From a recent speech by RADM Richard A. Riddell (Nautilus's last CO, who I served under):
"In the beginning of my three and a half years in command of Nautilus, we were involved with the Mobile Acoustic Communications System, or MACS, which was a research project to obtain fundamental data on long range propagation of sound. This project required a huge antenna on the after deck of the submarine. The antenna would transmit an acoustic signal to a receiving submarine or surface ship well over a hundred miles away, with multiple simultaneous signal frequencies, various antenna angles, different ocean bottom types, and a variety of weather conditions.
"My most vivid memory of our time with MACS was the ability of the human body to adapt to the ear-splitting acoustic signals that were transmitted every few seconds. This went on for days and weeks at a time. Somehow, we learned to live with this noise—to sleep, watch movies, and even play poker on Saturday nights."
I can confirm from personal experience the extreme tediousness of listening to this pinging, day after day, night after night.
