Experimental investigation of outdoor propagation of finite-amplitude noise

Propagation measurements of intense noise were made outdoors. A ground-mounted electroacoustic source transmitted broadband, octave band, or 13-octave band noise in the frequency range 2–10 kHz. Band levels (100-Hz bands) at the peak of the source spectrum were as high as 133 dB (re 20 μ Pa at 1 m). A microphone mounted on a boom extended from the elevator of an 85-m tall radio tower was used to measure power spectra as a function of propagation distance. Nonlinear propagation distortion caused substantial growth with distance of the high-frequency end of the noise spectrum, even at low source levels. For example, when the source band level peaked at 113 dB at 4 kHz, at a distance of 77 m the band at 20 kHz was 23 dB higher than would be predicted using linear theory. Preliminary calculations based on nonlinear acoustical theory show that observed spectral distortion is in line with that predicted for a homogeneous medium, even though winds with ground speeds as high as 25 km/h were present. [Work supported by NASA, ONR, AFOSR, and NOAA.]