Response of Infrasonic Microphone Field Array to A Controlled Source

A field test on a 3-microphone array at NASA Langley Research Center was conducted using a mobile controlled infrasonic source. A Helmholtz resonator, used to provide a simulated point source for infrasonic propagation studies, had an output SPL of 99 dB (at 1 m) at its resonance frequency of 9.45 Hz. The 3-microphone array was arranged as an equilateral triangle with microphones spacing of 30.48 m (100 ft) and at a distance of more than 85.3 m (280 ft) from the source. The signal level was 40 dB above the background noise in a 1-Hz band. Measurements of the acoustical response for each of the array microphones were recorded and the received signal was measured at the nearest microphone to be 60 dB (6 dB per doubling of distance).