The speed of sound increases with temperature up to 74 degrees Celsius

Scientific Explanation

In most liquids, the speed of sound decreases with rising temperature because the molecules move farther apart and the medium becomes more compressible. Water reverses this trend: the speed of sound increases from about 1400 meters per second at 0 degrees Celsius to a maximum of roughly 1555 meters per second at 74 degrees Celsius.

This anomalous behavior is directly connected to the decreasing compressibility (anomaly 28). The speed of sound c in a fluid is given by c = 1 / square root of (density times compressibility). Since water’s compressibility falls until 46.5 degrees Celsius and the density decreases only slowly over the same range, the speed of sound rises. Even above 46.5 degrees, the net effect remains positive until 74 degrees Celsius, when the increasing thermal expansion finally dominates.

Above 74 degrees Celsius, the speed of sound falls just as it would in a normal liquid. The maximum at 74 degrees marks the tipping point where normal thermal effects finally overcome the structure-driven anomalies.

Everyday Relevance

For underwater communication and sonar technology, this anomaly is critical. In the oceans, temperature and pressure layering creates so-called sound channels (SOFAR channels) in which sound waves can propagate over thousands of kilometers. Whales use these channels for long-distance communication, and submarines and marine researchers depend on understanding the temperature-dependent speed of sound.