Shallow water experiments have been conducted in Nantucket Sound with an autonomous underwater vehicle towed hydrophone array system in an area proximate to that of a previous experiment [Frisk and Lynch, J. Acoust. Soc. Am. 86, 1928–1939 (1989)]. Transmission loss was measured, for frequencies between 220 and 1228Hz in an ocean waveguide, under conditions of an isovelocity water column with an approximate depth of 13m over a sandy-silty bottom. Results obtained at 415Hz show classic isovelocity waveguide phenomena that include interference effects, mode stripping due to modal attenuation, and transmission loss proportional 15 to 10log10(R).
Several noise criteria methods commonly used in architectural acoustics have been quantitatively related to noise perception and task performance under a variety of ventilation systems-induced background noise conditions. Noise criteria, balanced noise criteria, room criteria, room criteria mark II, and A-weighted equivalent sound pressure level were examined. The first phase of the project included noise conditions controlled to be non-time-varying and nontonal, with neutral, rumbly, roaring, or hissy characteristics. An intermediate study examined exposure time length and types of performance tasks used. The final phase included noise conditions containing various levels of discrete tones from 120 to 595 Hz. Under each noise, subjects completed performance tasks and perception questionnaires. Results indicate task performance was significantly affected by perception of noise, but this relationship was not fully demonstrated by the criteria systems analyzed. The five criteria were generally well suited in describing subjective loudness perception, but some discrepancies in criteria spectral quality ratings and subjective perception existed. Finally, perception of annoyance changed based on the frequency and prominence of tones in noise, but these changes were not reflected in the criteria level or spectral quality ratings. Modifications to the existing criteria are recommended.
This paper summarizes evidence of a nonlinear frequency dependence of attenuation for compressional waves in shallow-water waveguides with sandy sediment bottoms. Sediment attenuation is found consistent with α(f)=α(fo)∙(f∕fo)n, n≈1.8±0.2 at frequencies less than 1kHz in agreement with the theoretical expectation, (n=2), of Biot [J. Acoust. Soc. Am. 28(2), 168–178, 1956]. For frequencies less than 10kHz, the sediment layers, within meters of the water-sediment interface, appear to play a role in the attenuation that strongly depends on the power law. The accurate calculation of sound transmission in a shallow-water waveguide requires the depth-dependent sound speed, density, and frequency-dependent attenuation.
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