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

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Discussion Overview

The discussion centers around whether the speed of sound is influenced by the relative motion of the observer with respect to a fixed medium. Participants explore the implications of relative motion on sound propagation, particularly in the context of the Doppler effect and the nature of sound waves in different reference frames.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that the speed of sound is fixed relative to the medium and does not depend on the observer's speed, while others argue that it does depend on the observer's motion relative to the medium.
  • One participant notes that the Doppler effect affects frequency but not the speed of sound itself, questioning the nature of sound propagation in relation to relative motion.
  • Another participant emphasizes that sound behaves differently than light, suggesting that sound waves add or subtract speeds "normally" based on relative motion.
  • Some participants highlight that the propagation speed of sound can be anisotropic depending on the observer's frame of reference and the motion of the medium.
  • There is a discussion about the need for a formula that incorporates the speeds of both the source and observer, with one participant noting that the standard definition of sound speed assumes a stationary medium.
  • Participants clarify that the speed of sound is defined for a medium at rest, and that vector addition is necessary when considering the motion of the medium.

Areas of Agreement / Disagreement

Participants express differing views on whether the speed of sound is affected by the observer's motion relative to the medium. There is no consensus, as some argue for its dependence on relative motion while others maintain it is independent.

Contextual Notes

The discussion reveals limitations in the assumptions about the medium's state and the conditions under which sound speed is defined. The implications of anisotropic propagation and the need for specific formulas are also noted but remain unresolved.

GenlyAi
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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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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.
 
GenlyAi said:
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:
phinds said:
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

Also see this thread:
https://www.physicsforums.com/threads/sound-doppler-shift-asymmetry-confusion.761291/

phinds said:
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.
 
Thank you for your answers. So , if the speed of sound is relative, can you provide a formula or equation showing its dependence on the speed of the source and/or of the observer? I wasn't able to find it on the Internet. In other words, the usual definition ## c = \sqrt{\frac{K}{\rho}} ## doesn't include the speed of the source or observer or both. It's like as if it is a definition for the speed of sound for a source at rest, but this is never specified. It would be like, for example, omitting the dependence on the acceleration when giving the general equation of motion for x: ## x = x_0 + vt + \frac{1}{2}at ##. That would be a special case where a = 0.
 
GenlyAi said:
It's like as if it is a definition for the speed of sound for a source at rest,
Not for source at rest, but for medium at rest. If the medium moves, vector addition determines propagation velocities in different directions.
 

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