Investigation of excess attenuation of noise in the atmosphere

Comparison of static-engine one-third-octave-band spectra projected to flight conditions with flyover-noise spectra has shown that at frequencies greater than blade passage, the static spectra were higher. Analysis of narrow-band spectra measured by microphones mounted inside the inlet and in the fan discharge duct of the CF6-6-powered DC-10-10 airplane under static conditions and in flight suggested that the high-frequency differences were not caused by differences between the fan noise generated in static operation and in flight, but were due to factors not accounted for in projecting the static-engine data to flight conditions. Specifically, the high-frequency differences were due to excess attenuation, which is the attenuation by the atmosphere over and above that obtained by using current procedures for predicting molecular and classical absorption. The results presented indicate that static-projected data should be adjusted for the effects of excess attenuation by using absorption coefficients determined as functions of frequency and altitude.