Limits of Coherence-Based Aeroacoustic Analysis in the Presence of Distributed Sources

Coherence-based analysis techniques utilizing a small number of microphones are often applied in aeroacoustic measurements.  These techniques can remove statistically incoherent noise, electronic or hydrodynamic, from acoustic signals measured by microphones, at significantly lower cost than array methods.  However, the assumptions involved in the usage of the ordinary coherence function technically limit analysis to a single-source field.  In the presence of multiple sources the coherence function breaks down and ordinary analysis techniques under-predict true acoustic levels.  This phenomenon is demonstrated mathematically and illustrated using experimental trailing edge noise data.