The Mysterious Bloop Sound Heard in the Ocean

Underwater microphones help fill in the picture of what is happening below the surface of our vast ocean, like giving data on migration patterns of marine animals or detecting underwater volcanic activity.

Some hydrophones record for a hours, but others can record sounds for years. And there are lots of different systems to house the hydrophones depending on how long it needs to be deployed and how deep. Some are anchored to the seafloor, some have a buoy on the surface, and others are dragged below a ship.Hydrophones also record sound over vast distances, as sound moves faster and farther in the ocean. Sound waves will move at 1500 meters/sec in water, but can only move about 340 meters/sec in the air. This is because water molecules are more dense than air molecules, allowing the vibrations of sound to quickly pass to the next molecule.

The SOFAR (Sound Fixing And Ranging) channel in the ocean is where sound can travel the furthest. There are different layers of salinity and temperature in the ocean. Saltier, colder water sinks below warmer, less salty water. Ocean sounds created in the SOFAR channel depth, which is between 600-1200 meters, will stay in the SOFAR channel layer and not dissipate into lower or higher ocean depths. This allows the sounds to travel further distances without losing much energy. Whales discovered this channel long before humans clued in, and use it as they migrate to communicate over hundreds and even thousands of miles.