Time frequency analysis of sound from a manuevering rotorcraft. Volume Phoenix, AZ, 2013. doi abstract bibtex 1 download A time-frequency analysis is presented in an effort to unveil the spectral properties of the acoustic signatures emitted by a full scale helicopter undergoing steady and roll right maneuvers. The signatures recorded at a single, stationary ground observer is investigated through the use of the wavelet transform. A coordinate transform is developed to relate the observers position to the relative direction of the maneuvering helicopter. Several mother wavelets are quantitatively evaluated and it is shown that the Morlet wavelet, with a non-dimensional frequency between 6 and 7, provides the best decomposition of the helicopter's acoustic signature. Wavelet analysis is successfully employed to understand the effects maneuvering has on the harmonics of the acoustic signal. Higher harmonics are seen to be greatly affected by maneuvering flight and peak when the vehicle is closest to the pertinent microphone. The overall sound pressure levels (OASPL) are also investigated, and it is shown that the two roll rate maneuvers have greater OASPLs then the steady level flyover. Furthermore, it is shown that these levels are greatest at the point of the maximum rate of roll change, and that the medium roll rate was louder then the fast roll maneuver. The pressure signature associated with blade vortex interaction is also investigated. At the point of maximum rate of roll change, the medium rate maneuver is shown to exhibit higher blade vortex interaction sound pressure levels (BVISPL) than the fast roll rate maneuver, and both maneuvers have higher BVISPLs then the steady level flight.
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