How Far Does Sound Travel in the Ocean? It Depends on Temperature and Pressure

Sound moves much faster in water than in air. How far it travels, though, depends mainly on two things: the temperature of the water and its pressure. And those two work against each other in a way that creates a natural highway for sound.

Water temperature and pressure determine how far sound travels in the ocean. Credit: NOAA.

Two forces in tension

Pressure keeps rising the deeper you go. Temperature falls with depth only up to a point, and below that it stays fairly constant. Sound speed responds to both — and that has a curious effect.

Follow a whale's call

Imagine a whale calling to its pod. Its call spreads through the water like ripples.

1. Bending down. As the sound goes deeper and the water gets colder, the sound slows, so the waves bend, or refract, downward.
2. The slowest point. At the bottom of the thermocline — a layer where temperature and pressure change quickly, found at different depths around the world — the speed of sound reaches its minimum.
3. Bending up. Below the thermocline, temperature holds steady but pressure keeps rising, so sound speeds up again and the waves refract upward.

The sound channel

The zone where sound waves bend up and down like this is called the sound channel. Trapped in it, sound can travel thousands of miles without losing much energy. Hydrophones — underwater microphones — placed at the right depth can pick up whale songs and human-made noise from many kilometers away. (The channel's discovery is told in SOFAR.)

Sources

Based on "How far does sound travel in the ocean?" from NOAA's National Ocean Service, a work of the United States government in the public domain.