phenomena. It is clear to us from an objective analysis (and the more difficult subjective evaluation of the recordings) that identification of tornadoes based on acoustic emissions is possible . A study of tornadic sounds can not only provide a new tool for gaining insight into electrical and mechanical disturbances within a tornadic storm, but will also allow acoustic detection of a tornadic storm. A study of the change in intensity of the sounds emitted by the approaching Guin storm at both high and low frequencies suggests that noisesgenerated by the high‐speed winds of the principal tornado vortex as its base scoured the ground might be discerned from noisesgenerated by the winds of one or more smaller vortices moving around the tornado, and/or electrical discharges aloft.
Subject Classification: 28.45, 28.65.
60(1976); http://dx.doi.org/10.1121/1.381133View Description Hide Description
Periodic vortex streets are formed in the wakes of blunted trailing edges on airfoils and struts. The pressures generated on the shedding struts by the vortices in these wakes are periodic in time with a frequency that is set by the shedding rate for the vortices. A simple analytical formulation is derived to relate wake‐induced pressures to the characteristics of the wake near the edge. The chordwise distribution and magnitude of the pressure is shown as a function of the circulation of shed vortices, as well as the formation distance and the spacing of the vortices in the street. Predictions from the theory are compared to some recent measurements which were obtained in the wakes downstream of different trailing edges. These measurements were made at Reynolds numbers, based on trailing edge thickness, on the order of 104 to 105.
Subject Classification: 28.65; 50.55.
60(1976); http://dx.doi.org/10.1121/1.381134View Description Hide Description
The theory of sound radiation from cylinders vibrating in resonance with vortex shedding is extended to consider the effects of vibration amplitude and mode shape. Farfield intensity and total radiated power are expressed as functions of given structural and flow parameters. Closed form solutions for intensity are obtained when cylinder vibration velocity is either much smaller than or comparable to mean flow velocity.
Subject Classification: 28.65, 40.26, 50.55.
60(1976); http://dx.doi.org/10.1121/1.381121View Description Hide Description
UltrasonicDoppler flowmeters can simultaneously obtain spatial and velocity distribution patterns of flow in a blood vessel. The factors fixing spatial and velocity resolution, however, must be determined if optimum utilization is to be realized. The parameters influencing spatial resolution have been described in the literature, but velocity resolution has received relatively little attention. This paper demonstrates analytically how optimum velocity resolution can be derived with a simple mathematical model and presents experimental data to verify theory. In addition, a ’’resolution product’’ is offered which characterizes pulsed Doppler flowmeters. This product shows explicitly the necessary compromise between position and velocity resolution for a given instrument.
Subject Classification: 28.60, 28.20; 80.70; 35.80.
60(1976); http://dx.doi.org/10.1121/1.381122View Description Hide Description
An experiment designed to measurenormal mode amplitude functions and attenuation coefficients was conducted in shallow water on Campeche Bank off the Yucatan Peninsula. Measurements were made at two locations on the bank in water of about 30 m in depth over a bottom consisting of consolidated limestone having a measured and sound velocity of 1900 m/sec. Pulsed cw signals with frequencies of 400, 750, and 1500 Hz were used. Theoretical calculations of the mode amplitude functions using a fluid model of the bottom were found to agree well with the measurements. In order to reconcile the measured mode attenuation coefficients with theory, it was necessary to assume that the shear velocity of the bottom was 1000 m/sec. The latter is lower than the minimum sound velocity in the water column so that the generation of propagating shear waves in the bottom was the dominant attenuation mechanism. Significant differences in the measured mode attenuation coefficients at the two stations were explained by the deepening of the low velocity channel at the bottom of the water column.
Subject Classification: 30.20, 30.50.
60(1976); http://dx.doi.org/10.1121/1.381123View Description Hide Description
The amplitude of a sonarecho from a fish depends upon the species and size of the fish, acoustic wavelength, aspect, position of the fish in the sonar beam, range and backscattering cross section. We simplify the problem to a single species and size of fish, vertically downward echosounding, single aspect, and nonoverlapping echoes. After removal of attenuation due to range and absorption two random functions remain. The position of the fish in the sonar beam is random and the scattering cross section for each trail is random. We assume that the fish have a uniform density (number/m3) and calculate the probability density function (PDF) for insonification and reception. We assume that the PDF of the envelope of the echo (excluding the variability of insonification and reception) has a Rayleigh PDF. Assuming two PDF’s are independent, we calculate the PDF of the echo envelopes w E (e). w E (e) depends upon the beamwidth of the sonar and the mean backscattering cross section. The theoretical PDF has the same shape as the measured PDF of echoes from alewife in Lake Michigan. We use the fit of the PDF’s to estimate the backscattering cross section and fish density. This calibrates the echo‐integration processing system. A profile of the density of alewife in Lake Michigan is shown.
Subject Classification: 30.40; 80.40.
60(1976); http://dx.doi.org/10.1121/1.381124View Description Hide Description
An improved technique has been developed for studies of the shear viscosity of fluids. It utilizes an acoustic resonator as a four‐terminal electrical device; the resonator’s amplitude response may be determined directly and simply related to the fluid’s viscosity. The use of this technique is discussed briefly and data obtained in several fluids is presented.
Subject Classification: 35.10, 35.68; 85.52.
Excitation, detection, and scattering of electroelastic surface waves via an integral equation approach60(1976); http://dx.doi.org/10.1121/1.381125View Description Hide Description
The problem of the excitation, detection, and scattering of electroelasticsurface (Bleustein) waves is solved exactly by determining the charge distribution on the fingers of an interdigital transducer. The approach is to solve an integral equation, in the Fourier transform domain, that relates the charge density on the fingers to the electric potential of the fingers. The solution of the integral equation is accomplished by expanding the charge distribution in a series of pulses and then transforming the problem to a vector matrix,one which is readily handled by a computer. In this manner the charge distribution is determined for a variety of conditions.
Subject Classification: 35,54.
60(1976); http://dx.doi.org/10.1121/1.381126View Description Hide Description
The dissipation in an elastic medium is represented by a dissipation mechanism which is similar to one used in an earlier paper [M. Caputo, Geophys. J. R. Astron. Soc. 13, 529–539 (1967)], but is simpler and has a frequency‐independent Q −1. The vibrations of a plate are studied by obtaining the eigenfrequencies, the amplitude of the displacement, the dispersion relation, the Q −1, the hysteresis cycle, and the yield stress.
Subject Classification: 40.24.
60(1976); http://dx.doi.org/10.1121/1.381127View Description Hide Description
This paper presents an analysis of the free vibrations of a disk–cable system spinning freely about a fixed axis through the disk center. Secondly, it is shown how the cable can be used as a vibration absorber to reduce the effect of torsional disturbances on a rotating system.
Subject Classification: 40.22, 40.70.
60(1976); http://dx.doi.org/10.1121/1.381135View Description Hide Description
The current method of measuringimpactnoise transmission involves the use of a standard hammer machine to produce a series of impact on the floor‐ceiling structure, and the measurement of the resulting noise produced in the room below. The method has been criticized on the ground that ratings based on the test data correlate poorly with the subjective judgments of people listening to real‐life impacts on the same floors. An alternative test method is proposed that uses a modified hammer machine whose internal impedance, intensity of impact, and striking frequency simulate those of real footfalls. The new method involves several changes from the present standard: short‐term rms impact sound levels are measured instead of long‐term rms levels; no normalization for the sound absorption of the receiving room is required; since the short‐term levels are higher than the long‐term levels usually measured, background noise is less of a problem than for the existing method. These proposed changes based on recent studies are expected to improve the correlation between test data and subjective judgments of floors.
Subject Classification: 55.80; 50.45.
60(1976); http://dx.doi.org/10.1121/1.381136View Description Hide Description
Measurements of the microphonic potential from the lateral line sensory organs of killifish (F u n d u l u s h e t e r o c l i t u s) are presented in this report. The potential consists of a dc shift and a dominant oscillatory component at twice the frequency of the stimulus, and it grows with the square of the stimulus at low amplitudes. An electrical circuit model of the microphonic is developed, assuming a simple quadratic nonlinearity in the electrical response of an individual hair cell as suggested by Flock. The model allows investigation of variable conductive or variable capacitive effects in microphonic generation, and the results obtained from the model are compared to the observed properties of the microphonic potentials. It is concluded that hair cell microphonics appear to be generated by a process involving current flow through a variable conductance, although capacitive effects of the hair cell are important in determining the waveform of the microphonic and its behavior as a function of frequency. A discrepancy between the observed low‐amplitude growth of the microphonic in killifish and the reported low‐amplitude growth of the microphonic in other acoustico‐lateralis preparations is also discussed.
Subject Classification: 65.28, 65.20, 65.40.
60(1976); http://dx.doi.org/10.1121/1.381137View Description Hide Description
An interferometric optical heterodyne technique has been developed especially for vibrational amplitude and phase measurements on auditory organs of live animals. Laser light diffusely scattered from the vibrating structure is used for the measurement. Continuous calibration and feedback compensation systems were developed to cope with the problems of drift in interferometer alignment and small background movements. Vibrational amplitudes from below 0.1 Å to above 400 Å have been detected on the posterior tympanic membranes of live crickets.
Subject Classification: 65.20; 40.60; 35.65.
60(1976); http://dx.doi.org/10.1121/1.381138View Description Hide Description
Loudness growth at 1000 and 3000 Hz was measured directly by magnitude estimation and production, and indirectly by loudness matches between tone and wide‐band noise and by interfrequency matching. The outcome of the three series of experiments does not reveal any systematic difference in shape of the loudness curves at 1000 and 3000 Hz. To a first approximation, above about 30 dB SL the loudness functions at 1000 and 3000 Hz are power functions of sound pressure with an exponent close to the accepted ISO standard of 0.60 (0.30 r esound intensity). Below 30 dB SL both loudness curves become progressively steeper than a simple power function and approach the same limiting slope, r esound intensity, of unity. Consistent with Steven’s calculation system [J. Acoust. Soc. Am. 51, 575–601 (1972)], the data also show that loudness equality is achieved when a 3000‐Hz tone is about 8 dB below the SPL of a tone at 1000 Hz.
Subject Classification: 65.50, 65.75.
60(1976); http://dx.doi.org/10.1121/1.381139View Description Hide Description
The identification of specific random waveforms, imbedded within random interference, was examined. Backward interference (interference after the specific waveform) was more effective than forward interference (interference before the specific waveforms). The accuracy of identification with combined interference (interference before and after the specific waveforms) is approximated by an independence model of interference. Under the present test conditions, interference with the identification of specific random waveforms is interpreted to be more nearly related to the interruption of auditory processing than to the masking of signal audibility.
Subject Classification: 65.58, 65.75, 65.52.
60(1976); http://dx.doi.org/10.1121/1.381140View Description Hide Description
Old World monkeys were trained with an operant conditioning technique to discriminate the natural speech sounds /ba/–/da/ and transferred to synthetic speech. Human and monkey difference thresholds for formant transitions were then compared along a seven‐step /ba/–/da/ continuum. Monkeys were not as sensitive as humans to differences in formant transition: the just noticeable difference for monkeys was about 320 Hz, and for humans, about 160 Hz. Although humans were more adept at intraphonemic discriminations than monkeys, their latencies to stimulus changes revealed evidence of ’’categorical perception’’ of the continuum: While latencies for the monkeys increased linearly as stimulus difference was decreased, human latencies were essentially constant for all interphonemic comparisons, but increased sharply for intraphonemic comparisons. We view these data as evidence for (a) similar sensory capacities in monkeys and humans, but (b) unique speech processing capacities in humans.
Subject Classification: 65.75; 70.30; 80.50.