Thomas G. Muir

The field of a focusing source

An analytical description for the field of a focusing source is derived. It is valid for spherically concave sources with small aperture angle and large ka (radius a, wavenumber k.) The solution furnishes easy access to the sound distribution along the axis and in the focal plane, as well as to parameters such as focusing gain, width of the focal spot, and phase shifting in the focal region. Experiments conducted with an f/2 lens coupled to a planar array are discussed. The results support the utility of the analytical model for describing the distribution of sound along the acoustic axis and across the focal plane.

Field of a finite‐amplitude focusing source

An analytical description for the field of a harmonic focusing source is derived. It is valid for spherically concave sources with small aperture angle and high ka, under conditions of quasilinear distortion (strong shocks precluded). The solution furnishes access to the phase and amplitude of second harmonic sound. Underwater experiments conducted with an f/2 lens coupled to a low‐density source array are discussed. The results support the utility and validity of the analytical model for describing the amplitude and phase distribution of the second harmonic along the acoustic axis. It is shown that phase shifting in the focal region differs from that of linear radiations, and that both nonlinear propagation and diffractive effects are important in determining phase properties.

Nonlinear Effects in Acoustic Imaging

The fundamental mechanisms of sound propagation are nonlinear. At low frequencies and/or intensities, linearity can usually be assumed but this is often not the case in ultrasonic imaging.

Acoustic saturation of spherical waves in water

The acoustic transmitting capacity of a medium is limited by nonlinear propagation effects. As the source amplitude is increased to very high levels, acoustic saturation sets in, a state in which the amplitude at a field point approaches a limiting value, independent of the source amplitude. A theoretical and experimental study of saturation of spherical waves is presented in this paper. Predicted saturation amplitudes for the fundamental component in an originally sinusoidal wave are obtained by matching the well known weak‐shock solution to a linear‐theory solution at the old‐age distance rmax. The saturation formulas are verified by experiments done in fresh water with a 3‐in.‐diam piston projector operating at 454 kHz at source levels up to 135 dB re 1 μbar at 1 yd. Amplitude response curves were measured on the acoustic axis at six distances ranging from 0.76 yd to 111 yd. At the longer ranges the curves become isotonic, thereby demonstrating saturation. High‐amplitude beam patterns show the effect o...

Measurement and localization of interface wave reflections from a buried target

It is demonstrated that seismic interface waves on the surface of a natural beach can be used to identify the position of a buried object. For this experiment, the waves were created with a sediment-coupling transducer and received on a three-element horizontal line array of triaxial geophones. The source and its coupling to the medium provided a high degree of signal repeatability, which was useful in improving signal-to-noise ratio. Reception of all three directions of particle velocity made it possible to augment conventional beamforming techniques with polarization filters to enhance interface-wave components. Reverberation in the beach was found to be large, though, and coherent background subtraction was required to isolate the component of the sound field reflected by the target. Propagation loss measurements provided comparisons of reflected signal power with predictions made previously, and the two were found to agree closely.

The Penetration of Highly Directional Acoustic Beams into Sediments.

Abstract A planar array of seven hydrophones buried in a sand sediment was used to study the propagation of 20 kHz sound waves through a water/sediment interface. Two different sources were used: one a parametric source with a beamwidth near 2°, and the other a linear source with a beamwidth near 10°. Measurements were made with each source at four grazing angles between 14° and 78°. It was found that at low grazing angles the behavior of the wave produced by the parametric source departed significantly from the prediction of plane wave theory for a lossy sediment. When the parametric source was used, the wave fronts penetrated more steeply into the sediment and the attenuation with depth was less than predicted by plane wave theory. It was observed experimentally that near the critical grazing angle the narrow acoustic beam is displaced along the interface before entering the sediment.

Second harmonic component of a nonlinearly distorted wave in a focused sound field

The second harmonic in a focused nonlinear field is examined with a numerical computation and with experiments done in water. The computation is accomplished by rewriting an expression derived by Lucas and Muir [J. Acoust. Soc. Am. 74, 1522–1528 (1983)] into a form appropriate for numerical computation at any point. The primary velocity distribution at the source is experimentally defined and utilized in the computation. The experiment is conducted with a 2‐MHz focusing sound source consisting of a circular transducer coupled to an acoustic lens. The experimental values for amplitude and phase of the second harmonic component are in reasonable agreement with the theory. The second harmonic component focuses more sharply than the fundamental component, and the amplitude ratio of the second harmonic component to the fundamental component attains the maximum value at the focus. The results lead to an interpretation that the component generated in the focal region is the dominant contributor to the propagatin...

Field of a parametric focusing source

An analytical description for the field of a parametric focusing source is derived. It is valid for spherically concave sources with small aperture angle and high ka, under conditions of quasilinear interaction (strong shocks precluded). The solution furnishes computations on the phase and amplitude of difference frequency sound along the axis and in the focal plane, as well as on the width of the radiation lobe in the focal region. Underwater experiments conducted with an f/2 lens coupled to a dual, interleaved primary array are discussed. The results support the utility and validity of the analytical model for describing the distribution of sound along the acoustic axis and across the focal plane. The difference frequency radiation was found to be effectively focused, in that the width of the beam became quite narrow in the focal plane.

Experimental investigation of the combustive sound source

In this paper, we describe a unique low frequency underwater sound source called the combustive sound source (CSS). In this device, a combustible gas mixture is captured in a combustion chamber and ignited with a spark. The ensuing combustion produces expanding gases which in turn produce high intensity, low frequency acoustic pulses. With high-speed motion pictures of the CSS event, we relate the motion of the bubble to the acoustic waveform. We also compare the measured first bubble period in the CSS pressure signature with the predictions of the Rayleigh-Willis equation, including the dependence of the radiated acoustic waveform on the volume and depth of the bubble. Measurements of the first bubble period agree with Rayleigh-Willis theory in trend, but not in absolute value. In addition, we discuss the variation of the acoustic output with the fuel/oxygen mixture. Finally, several other factors that affect the acoustic output of CSS are discussed. These include the shape of the CSS combustion chamber, the type of oxidizer and fuel, and the ignition source.

Beamforming on seismic interface waves with an array of geophones on the shallow sea floor

This brief communication relates a generalization to and additional comments on the above editorial*. The present author generally supports the editorial’s recommendations.