Spectral Characteristics of Acoustic Noise Generated by Turbulent Flow in the Free Atmosphere

This paper presents aeroacoustic noise spectra recorded at the top of Mount Washington Meteorological Observatory in New Hampshire, USA in Spring 2000. (The highest measured wind velocity on the surface of the Earth, 234 mph, was recorded here in 1934). The data is analyzed with reference to Lighthill's papers "On Sound Generated Aerodynamically" [Parts I and II, 1952 and1954]. In the 1950s, aeroacoustic experiments were performed in the laboratory using jets. The current data appears to be the first measurements of aeroacoustic noise generated by turbulence in the free atmosphere. The experimental spectra show a high amplitude near zero frequency that decreases rapidly with frequency. This has been observed in the past and is due to microphone-wind interaction. However there is also a distinct high frequency portion that is fairly small at intermediate frequencies that rises rapidly at a rate approaching frequency cubed to above 10kHz. The paper will examine this behavior with reference to Lighthill's papers and Kolmogorov's theory of turbulence.