Characterizing the effects of two ground-based outdoor microphone configurations

Multiple International and Federal regulations stipulate the acquisition of aircraft noise be conducted using inverted pressure microphones over a round ground board. Ground boards are used to provide an acoustically hard reflecting surface, limiting the effects of the potentially absorptive local ground. To determine the ground board effects on the measured acoustic signal, a measurement campaign was undertaken at NASA Langley Research Center. The experiments included multiple ground board configurations placed on top of a sand pit, in an otherwise anechoic chamber. Ground board configurations included a microphone inverted and offset over a ground board and a microphone offset and flush mounted in the ground board. White noise was used to investigate the ground board effects on the recorded signal. Normal impedance measurements were acquired to determine the reflection coefficients of the sand and ground boards. Results indicate that both microphone configurations perform adequately up to 10 kHz. When the ground substrate is a soft material and the sound comes in at angles near grazing incidence, the signal is attenuated above 1 kHz. Additionally, the plastic material used to construct the ground boards was found to be acoustically hard between 0.1 – 3.0 kHz, and likely extending to higher frequencies.