Sonic boom penetration under a wavy air–water interface

Abstract

The interaction of a sonic boom with a wavy air–water interface is investigated theoretically and experimentally to ascertain its effect on sonic boom noise penetration in deep water. The Mach number is assumed to be less than the water-to-air sound speed ratio of 4.53. Unlike the problem with a flat (nonwavy) interface, the interaction with a surface-wave train can produce everfescent (nonattenuating) wave components, in addition to evanescent wave components underwater. The overall effect results in an attenuation rate with a depth which is much lower than predicted by the flat-ocean (Sawyers) model; it overwhelms the flat-ocean wavefield in deep water, even though it has a secondary effect at the surface. The expectation is substantiated by both analysis and laboratory measurements. The latter were performed in a water channel with wavemakers and projectile over flights. Detailed measurements confirm both the wave packet characteristics of the deep water waveform and the frequency downshift property predicted by the theory. Underwater sound-pressure level, frequency range, and waveform characteristics from sonic booms of aircraft and space-launch operations are examined with the wavy-ocean model.