On the use of ground-board mounted microphones for outdoor noise measurements

Outdoor noise measurements made by a microphone mounted into or on an acoustically rigid thin board lying on the ground can be affected to some degree by diffraction from the edges of the board due to the thickness of the board or the finite impedance of the surrounding ground. This can result in the sound pressure level varying over the top surface of the board and deviating from the 6 dB increase (relative to the incident sound field) expected if the board was very thin and the surrounding ground was also acoustically rigid. In this paper, an experimental and numerical investigation is carried out to assess the variation in the sound pressure level over the top surface of the ground-board from sound incident at several different angles. The experiments utilise a probe microphone flush-mounted into a 0.4 m diameter circular thin metal ground-board slightly protruding from a simulated grass-covered field in an anechoic chamber. The experiments are repeated on an outdoor grass-covered field and compared. The effect of different installation methods such as the flush-mounted probe microphone, inverted microphone, and a side-set microphone are also investigated. The variation of the sound pressure level on the top surface of the ground-board due to diffraction from the edges of the board is shown. It is also shown that the presence of the microphone body begins to affect the noise measurements from a frequency of 3 kHz. The results presented in this paper suggest that narrow-band noise measurements using a microphone mounted on a circular ground-board installed on a grass-covered surface can deviate by more than 3 dB from the nominal 6 dB increase above the incident field level.