Inacoustics, thesound speed gradient is the rate of change of thespeed of sound with distance, for example with depth in theocean,[1]or height in theEarth's atmosphere. A sound speed gradient leads torefraction of soundwavefronts in the direction of lower sound speed, causing the sound rays to follow a curved path. Theradius of curvature of the sound path is inversely proportional to the gradient.[2]
When the sun warms the Earth's surface, there is a negativetemperature gradient in atmosphere. Thespeed of sound decreases with decreasing temperature, so this also creates a negative sound speed gradient.[3] The sound wave front travels faster near the ground, so the sound isrefracted upward, away from listeners on the ground, creating anacoustic shadow at some distance from the source.[4] The opposite effect happens when the ground is covered with snow, or in the morning over water, when the sound speed gradient is positive. In this case, sound waves can be refracted from the upper levels down to the surface.[3]
Inunderwater acoustics, speed of sounddepends on pressure (hence depth), temperature, and salinity ofseawater, thus leading to vertical speed gradients similar to those that exist in atmospheric acoustics. However, when there is a zero sound speed gradient, values of sound speed have the same "isospeed" in all parts of a given water column (there is no change in sound speed with depth).[1] The same effect happens in anisothermal atmosphere with theideal gas assumption.
