High‐powered parametric acoustic array in air.

Abstract

MIT Lincoln Laboratory has developed a prototype high‐powered parametric acoustic array (HPPAA) for standoff acoustic excitation in several applications. Parametric arrays offer a highly directional, narrow beam mechanism to deliver sound in air to desired targets typically within a 100 m range. However, a difficult challenge arises in generating sufficient sound power at difference‐frequencies below 1000 Hz at a range which can be critical for many target types. An important objective of the HPPAA design maximizes the difference‐frequency pressure amplitude at range by maximizing the end‐fire array length established by the PAA. The design optimizes the trade‐offs between the three characteristic lengths that control the resultant end‐fire array length including the pump or carrier wave attenuation, PAA aperture, and acoustic saturation of air. In field demonstrations, the HPPAA generated a carrier wave pressure power approximately 155 dB re‐20‐microPa one meter from the PAA face while generating a 300 Hz difference‐frequency SPL of 90 dB re‐20‐microPa 8 meters from the PAA. These sound pressure levels at a few hundred hertz may enable safe standoff excitation and detection of buried landmines and may be useful in standoff nondestructive testing (NDT) damage detection and imaging of structures.