Index of content:
Volume 113, Issue 3, March 2003
- ACOUSTICAL MEASUREMENTS AND INSTRUMENTATION 
Phase change measurement, and speed of sound and attenuation determination, from underwater acoustic panel tests113(2003); http://dx.doi.org/10.1121/1.1531984View Description Hide Description
A technique for measuring the change in phase produced by the insertion of a panel between a projector and receiver is described. Presented also is a procedure for determining the phase speed and attenuation of the panel material. Although the methods were developed over the frequency decade 10–100 kHz, they are not limited to that band. It was observed that a “settling time” of approximately 20 min is required to obtain reproducible phase measurements if the experiment is disturbed even slightly. For example, rotating the panel 10 degrees, then immediately returning to the original position, causes the observed phases to differ by up to 10 deg from those obtained prior to the disturbance. These differences are distributed randomly across frequency. Temperature stabilization within the medium as well as the material is also required before measurements can take place. After the stated 20 min settling time, however, the phases return to the values obtained prior to rotation, or after temperature stabilization, to within The sound speed and attenuation determination technique employs least-squares fitting of a causal model to the measurements. Four (or fewer) adjusted parameters accommodate the measurements over the stated frequency decade, even for samples that exhibit significant dispersion. The sound speed is typically determined to an accuracy of ±30 m/s, as judged from a propagation-of-error calculation. This model assumes single-layered panels.
113(2003); http://dx.doi.org/10.1121/1.1543851View Description Hide Description
The theoretical directivity of a single combined acoustic receiver, a device that can measure many quantities of an acoustic field at a collocated point, is presented here. The formulation is developed using a Taylor series expansion of acoustic pressure about the origin of a Cartesian coordinate system. For example, the quantities measured by a second-order combined receiver, denoted a dyadic sensor, are acoustic pressure, the three orthogonal components of acoustic particle velocity, and the nine spatial gradients of the velocity vector. The power series expansion, which can be of any order, is cast into an expression that defines the directivity of a single receiving element. It is shown that a single highly directional dyadic sensor can have a directivity index of up to 9.5 dB. However, there is a price to pay with highly directive sensors; these sensors can be significantly more sensitive to nonacoustic noise sources.