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I wonder how practical it'd be to have an underwater sound emitter to repel one. The use of sonar gets sometimes criticized for its impact on whales. You'd think that you could take advantage of that.

kagis

Probably not powerful enough. Looks like military sonar pulls down a lot of power:

https://www.quora.com/How-do-submarines-surface-ships-produce-such-loud-active-sonar-sound-emissions-from-their-transducers

The first ship I was on used a sonar system from the ‘60s. The system used the maximum amount of power, just short of causing the transducer (an underwater combination speaker and microphone) array to cavitate (boil the water). As you go deeper, it takes more power to cavitate, so submarine sonars were even more powerful (but seldom used, to keep from advertising their location). Our system used 288,000 watts (A powerful home stereo may use 250 watts, so this is like 1000 home stereos all going at the same time!) When the power supply for the amplifiers malfunctioned, it often erupted fireballs across the room (Our Division Officer was so frightened, after seeing one, that he refused to enter the room, or even come down the stairs to the room’s door!). In addition, besides the raw power, the signal can be electronically focused to go in a single direction, much like the powerful spotlights used for advertising (car dealerships, for example). This makes the signal strong enough, that you can bounce it off the bottom of the ocean and detect a submarine more than 40 miles away.

The sound is so loud, that you can hear it IN THE AIR while near a pier, when the ship was over 1,000 feet away (several city blocks). For a nearby diver in the water, it would extremely painful. In Vietnam, the ships in-port would run their sonars 24 hours a day, to keep enemy divers away from the ships.

Inside the ship, you could hear it, no matter where you were below decks, even in noisy places. Most of the crew hated it. Sometimes, we (the sonarmen) would light-off the system, with the most powerful beam pointed at the rest of the ship, at 6:00 AM for Reveille (“Damned %&$ sonarmen! *%#$%^%$!!!”).