Underwater measurement of the sound -intensity vector: Its use in locating sound sources, and in measuring the sound power of stationary and moving sources

Instrumentation was developed to measure the components of the sound-intensity vector (sound-power flow per unit area) in water. It consists of a probe with four hydrophones at the vertices of a regular tetrahedron and associated equipment including three two-channel FFT analyzers. Finite-difference approximations and the cross-spectral relation were used to determine the components of the sound-intensity vector. The hydrophones were closely spaced, allowing measurements to be made in a frequency range up to about 13 kHz. The accuracy of the probe in measuring the direction to a sound source was tested under different conditions. First gating methods were used to remove the effect of noise interference and it was shown that the accuracy of direction measurement was within +/- 2 degrees. Tests were then conducted to determine the effect of different types of interference: (a) boundary reverberation, (b) ambient background noise and (c) another sound source. Various simple signal-processing techniques were used to counteract these interferences. Data are presented in pitch-azimuth Cartesian direction of sound plots, where these plots provide a three-dimensional view of the ambient sound field the incident on the probe. Several techniques were used to separate the directions of multiple sound sources such as (a) spectral weighting and (b) spectral subtraction. In other tests, the probe was used to track a moving source and determine its sound power. The moving source was a driven hydrophone whose sound power was (a) determined from the measured voltage applied to the hydrophone and the response curves supplied by the hydrophone manufacturer and (b) measured when the hydrophone is stationary. These were compared with the sound power of the moving hydrophone which was found to be about 14--18% or 0.5--0.7 dB larger. It is believed that this is the first time the sound power of a moving source has been measured.