In-duct circumferential acoustic mode of a single stage fan was measured using equidistant microphone array with high back ground noise and high rigid wall reflection. Primary circumferential acoustic mode amplitude was obtained by utilizing the cross-correlated(CC) reference channel mode detection technique and the conventional root mean square-averaging (RMS) mode detection technique. The results were compared with Tyler and Sofrins duct mode theory

finding that both CC mode detection technique and RMS mode detection technique can obtain the ideal mode detection results with high back ground noise and high rigid wall reflection

and the mode amplitude of the former was lower than that of the latter. This is because CC mode detection technique decreases the impact of random noise on the mode amplitude. It is also found that the error of mode amplitude from CC mode detection technique is lower than 1dB when the number of microphone in the circular microphone array is more than four times of the circumferential acoustic mode order.