Ralph R. Goodman

Reflection and Transmission of Sound by Elastic Spherical Shells

The sound fields resulting from a plane wave incident on a spherical elastic shell are found. Both the internal and external fields due to the shell are considered and compared with well‐known results. The Rayleigh limit is discussed, and it is found that the external field is similar to that produced by a rigid sphere but with more complicated coefficients. The internal field is of a complicated nature but reasonable to consider for numerical computation. The sound fields for a hemispherical shell mounted on a rigid medium are obtained by the image method. Numerical results for the sound intensity at the center of the hemisphere are presented for various elastic materials.

Shallow-water bottom reverberation measurements

High-frequency bottom reverberation measurements were made at an experimental site in the Gulf of Mexico. The acoustic data were taken as a function of frequency (40-180 kHz) and grazing angle (40-33/spl deg/). The measured acoustic reverberation results are compared to predictions made by models developed by Jackson et al. (1986, 1996) and Boyle and Chotiros (1995). The models used inputs from the analysis of sediment cores and stereophotography. The model predictions show differences from each other and from the data. The results show reverberation-level variabilities as a function of frequency that cannot be accurately predicted by these models.

An iterative approach for approximating bubble distributions from attenuation measurements

A precise theory exists, based on an integral equation, by which acoustic signal attenuation versus frequency, due to a known bubble-density distribution versus bubble radius, may be calculated. Lacking a simple inversion scheme for the integral equation, an approximation which accounts only for attenuation due to resonant bubbles is available (and often applied) to calculate a bubble distribution. An iterative approach for improving on that resonant bubble approximation is presented here. That new approach is based on alternating calculations and corrections between attenuation data and the bubble distribution presumed to have produced it. This iterative technique is tested, first, on two simulated data sets of bubble distributions. It is then applied to attenuation data measured as a function of frequency from 39 to 244 kHz during the Scripps Pier Experiment [Caruthers et al., Proc. 16th Int. Cong. on Acoust., pp. 697–698 (1998)]. The results of the simulations demonstrate the validity of the method by ...

Acoustic attenuation in very shallow water due to the presence of bubbles in rip currents

An experiment was performed just off the research pier at the Scripps Institute of Oceanography to determine the acoustic effects of small bubbles in very shallow water (∼6 m depth). The distance offshore was ∼300 m. The propagation lengths were 2–10 m, and the frequency range was from 39 to 244 kHz. During the experiment, rip currents passed through the field of measurement instruments. These rip currents were laden with bubbles created in the surf between the instruments and the shore. The effects of these rip currents on the spatial distributions of the resulting acoustic attenuation are discussed. From the attenuation data, the bubble distributions are calculated using a new iterative approach [Caruthers et al., in press, J. Acoust. Soc. Am.] that is based on the well-known resonant bubble approximation. Calculated bubble distributions varied from an essentially uniform lack of bubbles during quiescent periods to highly inhomogeneous and dense bubbly regions within rip events. Such observed distributi...

Measurements of high-frequency shallow-water acoustic phase fluctuations

A shallow-water high-frequency (HF) acoustic propagation experiment was conducted just off shore in Panama City, FL. Several broad-band high-resolution sources and receivers were mounted on stable platforms and deployed in water depths of 8-10 m. Signals covering the frequency range from 20 to 200 kHz were transmitted from the sources to two spatially separated receivers. The data were analyzed to provide estimates of the signal phase variances as a function of frequency and source-to-receiver range. These phase variabilities are correlated with small-scale water column thermal variabilities and ocean swell conditions.

Observation of high-frequency sound propagation in shallow water with bubbles due to storm and surf

An experiment was performed off the shore of Panama City, FL, to measure the spatial and temporal coherence of high-frequency signals that were transmitted between fixed towers. Transmission was along paths at mid depths in about 10 m of water. During the time of the experiment, there were two stormy days with breaking waves and nearby high surf. It was observed that pulse-to-pulse variations (over seconds) in travel times, over a range of frequencies, increased dramatically from those observed on quiet days. Average travel times also increased by about 3%. Dispersion was also observed. The distance between the source and the receiver towers was approximately 60 m. By assuming that bubbles were either generated by breaking waves and advected down ward and/or generated by surf and advected outward, these results are explained. Estimates of the average bubble density and bubble-density variations are made.

Energy of the acoustically excited surface wave on a flat semi‐infinite elastic medium

An analysis is made of the distribution of intensity of the evanescent waves in a semi‐infinite elastic medium when a plane acoustic wave in a contiguous fluid medium is incident upon it. It is found that the maximum intensity and total energy in the evanescent wave occurs at the angle of incidence corresponding to the Rayleigh angle [θR = sin−1(C 0/CR), where CR is the Rayleigh wave speed and C0 is the sound speed in the fluid medium]. This result gives insight both mathematically and physically as to what should be expected for surfaces with curvature as well as for plates.

Near‐bottom hydrophone measurements of ambient noise and sperm whale vocalizations in the northern Gulf of Mexico

Three bottom‐moored hydrophones, 50 m above the bottom, were placed on a downslope line, ending at the largest concentration of sperm whale sightings in the northern Gulf of Mexico, in 600 m, 800 m, and 1000 m water depths. These depths were chosen after upslope propagation modeling, using historical databases, showed transmission losses greater than 110 dB at hydrophones near the bottom in water shallower than 600 m for a 500 m deep source at the 1000 m contour. These autonomously recording hydrophones were environmental acoustic recording system (EARS) buoys obtained from the Naval Oceanographic Office. They were capable of recording signals up to 5500 Hz continuously for 36 days and were deployed from July 17 through August 21. During this period a major marine mammal exercise was being conducted at the surface by the Minerals Management Service and the National Marine Fisheries Service, with other government and university scientists, in which temporary acoustic recording devices were attached to the ...

Effects of bubbles on high‐frequency sound propagation in very shallow water

The results of an experiment performed off the research pier at the Scripps Institute of Oceanography that measured the acoustic effects of small bubbles in very shallow water (6 m) are discussed. The results presented are in coordination with researchers who conducted other measurements simultaneously. During the experiment, rip currents passed through a field of measurement instruments 300 m offshore. These rip currents were laden with bubbles created in the surf between the instruments and the shore. Pulse signals, between 39 and 244 kHz, were propagated to 10 m through the bubble clouds. The effects of these rip currents on the spatial distributions of the resulting acoustic attenuation are discussed. From the attenuation data, bubble populations are calculated by an iterative procedure based on the well‐known resonant bubble approximation. [Work supported by the Office of Naval Research.]

The Scripps Pier Bubble Experiment of 1997 and overview of rip events and their effects on acoustics measurements

An experiment to determine the dynamics, distributions, and acoustic effects of bubbles in shallow water just offshore from active surf was performed in the Spring of 1997. The region of interest was just North of the pier at the Scripps Institution of Oceanography. An area from a few tens of meters to a thousand meters from the beach was instrumented by several researchers. An effort was made to acquire a comprehensive dataset of coastal dynamics, bubble distributions, and acoustic propagation. A major emphasis was placed on a smaller region approximately 400 m2 in area and about 300 m offshore, where most of the instruments were clustered. The purpose of this paper is to provide an overview of the experiment, a summary of the runs made over the several days of experiments, and a context for the observed rip events based on an acoustic device known as the ‘‘Delta‐Frame.’’ Two rip events (Run 5, event starting about 14:48 PST 7Mar97 and Run 7, event starting about 15:38 PST 8Mar97) selected for coordinate...