Volume 53, Issue 3, March 1973

Comparable tests on linear‐ and carbon‐type microphones
View Description Hide DescriptionA carbonmicrophone is shown to have 5 dB less low‐frequency sinusoidal output than a magnetic microphone, both judged to have approximately the same voice output level and tonal quality. Evidence is offered to show that the sinusoidal characterization of a carbonmicrophone does not properly describe its response to a complex, wide‐band signal having a high peak factor, such as speech. This results from a greater‐than‐unity slope for the input‐output curve of the carbon instrument. An improved characterization for estimating the subjective performance with speech input was obtained with an excitation signal consisting of a sinusoid plus white noise, the latter being filtered from the output. With this technique, the response curves for the two microphones became comparable and reasonably similar. This information is particularly important for testingcarbonmicrophones designed to have a steep input‐output characteristic to exclude farfield sounds.

Dichotic speech perception: An interpretation of right‐ear advantage and temporal offset effects
View Description Hide DescriptionIn two experiments on normals we presented CV nonsense syllables both dichotically and monotically, with onsets of the syllables separated by 0, 15, 30, 60, and 90 msec (first experiment) and 0, 90, 180, 250, and 500 msec (second experiment). We found that when one of the CV's trailed the other by 30–60 msec, the trailing CV became more intelligible than when it was given simultaneously; the leading syllable's intelligibility dropped from its “simultaneous” level when leading by 15 and 30 msec. The leading message was more intelligible between 15 and 250 msec when the two channels were mixed monotically. In the dichotic simultaneous conditon, voiceless consonants were more intelligible than voiced, especially in voiced‐voiceless pairs. When the voiced CV trailed the voiceless CV, the former became almost as intelligible as its voiceless counterpart. A left hemisphere “speech processor” was postulated, with suppression of information from ipsilateral sources during contralateral stimulation. The postulated “speech processor” may be involved in acoustic‐signal‐vocal‐tract control functions.

Electromagnetic leakage in high‐frequency continuous‐wave ultrasonics
View Description Hide DescriptionElectromagnetic leakage or cross talk between transmitter and receiver represents a severe limitation on the usefulness of continuous‐wave (CW) ultrasonic methods at high frequencies. The problem of leakage is, however, entirely eliminated by use of a scheme known as the sampled‐CW technique. The role of the sampled‐CW technique in eliminating leakage is explicitly demonstrated by contrasting the results of CW and sampled‐CW measurements under a variety of conditions.

Ultrasonic studies in binary solutions of pyridine with water, methanol, and ethanol
View Description Hide DescriptionThe velocity and absorption of ultrasound at 19.5 MHz were studied as a function of the concentration in binary solutions of pyridine with water, methanol, and ethanol. In addition, the compressibility and volume viscosity were calculated. Molecular processes are suggested to explain the variation of the ultrasonic properties of these binary solutions with respect to concentration.

Scattering from a periodic corrugated surface: semi‐infinite alternately filled plates
View Description Hide DescriptionAn incident scalar plane wave is scattered from a surface, corrugated in one dimension, and given by an infinite number of periodically spaced semi‐infinite parallel plates of zero thickness (thin plates). The surface is a parallel‐plate grating. An additional plate is placed between two adjacent (in period) plates, thus dividing the periodicity interval into two regions, one of which has the same (constant) density and wavenumber properties as the material above the plates, while the second (inhomogeneous) region is filled with a material which has a different (constant) wavenumber and density from the surrounding media. The solutions of the Helmholtz equation are assumed to be upgoing plane waves above the plates and, between the plates, standing waves along the periodicity direction for either soft (Dirichlet) or hard (Neumann) boundary conditions, and downgoing waves down the slots between the plates. Continuity of pressure and velocity across the common boundary yield linear equations relating the wave amplitudes in the various regions. The equations are shown to be similar to the residue series of integrals of certain metamorphic functions. The amplitudes are expressed as values or residues of these functions, which are explicitly constructed from properties of the linear equations. Two examples are done in detail: (1) no inhomogeneity structure with arbitrary incident angle, and (2) arbitrary inhomogeneity values at normal incidence. Numerical results are presented for backscatter (specular) reflection coefficientsR _{0} for normal incidence as a function of frequency for various values of the inhomogeneity parameters. The Rayleigh anomaly and the wavenumber threshold effects for the modes between the plates are the most prominent effects.

High‐speed method for computing the exact solution for the pressure variations in the nearfield of a baffled piston
View Description Hide DescriptionA method is presented whereby the pressure variations at any point in the field of a baffled piston may be efficiently calculated. If a solution for the impulse response of a piston of a given geometry is known, then for harmonic excitation the steady‐state field may be computed by evaluating the driving‐frequency component of the Fourier transform of the impulse response. This method involves a single integration, whereas the direct numerical solution requires a double numerical integration. An exact, closed‐form solution for the impulse response of a rectangular piston is derived. With this solution and the known solution for the impulse response of a circular piston the steady‐state solutions for these two geometries are obtained. Three‐dimensional and contour plots of data obtained for a circular piston and for a plane of symmetry of a rectangular piston field are presented. The plots for the circular piston compare favorably with previously published plots of data calculated by a double integration.

Low‐frequency scattering
View Description Hide DescriptionThe leading terms in the low‐frequency expansions for acoustically soft and hard bodies are examined and the relevance of the magnetic polarizabilitytensor is discussed. For a hard, rotationally symmetric body, two tensor elements, functions only of the geometry, are now sufficient to specify the entire low‐frequency scattering behavior in just the same way as the electrostatic capacity suffices for a soft body. Even these quantities are subject to known constraints and computed data for a variety of bodies are presented.

Free and forced response of a laminated ring
View Description Hide DescriptionA free‐ and forced‐vibration analysis is made of a three‐layer laminated ring consisting of an inner and outer elastic layer with a core capable of transmitting only shear. The Variational Method is used to obtain the equations of motion. Analytical expressions are obtained for the response and the natural frequencies of the laminated ring having an elastic core material. Curves are obtained which demonstrate the variation of the natural frequencies of the laminated ring for specific values of the core thickness and shear parameters with a change in the elastic thickness parameter for the first four circumferential bending modes. The resonance response is investigated and analytical expressions are obtained for the system having a viscoelastic core. Curves are obtained relating the resonant‐frequency parameter and the composite‐loss factor to the geometrical and physical properties of the ring vibrating in the fundamental circumferential bending mode.

Fractional‐harmonic frequency pairs in nonlinear systems
View Description Hide DescriptionUnder proper conditions, the response of two coupled nonlinear oscillators, driven by a sinusoidal input, contains a pair of fractional‐harmonic frequency components. The frequencies f _{1} and f _{2} of the fractional‐harmonic pair correspond approximately to the resonance frequencies of the system, and the sum f _{1} + f _{2} equals the driving frequency. Conditions for the occurrence of fractional harmonics are examined by an analysis that relates them to a pair of coupled Mathieu equations. Fractional‐harmonic components were observed in several nonlinear electrical, mechanical, and acoustical systems. The observations are consistent with qualitative predictions of the analysis.

Nonlinear transverse vibrations of beams with properties that vary along the length
View Description Hide DescriptionThe method of multiple scales is used to analyze the nonlinear transverse vibrations of inhomogeneous beams including the effects of transverse shear and rotary inertia. First‐order uniform perturbation expansions are derived for the displacements, shear force, and frequency in terms of the ratios of maximum transverse displacement to cross‐sectional radius of gyration and cross‐sectional radius of gyration to wave length.

Vibration analysis of thick‐walled spheres and cylinders
View Description Hide DescriptionA wave‐propagation approach is employed to determine the fundamental frequency of hollow spheres and long hollow cylinders (plane strain state). The study was initiated in order to study the periodic response of such specimens as a function of the number of wave reflections off the inner and outer surfaces as well as the thickness‐to‐inside radius ratios, (h/R_{i} ). It was hoped to achieve a relationship similar to that obtained in the classical longitudinal step loading bar problem. In the bar case, the material response is periodic for either two or four complete reflections, depending on whether the bar has a free or a fixed end. In the hollow sphere and cylinder problem, however, the analysis is not as simple. The motion is approximately periodic. It was found that the number of wave reflections for one cycle was some irrational number, the number being a function of (h/R_{i} ). This unusual observation led to the development of useful design curves and equations for determining the fundamental frequencies of thick‐walled spheres and cylinders. Comparisons of results from the vibration and wave propagation approaches are used to establish regions of validity of the approaches as a function of (h/R_{i} ).

Mapping the underside of arctic sea ice by backscattered sound
View Description Hide DescriptionA narrow‐beam scanning sonar was used to measure the relative backscattering strengths at 48 kHz of the undersurface of arctic sea ice. The graphic records displaying the range and relative scattering levels were assembled into a sonar map that displays the location and shape of under‐ice features. The data indicate that there are two distinct types of backscattering: (1) very high level backscattering from well‐defined under‐ice ridges and (2) very low level backscattering from between the ridges. The higher scattering the ridges is probably due to the increase in roughness and the tilting of the average plane of the scattering surface. Comparison of the sonar map and the aerial photograph shows that most surface features have subsurface expressions and that their relationship can be complex.

Pressure dependence of sound attenuation in the Pacific Ocean
View Description Hide DescriptionMeasurements of the attenuation of sound at a nominal frequency of 75 kHz have been made over vertical and horizontal acoustic paths located between depths of 700 and 3400 m in the Pacific Ocean. Results indicate that the equation used to predict attenuation overestimates the values actually encountered at these frequencies and depths. (Compare calculated values of 27.3 and 22.7 dB/km to measured values of 19.9±0.5 and 13.3±0.5 dB/km at depths of 910 and 3350 m, respectively.) The measurements definitely establish the decrease of attenuation with increasing pressure. However, the magnitude of the pressure coefficient of attenuation is almost twice as large as previously suspected (12.3±1.1×10^{−4}/bar, or approximately 12.3×10^{−4}/atm, compared to 6.54×10^{−4}/atm).

Reverberation at 75 kHz to a depth of 1 km in the Pacific Ocean—a negligible factor in attenuation
View Description Hide DescriptionUsing the results of an acoustic reverberation experiment at 75 kHz, in which the active volume was localized to the vicinity of the source and receiver, the volume scattering coefficient is calculated and compared to the attenuation coefficient. Assuming isotropic scattering, the conclusion is reached that scattering does not significantly affect the pressure dependence of acoustic attenuation in the ocean at this frequency.

Forward and specular scattering from a rough surface: theory and experiment
View Description Hide DescriptionA heuristic rough‐surface scattering model is given and compared with experimental results obtained from model rough surfaces. The theoretical model is developed from separate calculations of the coherent and incoherent parts of the scattered field. This allows a direct comparison between theoretical and experimental coherent and incoherent scattering coefficients to be made. The model is consistent in that the correct limiting forms for zero roughness and for large‐scale roughness are obtained. Non‐Gaussian surface statistics are incorporated in the model and the resulting theoretical predictions agree closely with specular‐ and forward‐scattering experimental data.

Refracted, bottom‐reflected ray propagation in a channel with time‐dependent linear stratification
View Description Hide DescriptionA model study has been undertaken of refracted, bottom‐reflected (RBR) ray propagation in a time‐dependent, linearly stratified acoustic channel. The analysis is relevant to the acoustic propagationeffects of large‐scale variations in the channel and pertains to the influence of these variations upon sound‐speed stratification. Such ocean phenomena as surfacetides, low‐frequency internal waves, and advected intermediate and large‐scale inhomogeneities—all of which may have scales comparable to or larger than representative acoustic path lengths of a few kilometers to a few hundred kilometers—are included. For numerical work, a measure of the influence of large‐scale variations upon sound‐speed stratification is provided by a thermistor string installation in the Straits of Florida. The analysis shows that any refracted, bottom‐reflected ray contributes a signal that is very nearly constant in spreading loss, but phase modulated at the frequency of the environmental variation in the channel. Thus, for a simple harmonically time‐dependent channel the signal at the receiver is a sum of constant amplitude, but sinusoidally phase modulated signals. Comparisons with data from a 7‐NM propagation multipath in the Straits of Florida demonstrate qualitative agreement between theory and experiment.

Acoustic transmission in an ocean surface duct
View Description Hide DescriptionAn analysis is presented of acoustical transmission measurements which were performed in the oceanic surface duct in two areas of the Pacific Ocean. Three frequencies, three transmitter depths, and three receiver depths were employed. Both areas demonstrated a persistent surface duct but the water mass was more variable in one area than the other. The thermal surveys were inadequate to establish that either area represented the actual thermal front between different water masses. Experimental transmission‐loss curves gave good agreement with theoretical curves based upon normal‐mode theory. It was not possible to distinguish between the acoustic properties of the two areas either by the experimental transmission‐loss data or by an analysis of the observed signal variability.

Focusing properties of solid ultrasonic cylindrical lenses
View Description Hide DescriptionThe sound velocity, thermal coefficient of sound velocity, and attenuation coefficients of 20 common plastics were measured and used as a basis for the design of several lenses.Materials selected were polyethylene, nylon, polystyrene, polyphenylene oxide, and a low‐sound‐velocity silicone rubber compound. Using common geometrical optical techniques, lenses were designed to minimize off‐axis aberrations for focal‐length‐to‐aperture ratios ranging from 1 to 3 and aperture‐to‐wavelength ratios up to 150. Using methods analogous to chromatic aberration correction in optics, lenses were designed for constant focal length over a temperature range of 0° to 30°C. Two‐ and four‐element lens systems with apertures up to 30 cm were fabricated from the selected materials.Measurements were made in the frequency range 600 kHz to 1.2 MHz and good agreement with theoretical predictions of focal length and directional response was observed. The effects of shear waves were not evident. The theoretical model used to evaluate lens performance was based on the numerical evaluation of the Fresnel‐Kirchhoff diffraction integral. This work shows that it is possible, using established optical design methods and commonly available solid materials, to fabricate athermal acoustic lenses with good performance characteristics at aperture‐to‐wavelength ratios up to 150.

An environmental‐acoustics model for sound propagation in a geostrophic flow
View Description Hide DescriptionA model for an ocean region experiencing geostrophic motion is considered. Under suitable simplifying assumptions and boundary conditions,equations of motion are solved for the density and pressure. Next, temperature is found by means of a state equation. Then, the pressure and temperature are used to determine the sound‐speed structure induced by the presence of the current. The sound speed is found to depend upon both the magnitude of the current and one's position in the current. For a bottom‐mounted source and receiver in the current direction, the model is used to investigate ray geometry, travel time, spreading loss, and the total acoustic field at the receiver. It is shown that the phase and amplitude of the received signal are significantly affected by variations in the current. Further, the dependence of sound speed on current is shown to be important so that these two quantities should not be considered separately as has been done in the past.

Acoustic backscattering from a rough surface at extremely low grazing angles
View Description Hide DescriptionThis study is concerned with the wide discrepancy between published experimental data and existing theories for acoustic backscattering at low grazing angles. A new theoretical development based on the Helmholtz theorem is presented which is valid at low grazing angles and which includes the effect of shadowing. At low grazing angles a scattering strength nearly independent of grazing angles and an inverse relationship between scattering strength and wind speed is predicted. A low‐grazing‐angle experiment was performed at sea on a stable platform where conditions approximated those found in the open ocean. Observations were obtained on the acoustic backscattering of the sea surface as a function of grazing angle down to 1.5° and wind speed from 5 to 20 knots. The effect of shadowing was observed with acoustic radiation. The sea surface was measured with a wave staff to obtain the rms slope of the surface and the wave‐height spectrum. Previous theories are shown to give incorrect results because of improper use of rough‐surface boundary conditions. It is also shown that the effect of shadowing needs to be included, but a relatively simple correction is adequate.