Sound speed

Sound speed is the rate at which small pressure disturbances propagate through a material. It depends on the material’s elastic properties, density, and, in gases, temperature.

Bulk modulus: A measure of how resistant a substance is to uniform compression. Greater resistance to compression generally lets pressure disturbances travel faster.
Ultrasonic testing: A method that uses high-frequency sound waves to inspect materials for internal flaws. Travel-time measurements use sound speed to locate defects inside solids.
Group velocity: The speed at which a wave packet’s envelope and associated energy propagate. It can differ from phase velocity when sound is dispersive.
Mechanical wave: A disturbance that transports energy through matter without transporting matter overall. Sound speed is one example of a propagation speed for mechanical waves.
Density: Mass per unit volume of a substance. For a given stiffness, greater density lowers the speed of sound.
Time-of-flight sonar: A method that estimates distance from the travel time of an emitted and returning sound pulse. Distance estimates require the sound speed in the surrounding water.
Phase velocity: The speed at which a fixed phase of a periodic wave travels. It describes individual wave crests, not necessarily a sound pulse’s overall travel.
Longitudinal wave: A wave in which particles oscillate parallel to the direction of propagation. Sound in fluids travels through successive compressions and expansions.
Equation of state: A relation connecting the pressure, volume, temperature, and other state variables of a material. Its pressure response determines the effective stiffness that sets sound speed.
Seismic wave: A wave of energy that travels through Earth or along its surface after a disturbance. Seismic speeds reveal properties and structure within Earth.

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