Divers from Brennan Marine installed five transducers on the downstream lock gates on July 28, and they have been operational for the past week. Powered by five 2,000-watt amplifiers, the speakers emit sound frequencies between 500 and 1,500 hertz with a sound pressure level of about 180 decibels -- about the equivalent of 20 outboard motors.

From the surface, the recording sounds like a faint, screechy growl. But underwater, it’s “like fingernails on a chalkboard,” Sorensen said.