Comparison of upside-down microphone with flush mounted microphone configuration

Measurement of fly-over aircraft noise is often performed using the microphones mounted in an upside-down configuration, with the microphone placed 7 mm above a hard reflecting surface. This method assumes that most of the sound is coming from the back of the microphone within an angle of + -60 degrees. The same microphone configuration is proposed for installed and un-installed jet-engine test in which case, however, the incidence angle for the microphone may be in the range of 60–85 degrees. The response of the upside-down microphone configuration is compared with flush mounted microphones as reference. The influence of microphone diameter (ranging from 1/8 in. to ½ in.) is compared in the different configurations and the effect of windscreens is investigated.