Accurate knowledge of trends in ocean ambient noise is important for predicting future ambient noise levels. Concern about the level of ambient noise at low frequencies and its impact on marine mammals that arose with the Heard Island Feasibility test and the subsequent Acoustic Thermometry of Ocean Climate project, both conducted in the 1990's, continues today. Ocean noise generated at low frequencies is dominated by noise from ships, and the number and size of world shipping vessels has increased significantly since the first documentation of noise levels in the 1950s. This paper addresses the issue of the trend in low frequency ambient noise levels in the ocean over the past 50 years, and presents measurements from sites in the North Pacific Ocean that were taken in the intermediate years between the mid-1960s and the present time.
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