To summarize, according to this study the best microphone
configuration is obtained by selecting the distances s between
the three measurement cross-sections using 0 :1p < ks < 0 :8p
[22] and using angular twist of p =4 in the spiral microphone configuration. Using the proposed procedure the in-duct sound power
beyond the plane wave range, can be estimated with an average
standard deviation of approximately 3 dB in each 1/3 octave band