Understanding the mechanisms by which the ocean sediment redirects impinging sound back into the ocean is necessary in developing propagation models for sonar performance prediction. The Naval Underwater Systems Center (NUSC) has (1) conducted controlled, self‐calibrating acoustic measurements where the ocean bottom interacted signal is isolated in time for analysis, (2) developed deconvolution processing techniques to aid in describing the impulse response of the ocean sediment, and (3) performed modeling to study the interaction of acoustic waves at the ocean bottom. This paper presents a synopsis of studies showing the necessity of considering the refraction of sound by the ocean sediment when predicting low‐frequency propagation loss. Constructive interference between nonplanar wave sediment refracted sound and sound reflected by the ocean‐sediment interface and subbottom layering can cause negative values of bottom loss when using plane‐wave models to interpret measured data. These models cannot account for all possible acoustic arrivals at a receiver. In addition, for a given frequency and constant ocean bottom grazing angle, bottom loss can be dependent upon both processing bandwidth and source/receiver depth. Deconvolution has aided in time resolution of signals that make up the bottom‐interacted signals. Resolution of these signals aids in interpreting results. A modeling effort utilizing the Fast Field Program (a computer technique for evaluating the field integral by the fast Fourier transform) provides quantitative evidence for the necessity of accounting for the refraction of sound by subocean sediments to interpret properly low‐frequency propagation loss measurements.

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The perissodactyls (odd-toed ungulates) comprised of equines, rhinos, and tapirs have vocalizations during which the frequency spectra can change, in contrast to the artiodactyls (even-toed ungulates), comprised of cows, sheep, deer, etc., whose vocalizations are primarily tonals. This suggests that a simple vocal expression by perissodactyls may be possible, and a recent study of horse whinnies under different conditions indicates that this may be the case. The vocalizations of tapirs, perhaps the least studied members of this group, are compared to those of equines and rhinos.

Under downward refracting conditions, raypafth fromi a shallow source will tend to have grazing angles at a hard bottom that are greater than the critical angle and. therefore, suffer a relatively large loss per bounce. As the source depth Increases, lower grazing angles can be obtained. When the grazing angle becomes less than the critical angle, bottom loss per bounce Is significantly reduced. allowing a possible reduction In propagation loss. analysis Is made for a North Atlantic shallow water area south of Long Island under summer conditions to detenrine the sensivty of transmission loss to changes in source and recelver depth for ranges up to 50 kmn. The results are compared to the previous results of Cole and Podeszwa [B. F.

The principal mechanism for low frequency absorption in seawater is a boron reaction that is pH dependent; the lower the pH, the lower the absorption. Twenty seven years ago, Mellen et al. [J. Acoust. Soc. Am. 82, S30 (1987)] computed the low frequency sound absorption for the Arctic Ocean. Since the time the carbon dioxide (CO2) level in the atmosphere has been continually increasing. Experts predict that the resulting ocean acidification may increase by up to 170% this century. Acoustically, the Arctic Ocean is most sensitive to rapid change not only because the cold water readily absorbs CO2, but also because the sound channel axis is at or near the surface. The range of reduction in the low frequency sound absorption is presented based on possible future acidification scenarios, mindful that this is just one component of a complex evolution that is occurring in the Arctic.