Noise source identification in airplane cabins using acoustic intensity technique

A technique to measure surface radiation from an airplane cabin in flight using a two microphone acoustic intensity system is described. The technique addressed the problems of high background levels, surface irregularity, and surface absorption that have complicated cabin radiation measurements. Two probe types were used: a bare probe for the reflective sidewall and ceiling regions, and a shielded probe for the absorptive carpet. Laboratory tests were conducted to establish the accuracy and tolerance to background noise for the flight measurement system. From these tests the operating range for each probe was determined in terms of the ratio of pressure to intensity. This ratio, called the Signal-to-Noise Indicator (SNI), was used to screen out flight data saturated by the background field. From the cabin surveys several strong radiation areas, such as the ceiling panel and the air distribution and air return grills, were measured quantitatively. Small area sources were distinguished from adjacent areas and other weak sources were identified.