Abstract:
This paper attempts to characterize the high-frequency components of in-flight, near-field noise measurements of a Bell206B helicopter. This noise is shown to be largely broadband, and displays a pattern of modulation that appears to bean audible and salient feature of the noise. The development of a frequency domain technique is given that uses veryshort (in time) discrete Fourier transforms with windowing and overlap in order to produce a representation of how the1/3rd octave band spectrum of the noise varies over time scales shorter than a blade passage. The output of this methodcan be used to resynthesize the source noise. This resynthesis evidences natural variations that are perceptually similarto those seen in the original recording, and which would not be predicted by contemporary rotorcraft noise analysistechniques. These concepts are then applied in an attempt to synthesize helicopter source noise from a physics-basedprediction that includes such modulating high frequency components. The elevated significance of broadband noiseon future prediction efforts is discussed.