The statistics of transmission fluctuations in the ocean are examined theoretically for both multipath and scattering processes. It is shown that, when both are present, multipath propagation dominates the fluctuations and leads to quite broad statistical distributions. In such cases, the mean and standard deviation for logarithmic measures such as transmission loss differ considerably from corresponding mean-square measures. Results for a single tone have been generalized to include multitones, as a model of ambient noise. Statistics of a signal combined with such noise have been derived; the standard deviation of the combination may be several decibels higher than the 5.6 dB of signal alone. The distributions of logarithmic quantities are generally found to be log transformations and combinations of chi-square distributions, and not log-normal distributions as commonly supposed.
Low-frequency ambient noise under pack ice of the central Arctic Ocean has long-term variations (periods greater than 1 h) which correlate highly with composite measures of stress applied to the ice by wind, current, and drift. These composites are the horizontal ice stress and the stress moment, and are derived from meteorological and oceanographic data observed simultaneously with the noise. Atmospheric cooling, a known high correlate of midfrequency noise under the ice, is not important at low frequencies.
Flexural waves in a beam or plate structure can be attenuated by a distribution of masses resiliently mounted to the structure's surface. Wave attenuation and the bandwidth within which it acts are affected largely by the resistance of the resilient mounting and the attached mass, while the center frequency of the attenuation band is affected largely by the spring constant and the mass. We report experimental results obtained on a beam which show good agreement with an analytical model. The attenuation process is physically related to vibration control via dynamic absorption. Significant values of attenuation can be achieved in relatively wide bandwidths for practical combinations of attached mass and resilient mounting.
Temporal variations of low-frequency, broadband ambient noise measured in early summer under drifting ice floes of the marginal ice zone (MIZ) are cross correlated with local environmental forces and ice field descriptors. Surface gravity wave forcing is the primary correlate of the noise; its interaction with ice floes generates sound, most likely via flexural floe failure, and unloading motion, within a few kilometers of the ice edge. Ice concentration is also well correlated with the noise, most likely parametrically. That is, as ice concentration increases, so does the density of potential sound sources in the ice field. Ice stress, ice moment, and wind stress magnitude, while highly correlated with low-frequency noise in the fully ice-covered Arctic, are poor correlates in the MIZ. Lateral melt rate, as a surrogate for thermally induced ice stress, is also poorly correlated with low-frequency MIZ noise. When surface wave forcing is weak, about 1/2 the time in this experiment, episodes of high noise are sometimes observed. These episodes are roughly 3–6 h in duration and 12 h in periodicity. Evidence suggests that they are related to the formation and advection of bands of highly concentrated ice by internal waves during off-ice winds.
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