Airborne noise contributions to the underwater noise sound field.

Abstract

Contributions of airborne noise sources to the underwater noise field are the result of two acoustic fields: the transmitted and evanescent. The transmitted field can be represented by only those rays confined to a small cone (about 26 deg) where the reflection coefficient is real‐valued. The evanescent field, which arises when rays are totally reflected from the surface, can also contribute to the underwater noise field. Unlike the transmitted field, the evanescent field does not propagate and decays exponentially with depth with a decay rate as a function of frequency. Determining the individual contribution of these two fields to the overall sound field is experimentally difficult to observe. One situation where these two fields can be observed individually occurs when an airplane flies overhead. The Doppler shift associated with tonal propeller noise is dependent on the acoustic path. The frequency separation of the two fields allows for separate analysis of the two fields. Measurements from aircraft (altitude 1000 ft) passing over a buoy equipped with a microphone 3 m above the surface and a hydrophone 2.5 m below the surface will be presented. Numerical simulations are presented along with the experimental observations. [Research supported by the Washington Sea Grant and the U.S. Office of Naval Research.]