# Does the speed of sound depend on the observer's relative motion?

## Main Question or Discussion Point

Hi,
I wanted to ask if the speed of a sound wave, given a fixed medium, depends on the speed of the observer. That is to say, does the speed of sound obey to the laws of relative motion, implying addition of relative speeds? In case it doesn't depend on relative motion, can you explain why? I know that the Doppler effect accounts for the speed of the observer, but it only affects the frequency of the wave, not its speed. Why is it so (in case it actually is)?
Thanks.

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phinds
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Yes it does. Unlike light, sound adds/subtracts "normally". The "Doppler Effect" applies equally well to both source and observer. There would be no difference between a tuning fork on a moving train heard by an observer at the station vs the same tuning fork on the station heard by someone on the train. The situations are symmetric because motion is relative.

A.T.
Hi,I wanted to ask if the speed of a sound wave, given a fixed medium, depends on the speed of the observer.
The speed is fixed relative to the medium, so it depends on the velocity of the observer relative to the medium, and the propagation direction. in frames where the medium moves, the propagation speed is anisotropic.

Last edited:
A.T.
There would be no difference between a tuning fork on a moving train heard by an observer at the station vs the same tuning fork on the station heard by someone on the train.

Wrong. There would a difference in the heard frequency. See the two
different formulas for movement of the source vs. movement of the receiver relative to the medium:
http://en.wikipedia.org/wiki/Doppler_effect#Analysis

The situations are symmetric because motion is relative.
The relativity of motion
implies symmetry for 2 objects. But here you have 3: source, receiver and medium.

phinds