Richard K. Johnson

Sound scattering from a fluid sphere revisited

The backscattering frequency responses for euphausiids and copepods are predicted using a fluid‐sphere model and measured physical properties for the zooplankters. The fluid‐sphere model is also compared with the resonant gas bubble equation.

Physical and acoustical properties of zooplankton

The target strengths of marine zooplankton are often predicted from fluid sphere scattering models. These models require estimation of certain physical properties of the individual zooplankters: radius of the equivalent sphere, mass density, and either sound speed or compressibility. This report summarizes previously published measurements of these parameters and recent, unpublished data.

Laboratory measurements of shear wave attenuation in saturated sand

Laboratory measurements of shear wave attenuation as a function of frequency were made using recently developed ceramic bimorph bender transducers to excite transverse particle motion in a medium grain water‐saturated sand. The measurements were made at 13 frequencies from 450 to 7000 Hz. Multicycle sine‐wave pulses were used to insure steady‐state vibration at the measurement frequency. Attenuation was determined from the slope of a linear least‐squares fit to the maximum received level versus transducer separation data. This not only affords an estimate of the attenuation but allows confidence intervals to be placed around that estimate. The attenuation values’ which increased with frequency from 5 dB/m at 450 Hz to 120 dB/m at 7000 Hz’ did not exhibit a simple first‐power frequency dependence. The results were compared with predictions based on a two‐component model developed by Stoll and were consistent in both amplitude and frequency dependence.

Acoustic estimation of scattering‐layer composition

A method is presented for the estimation of the abundances of scatterers from their backscattering spectra. It uses constrained steepest descent and gives optimal estimates in the least‐squares sense. The population distributions of fish with gas‐filled swim bladders, calculated from a set of backscattering measurements taken in the ocean near Oregon, are generally consistent with midwater‐trawl samples in the same area.

Acoustic wave scattering from a fluid/solid periodic rough surface

A rigorous theory based on the extended boundary condition method is proposed to solve the problem of elastic wave scattering from a periodic fluid/solid interface. The diffraction efficiencies of the reflected compressional wave in the fluid and the transmitted shear and compressional waves are calculated. The energy conservation criterion is used to check the accuracy of the numerical results. The effect of loss (viscoelasticity) in the solid is also included. The wave diffraction from a water and acrylic rough surface is measured. Good agreement between the theory and the experiment is obtained.

Crosscoupling effects on large wideband arrays

The effects of crosscoupling on array performance can be well described by spatiotemporal or spatiofrequency correlation matrices. The data acquisition and analysis required for this approach, however, are very demanding tasks for large wideband arrays. The most important effects of crosscoupling within an array are latent in single element beam patterns which are relatively simple to measure. This paper describes the use of these single element beam patterns to predict array performance and to establish crosscoupling requirements.

Scattering by a periodic rough surface

An exact theory based on the extended boundary conditions method [S. L. Chuang and R. K. Johnson, J. Acoust. Soc. Am. 71, 1368–1376 (1982)] for acoustical scattering from a periodic rough interface between a fluid and a solid is reviewed. Good agreement between theoretical calculations and laboratory measurements is demonstrated. Theory and measurements are used to explore the effects of roughness height and shape as well as material properties on the scattering behavior. Interesting phenomena involving surface wave generation and finite aperture transducer effects are also discussed.

Medical flow imaging with ultrasound

Recent technology advances have made possible commercial machines which produce real‐time images of blood flow for medical applications. Design approaches will be discussed for the issues of velocity range and resolution, tissue and flow differentiation, two‐component image processing, and frame rate. A videotape of clinical flow images will be shown.

Angular spectrum characterization of acoustical transducers

The transmitting and receiving responses of an acoustical transducer in a fluid are characterized using the angular spectrum of plane waves representation. Nearfield and farfield calibration procedures for determining the parameters of the representation are described. Some of these procedures have been implemented, and results are shown. A simplified description is developed for the case of a circularly symmetric transducer which permits a significant reduction in computation. Forward and backward propagation of the radiated acoustical field can be performed using Fourier transform methods. This characterization is very effective for analysis of scattering problems, particularly when they are expressed in the T‐matrix formalism. An example is shown for normal incidence backscattering from a layered viscoelastic half‐space.

Scattering behavior of a solid with a periodic rough surface

A theoretical approach based on the extended boundary condiiton for wave scattering from a liquid and solid periodic interface has been presented and compared with experimental data by Chuang and Johnson [J. Acoust. Soc. Am. Suppl. 1 70, S80 (1981)]. This theory is now applied to study scattering effects in water for several cases in order to demonstrate the physics of the problem. (1) The scattering from two different surface profiles: sinusoidal and triangular. (2) The diffraction efficiencies as functions of the frequency. (3) The effect of viscoelasticity on the diffraction efficiencies. The effects of frequency are also compared with experimental data measured for an acrylic block with a triangular surface profile. We observe peaks in the specular diffraction efficiency at two incidence angles both theoretically and experimentally. One peak occurs near the critical angle of the compressional wave of the solid. The second peak occurs at the angle of incidence such that the surface wave of the liquid a...