Practical application of a tri-axial intensity array

Abstract

Sound intensity is a vector quantity representing the magnitude and direction of propagating energy within an acoustic field. In an underwater environment, a single omni-directional hydrophone can be used to measure instantaneous acoustic pressure and a finite difference approximation applied to the pressure signals from a pair of such hydrophones can be used to calculate particle velocity in a single direction. Because the time average of the product of instantaneous pressure and particle velocity is intensity, a pair of hydrophones is all that is required to measure a single component of the intensity vector. The complete three-dimensional intensity vector can be calculated using three orthogonal pairs of hydrophones. To evaluate this concept a tri-axial array consisting of three orthogonal pairs of omni-directional hydrophones has been developed and tested on both calibrated sources at a laboratory facility and sources of opportunity during sea trails in littoral waters. The use of this array to calculate the intensity vector and thereby localize both near-field and far-field acoustic sources and characterize the directionality of ambient noise fields will be discussed. The impact of signal-to-noise ratio and the effect of self-noise will also be examined.