Regarding the perceptual significance and characterization of broadband components of helicopter source noise

This paper attempts to characterize the high-frequency components of in-flight, near-field noise measurements of a Bell 206B helicopter. This noise is shown to be largely broadband, and displays a pattern of modulation that appears to be an audible and salient feature of the noise. The development of a frequency domain technique is given that uses very short (in time) discrete Fourier transforms with windowing and overlap in order to produce a representation of how the 1/3^rd octave band spectrum of the noise varies over time scales shorter than a blade passage. The output of this method can be used to resynthesize the source noise. This resynthesis evidences natural variations that are perceptually similar to those seen in the original recording, and which would not be predicted by contemporary rotorcraft noise analysis techniques. These concepts are then applied in an attempt to synthesize helicopter source noise from a physics-based prediction that includes such modulating high frequency components. The elevated significance of broadband noise on future prediction efforts is discussed.