Arnold W. Mueller
NASA---Langley Res. Ctr., MS 460, Hampton, VA 23665
Charles D. Smith
North Carolina State Univ., Raleigh, NC 27695
Acoustic tests were conducted to study far-field noise levels and aural detection ranges associated with a Sikorsky S-76A helicopter flying at various advancing blade tip Mach numbers spanning the range from 0.586 to 0.875. The responses of a sound jury of 12 persons, located along the flight path, were recorded at the moments in time when the aircraft was first heard. Simultaneously recorded with the subjective data were acoustic and aircraft flight operations data. Additionally, ground impedance characteristics were measured at numerous sites along the flight track. This paper presents a comparison between the normalized aural detection results obtained from the prediction code ICHIN (``I Can Hear It Now'') and the sound jury measurements. Predicted detection distances, which compare favorably to measured detection distances, are shown to improve by using measured ground impedances in the propagation module section of ICHIN.