Sound speed gradient

In acoustics, the sound speed gradient is the rate of change of the speed of sound with distance, for example with depth in the ocean,cite book
title=Navy Supplement to the DOD Dictionary of Military and Associated Terms
month=August
year=2006
publisher=Department Of The Navy
url=http://www.nwdc.navy.mil/library/documents/1-02_(Aug_2006)_(NTRP).pdf
id=NTRP 1-02] or height in the Earth's atmosphere. A sound speed gradient leads to refraction of sound wavefronts in the direction of lower sound speed, causing the sound rays to follow a curved path. The radius of curvature of the sound path is inversely proportional to the gradient. [cite book
title=Noise Control
chapter=10. Outdoor sound propagation
series=ME 458: Engineering Noise Control
year=2000
first=J. S.
last=Lamancusa
url=http://www.mne.psu.edu/lamancusa/me458/10_osp.pdf
publisher=Penn State University
city=State College, PA
format=pdf
pages=pp. 10.6-10.7]

When the sun warms the Earth's surface, there is a negative temperature gradient in atmosphere. The speed of sound decreases with decreasing temperature, so this also creates a negative sound speed gradient.cite book
title=Sound Reinforcement Engineering
first=Wolfgang |last=Ahnert
coauthors=Steffen, Frank
publisher=Taylor and Francis
place = London
year=1999
pages=p. 40
isbn= 0415238706] The sound wave front travels faster near the ground, so the sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. [cite book
last = Everest | first = F.
title = The Master Handbook of Acoustics
publisher = McGraw-Hill
location = New York
year = 2001
isbn = 0071360972
pages = pp. 262-263 ] 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.

In underwater acoustics, speed of sound depends on pressure (hence depth), temperature, and salinity of seawater, 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). The same effect happens in an isothermal atmosphere with the ideal gas assumption.

References

ee also

* SOFAR channel
* Wind gradient