Index of content:
Volume 120, Issue 2, August 2006
- UNDERWATER SOUND 
120(2006); http://dx.doi.org/10.1121/1.2211427View Description Hide Description
Frame bulk modulus is important for analyzing the acoustic wave propagation in porous water-saturated marine sediments such as sands. Previous measurements of the longitudinal wavevelocities in air-saturated glass beads of uniform grain size showed that the longitudinal wavevelocity increased with the grain size. This result cannot be explained by using a classical contact theory such as the Hertz-Mindlin model. It was speculated that this phenomenon is due to the effect of air elasticity between the grains. In this study, the longitudinal and shear wavevelocities in samples of vacuum-, air-, and water-saturated glass beads as well as beach sands at a lower stress were measured. The results obtained were used to estimate the corresponding values of the frame bulk modulus. In the water-saturated samples, these values are about at a frequency of and about ten times greater than those in air-saturated samples at a frequency of . The grain size dependence was also observed. These measurements are explained in terms of the effect of fluid elasticity at the grain-to-grain contact in the context of a modified gap stiffness model.
Increases in deep ocean ambient noise in the Northeast Pacific west of San Nicolas Island, California120(2006); http://dx.doi.org/10.1121/1.2216565View Description Hide Description
Recent measurement at a previously studied location illustrates the magnitude of increases in ocean ambient noise in the Northeast Pacific over the past four decades. Continuous measurements west of San Nicolas Island, California, over , spanning 2003–2004 are compared to measurements made during the 1960s at the same site. Ambient noise levels at were higher (95% ) in 2003–2004 than in 1964–1966, suggesting an average noise increase rate of per decade. Above the noise level differences between recording periods gradually diminished to only at . Above the 1964–1966 ambient noise levels were higher than in 2003–2004, owing to a diel component which was absent in the more recent data. Low frequency ocean ambient noise levels are closely related to shipping vessel traffic. The number of commercial vessels plying the world’s oceans approximately doubled between 1965 and 2003 and the gross tonnage quadrupled, with a corresponding increase in horsepower. Increases in commercial shipping are believed to account for the observed low-frequency ambient noise increase.