Slowing the speed of sound in a dispersionless environment and consequences regarding source location.

Abstract

Theoretical considerations and simulations indicate temporal interference between direct and boundary-reflected paths induce large variations in the speed, c3d, of an acoustic pulse between two points. This occurs even in a dispersionless medium, where the phase and group speeds are equal. The effect occurs when the source and receiver approach one another and are within cδt̃/2 of the boundary, where c is the in situ speed of sound and δt̃ is the smallest temporal separation between the paths at which interference initiates. At small separations, the c3d might approach zero. The effect diminishes far from a receiver as the size of the delay shrinks relative to the overall time of propagation. The phenomenon may be of importance for methods designed to locate sounds from whales via time differences of arrival as the c3d may significantly differ from the in situ value, and the c3d between a source and each receiver may differ by large amounts, a phenomenon invalidating the geometrical interpretation of location by hyperboloids and validating isodiachronic geometries instead.