1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Oct 23, 2014 #1
    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.
     
  2. jcsd
  3. Oct 23, 2014 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    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.
     
  4. Oct 23, 2014 #3

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    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: Oct 23, 2014
  5. Oct 23, 2014 #4

    A.T.

    User Avatar
    Science Advisor
    Gold Member


    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/


    The relativity of motion
    implies symmetry for 2 objects. But here you have 3: source, receiver and medium.
     
  6. Oct 23, 2014 #5

    phinds

    User Avatar
    Gold Member
    2016 Award

    Interesting. Thanks.
     
  7. Oct 24, 2014 #6
    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.
     
  8. Oct 24, 2014 #7

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Not for source at rest, but for medium at rest. If the medium moves, vector addition determines propagation velocities in different directions.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Does the speed of sound depend on the observer's relative motion?
Loading...