Correlation microphone for measuring airframe noise in large-scale wind tunnels

A correlation microphone arrangement was used in the test section of the Ames 7- by 10-Foot and 40- by 80-Foot Wind Tunnels in an attempt to reject the reverberant and wind noise. The tests in the 7- by 10-foot wind tunnel covered a frequency range from 400 Hz to 8 kHz while the tests in the 40- by 80-foot tunnel covered the range from 31.5 to 800 Hz. Examination of the experimental data plus calculations of cross-correlations due to reflective noise indicate that the correlation microphone is effective in rejecting microphone wind noise and reverberant noise even at low frequencies, and that the low-frequency background noise monitored by the correlation microphone is reflected noise with a preferred direction from the tunnel drive system. Design studies indicate that this tunnel drive noise can be reduced substantially through suitable modification of the drive fans and acoustical treatment of the nacelles that house these fans.