The ATOC (later the North Pacific Acoustic Laboratory [NPAL]) project was initiated in the early 1990s to
study ocean warming, and received much attention from regulatory agencies, the public, and the scientific
community due to concerns regarding the potential impact of its sound source on marine mammals
(Baggeroer et al. 1998). This program was extensively discussed in two National Research Council reports
(NRC 1994, 2000). The ATOC source has a 195 dB re 1µPa @ 1 m level and is deployed at 939 m, near
the axis of the deep sound channel (Howe 1996). It is designed to study the entire North Pacific basin, with
the sounds being received by the U.S. Navy’s fixed hydrophone arrays. The transmitted signal is centered
at 75 Hz with a bandwidth of 37.5 Hz. It broadcasts on 4-hour intervals with a “ramp-up” period of 5
minutes and a full power signal duration of 20 minutes. The long time frame for operation of this
experiment was a key aspect that led to questions regarding its potential impacts on marine mammals
(Potter 1994)