Low‐frequency noise fields

Abstract

Since the classic paper of Wenz [J. Acoust. Soc. Am. 34, 1936 (1962)], ambient noise has been an extensively studied phenomena. Morris [SIO Ref. 75‐34, MPL/Scripps (1975)] emphasized the importance of ships as their signals are enhanced when they cross over seamounts or proceed over the continential slopes. Wagstaff [J. Acoust. Soc. Am. 69, 1009 (1981)] showed by the comparison of measurements and calculations that coastal shipping (ships over the continental slope and on the shelf near the slope) must be considered in order that the horizontal directionality be correctly described. He showed that these coastal sources would also affect the vertical directionality. This paper reinforces these findings with new results from down‐slope transmission loss (TL), coherence, and noise directionality experiments. High‐resolution noise measurements show long‐term persistent directional characteristics associated with distant shipping lanes and density patterns. Short‐term averages show a temporally dynamic field composed of resolved distant shipping and uncorrelated background noise. These results emphasize the importance of the coherent contribution from shipping to the mid‐ocean noise field. Vertical directionality measurements by Anderson et al. (1972) show a broad angular distribution of noise intensity near the horizontal at low frequencies and a peaked distribution about the horizontal at high frequencies. This broad angular distribution near the horizontal was found smooth and indicates that in addition to surface ships, environmental noise influences the vertical directionality.