Volume 130, Issue 2, August 2011
Index of content:
- UNDERWATER SOUND 
130(2011); http://dx.doi.org/10.1121/1.3605666View Description Hide Description
It has been hypothesized that at sufficiently high levels of oceanic salinity turbulence it should be possible to observe acoustic backscattering. However, there have been limited in situ measurements to confirm this hypothesis. Using an autonomous underwater vehicle equipped with upward and downward looking 1.2 MHz acoustic Doppler current profilers and with turbulence and fine scale sensors, measurements were performed in a region of intense turbulence and a strong salinity gradient. The approach taken was to correlate variations in the backscattered acoustic intensity, I, with a theoretical acoustic backscattering cross section per volume for salinity turbulence, , to obtain an estimated scattering cross section per volume, . Results indicated that of order 50% of the observed region was characterized by salinity turbulence induced backscattering.
Adaptive projection method applied to three-dimensional ultrasonic focusing and steering through the ribs130(2011); http://dx.doi.org/10.1121/1.3607419View Description Hide Description
An adaptive projection method for ultrasonic focusing through the rib cage, with minimal energy deposition on the ribs, was evaluated experimentally in 3D geometry. Adaptive projection is based on decomposition of the time-reversal operator (DORT method) and projection on the “noise” subspace. It is shown that 3D implementation of this method is straightforward, and not more time-consuming than 2D. Comparisons are made between adaptive projection, spherical focusing, and a previously proposed time-reversal focusing method, by measuring pressure fields in the focal plane and rib region using the three methods. The ratio of the specific absorption rate at the focus over the one at the ribs was found to be increased by a factor of up to eight, versus spherical emission. Beam steering out of geometric focus was also investigated. For all configurations projecting steered emissions were found to deposit less energy on the ribs than steering time-reversed emissions: thus the non-invasive method presented here is more efficient than state-of-the-art invasive techniques. In fact, this method could be used for real-time treatment, because a single acquisition of back-scattered echoes from the ribs is enough to treat a large volume around the focus, thanks to real time projection of the steered beams.
130(2011); http://dx.doi.org/10.1121/1.3605670View Description Hide Description
The acoustic behavior of an acoustically transparent polyurethane and an interpenetrating polymer network of polyurethane with polydimethyl siloxane were studied using dynamic mechanical analysis, finite element modeling, and experimental evaluation of acoustic properties in a water-filled pulse tube setup. Dynamic mechanical measurements in the temperature range −50 °C to +70 °C were carried out, and the data were used for time temperature superposition to generate material behavior at high frequencies. These inputs were used for modeling the acoustic behavior of these materials using atila, which is a commercial finite element code, capable of computing transmission and reflection characteristics of materials. From this data, absorption characteristics were computed. The results were compared with the experimental results obtained using a water-filled pulse tube facility.
Analysis and localization of blue whale vocalizations in the Solomon Sea using waveform amplitude data130(2011); http://dx.doi.org/10.1121/1.3605550View Description Hide Description
During the Woodlark Basin seismic experiment in eastern Papua New Guinea (1999–2000), an ocean-bottom seismic array recorded marine mammal vocalizations along with target earthquake signals. The array consisted of 14 instruments, 7 of which were three-component seismometers with a fourth component hydrophone. They were deployed at 2.0–3.2 km water depth and operated from September 1999 through February 2000. While whale vocalizations were recorded throughout the deployment, this study focuses on 3 h from December 21, 1999 during which the signals are particularly clear. The recordings show a blue whale song composed of a three-unit phrase. That song does not match vocalization characteristics of other known Pacific subpopulations and may represent a previously undocumented blue whale song. Animal tracking and source level estimates are obtained with a Bayesian inversion method that generates probabilistic source locations. The Bayesian method is augmented to include travel time estimates from seismometers and hydrophones and acoustic signal amplitude. Tracking results show the whale traveled northeasterly over the course of 3 h, covering approximately 27 km. The path followed the edge of the Woodlark Basin along a shelf that separates the shallow waters of the Trobriand platform from the deep waters of the basin.