The acoustic propagation losses between a 230-Hz cw source and a multi-hydrophone receiving system were measured over Dickins Seamount in the Northeast Pacific. The source was towed at depths of 18 and 184 m. The receiving system had hydrophones spaced in depth from 323 to 633 m. The measurements were made to a maximum range of 130 km with the receiver located at a range of 60 km from the seamount peak. The results show that the seamount cast an acoustic shadow over the receiver, increasing the propagation loss by up to 15 dB, when the source was shallow and in a position which enabled the seamount to intercept all of the deep refracted source energy. Back reflections from the seamount with levels 6 to 13 dB below the direct signal level were present when the shallow source was 3 to 5 'km from the seamount peak. Downslope reflections enhanced the direct signal by up to 10 dB when the shallow source was within 3 km of the peak. Acoustic shadowing and reflection effects were minimal in the results for the deep source because most of the source energy propagated along the sound-channel axis above the seamount peak. The analysis indicates that ray theory is adequate for describing the reflection effects of the acoustic propagation but does not account for all of the acoustic energy in the shadow zone.

The frequency dependence of acoustic volume backscattering strength has been measured at depths of 100, 200, 300, and 400 ft in Saanich Inlet, British Columbia, Canada, over a 24-h period. The measurements were made for comparison with open-ocean (Pacific) results, since the biology of the inlet has been extensively studied, and it serves to some extent as a reference water volume. In the course of carrying out the above measurements, the backscattering spectral characteristics and strength of a school of young hake, of apparent mean size of 11 in., was obtained. The time series measurements revealed unchanging scattering characteristics at the 300- and 400-ft depths, with broad peaks near 1.2 and 4.8 kHz superimposed on otherwise flat spectra. Greater variability was observed at the 200-ft depth, while the greatest variability was observed near the surface, i.e., at the 100-ft observation depth. The fish school, which was observed to be closely packed (less than a fish length spacing near the surface) and randomly oriented, was shown to have a flat backscattering spectrum between 1 and 9 kHz and an estimated backscattering strength of −47±3 dB.

Long-range propagation losses were measured at frequencies from 0.025 to 0.8 kHz to the north and south of a receiver at 46 °N, 143°30′W in the Northeast Pacific. To the south of the station, the attenuation losses experienced a pronounced minimum near 50 Hz and approached the one-half Thorp value at 0.8 kHz. An examination of the behavior of the attenuation coefficient as a function of range indicated that a range of at least 700 km would be required to make meaningful measurements of attenuation in this area. To the north of the receiving station, focusing of the sound due to a rapidly shallowing sound channel prevented reliable measurement of the attenuation coefficient at any range.

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A macroscopic theory is presented to explain the electromagnetic generation of acoustic waves in a metal in the presence of a steady magnetic field, and to account for the effect of these acoustic waves on the surface impedance. The theory is valid for the limiting case where the skin depth is much smaller than the wavelength of sound. In particular, the size and shape of the surface impedance singularities at frequencies corresponding to the formation of standing acoustic waves in a metal plate are calculated. Absolute measurements of the surface resistance and reactance of gallium single crystals are reported which are in quantitative agreement with the predictions of the theory.