Rotorcraft External Far-Field Noise Measurement Using a Hot-Air Balloon

An innovative and fundamental approach to measure far field noise of a helicopter using a hot-air balloon as an acoustic measurement platform is described. The technique, featuring a microphone suspended beneath the basket of a hot-air balloon, is used to obtain acoustic data above, in, and below the horizon plane of a helicopter in forward flight. This approach is postulated to have many advantages over previous measurement approaches: very high signal-to-background noise levels, the ability to test at specific altitudes, and the ability to gather acoustic data above the horizon plane of the helicopter a first. Successful preliminary flight tests obtained acoustic time histories of a Bell 206B-3 helicopter flying steady maneuvers around a microphone suspended beneath a hot-air balloon at altitudes between 1000 and 3000 feet. Recorded time histories were free of ground and balloon reflections and showed low levels of background noise, suggesting that the technique could readily be applied to acquire pristine seminal acoustic data. The concept will be further refined over the course of the Phase II effort. Custom instrumentation will be developed for the subject helicopters and the balloon and integrated with the balloon measurement platform so that quantitative acoustic data may be obtained.