Index of content:
Volume 130, Issue 4, October 2011
- UNDERWATER SOUND 
The relative effects of particles and turbulence on acoustic scattering from deep-sea hydrothermal vent plumes130(2011); http://dx.doi.org/10.1121/1.3624816View Description Hide Description
Acoustic methods are applied to the investigation and monitoring of a vigorous hydrothermal plume within the Main Endeavor vent field at the Endeavor segment of the Juan de Fuca Ridge. Forward propagation and scattering from suspended particulates using Rayleigh scattering theory is shown to be negligible (log-amplitude variance ) compared to turbulence induced by temperature fluctuations (). The backscattering from turbulence is then quantified using the forward scattering derived turbulence level, which gives a volume backscattering strength of sV = 6.5 × 10−8 m−1. The volume backscattering cross section from particulates can range from sV = 3.3 × 10−6 to 7.2 × 10−10 m−1 depending on the particle size. These results show that forward scatter acoustic methods in hydrothermal vent applications can be used to quantify turbulence and its effect on backscatter measurements, which can be a dominant factor depending on the particle size and its location within the plume.
130(2011); http://dx.doi.org/10.1121/1.3621514View Description Hide Description
A four hydrophone linear array was used to localize calling black drum and estimate source levels and signal propagation. A total of 1025 source level estimates averaged 165 dBRMS relative (re:) 1 μPa (standard deviation (SD) = 1.0). The authors suggest that the diverticulated morphology of the black drum swimbladder increase the bladder’s surface area, thus contributing to sound amplitude. Call energy was greatest in the fundamental frequency (94 Hz) followed by the second (188 Hz) and third harmonics (282 Hz). A square root model best described propagation of the entire call, and separately the fundamental frequency and second harmonic. A logarithmic model best described propagation of the third harmonic which was the only component to satisfy the cut-off frequency equation. Peak auditory sensitivity was 300 Hz at a 94 dB re: 1 μPa threshold based on auditory evoked potential measurements of a single black drum. Based on mean RMS source level, signal propagation, background levels, and hearing sensitivity, the communication range of black drum was estimated at 33–108 m and was limited by background levels not auditory sensitivity. This estimate assumed the source and receiver were at approximately 0.5 m above the bottom. Consecutive calls of an individual fish localized over 59 min demonstrated a mean calling period of 3.6 s (SD = 0.48), mean swimming speed of 0.5 body lengths/s, and a total distance swam of 1035 m.
130(2011); http://dx.doi.org/10.1121/1.3628325View Description Hide Description
The phase angle component of the complex frequency response of a sonar system operating near transducer resonance is usually distorted. Interpretation and classification of the received sonar signal benefits from the preservation of waveform fidelity over the full bandwidth. A calibration process that measures the phase response in addition to the amplitude response is thus required. This paper describes an extension to the standard-target calibration method to include phase angle, without affecting the experimental apparatus, by using dual-frequency transmission pulses and frequency-domain data processing. This approach reduces the impact of unknown range and sound speed parameters upon phase calibration accuracy, as target phase is determined from the relationship of the two frequency components instead of relying on a local phase reference. Tungstencarbide spheres of various sizes were used to simultaneously calibrate the amplitude and phase response of an active sonar system in a laboratory tank. Experimental measurements of target phase spectra are in good agreement with values predicted from a theoretical model based upon full-wave analysis, over an operating frequency of 50–125 kHz.