# Redshift of Sound in a Gravitational Field

1. Jun 20, 2009

### arhanbezbora

Hi,

Do sound waves experience a redshift similar to that of light in a gravitational field?
Does anyone know the equation governing the change in frequency of the sound?
Is this the same as the Doppler shift for an accelerating source of sound?

Thanks.

2. Jun 22, 2009

### bucher

I believe it's just the Doppler shift for sound waves.
I'm not quite sure what you mean by a "gravitational field" though.

3. Jun 22, 2009

### Bob S

You will find that the velocity of sound is proportional to the sqrt(pressure/density).
Check the units to verify: sqrt[(kilogram-meters/sec2)(meters3/kilograms)] = meters/sec.

4. Jun 23, 2009

### Jonathan Scott

If sound waves are emitted at one location (within a medium which will propagate sound, such as air) and received at another where the two locations differ in gravitational potential, then the sound will be apparently shifted in frequency because of the different time rates at the two locations, exactly as for any other form of signal. Exactly the same shift would occur if the sound was transmitted live over a radio link instead (in which case the radio signal frequency would also be shifted).

If the field is of approximately constant strength g and the displacement in the direction of the field is h, then the fractional shift is gh/c2 (so the frequency appears to be increased or blue-shifted when it reaches an observer at a lower potential, and decreased or red-shifted at a higher potential). This effect would obviously be far too small to measure for conventional sound frequencies except in the vicinity of an extremely dense mass.

5. Oct 12, 2011

### sunroof

It is resolved in a paper "Graphene,neutrino mass and oscillation". The gravitational shift df or sound waves in air on the surface of the earth is f.dr/R (f is frequency, dr is height difference and R=6,400km is radius of the earth.) The result is dissimilar to that of light because the gravitational field is a weak field for light and strong for sound waves.