Observations of acoustic surface waves in outdoor sound propagation

Acoustic surface waves have been detected propagating outdoors under natural conditions. Two critical experimental conditions were employed to ensure the conclusive detection of these waves. First, acoustic pulses rather than a continuous wave source allowed an examination of the waveform shape and avoided the masking of wave arrivals. Second, a snow cover provided favorable ground impedance conditions for surface waves to exist. The acoustic pulses were generated by blank pistol shots fired 1 m above the snow. The resultant waveforms were measured using a vertical array of six microphones located 60 m away from the source at heights between 0.1 and 4.75 m. A strong, low frequency "tail" following the initial arrival was recorded near the snow surface. This tail, and its exponential decay with height (z) above the surface (approximately e(-alpha z)), are diagnostic features of surface waves. The measured attenuation coefficient alpha was 0.28 m(-1). The identification of the surface wave is confirmed by comparing the measured waveforms with waveforms predicted by the theoretical evaluation of the explicit surface wave pole term using residue theory.