Science: Davy Jones’s Sound Effects

Early in 1942, the Army fitted some of its coastal minefields with underwater microphones. Its purpose: to listen for enemy craft, and blow them sky-high by exploding appropriate mines. For a while the minefields were quiet. Then, with spring, the microphones under an empty sea picked up an “awful racket.” To some it sounded like a pneumatic drill, to others like laden freighters coming up the channel.

Baffled, the Army called in the Navy, and the Navy, which had noise trouble of its own, appealed to Biologist Charles H. Blake of M.I.T. What subsea gremlin, they asked, was making the uproar? “Fish,” said Professor Blake.

So fish make noises under water? Well, said the Navy, that’s interesting: perhaps some are noisier than others. To find out, the Navy made a “sonic survey” of underwater noisemakers. Some findings:

Noisiest U.S. fish are the croakers, which cruise in racketing schools. They have special muscles to make their swim-bladders resonate. The hogfish grinds its teeth. The spot gives a raucous honk. The sea-catfish makes a noise like a tom-tom. The searobin cackles, while the toadfish toots a musical warning to leave its eggs alone.

Ultrasonic Shrimp. As” one means of detecting enemy submarines, the Navy used sonar, a device which bounced off them beams of sound waves too short for reception by the human ear. But in subtropical waters a kind of shrimp interfered: the snapping of their claws made these same “ultrasonic” sounds. Enemy submarines, the Navy feared, might hide behind this interference. As it turned out, “ultrasonic” shrimp did not exist where German U-boats most commonly cruised; but the)’ did live in the waters off Japan, and U.S. submarines hid from listening Japanese behind the noise of the shrimps’s snapping claws.

Japanese croakers were not as helpful. The U.S. laid acoustic mines in Tokyo Bay, and some of them, according to Japanese officers, exploded mysteriously without damage. The Navy’s suspicion: Kamikaze croakers thus died for their Emperor.