I googled the speed of sound and found that it only depends on the medium (just like the speed of light but with different parameters). I can't see how it doesn't pick up the speed of its source! I mean for the constancy of the speed of light the hole addition rule of velocities was modified to the relativistic one. So how to maintain the constancy of the speed of sound?

p.s. I'm so grateful to every single one answered this question cause she/he truly induced me to understand the subject.


Speed of sound in air is the speed with which a pressure wave propagates. If you look at the wave itself, without considering its source, you have all the information you need to determine the speed of propagation - since it's only the local conditions (things like the density, and the local rate of change of pressure) that determine this.

The same is true with light: if you "look at a photon passing you", you don't need to know anything about the source - once the signal (sound or light) has left its source, it loses all memory of that, and just becomes a wave that propagates.

Now the laws of addition of velocities that we use in relativity are related to the postulate that the speed of light will be the same in any inertial frame of reference; the Lorentz transformation follows directly. But there is no equivalent claim for sound. The fact that supersonic travel exists pretty much proves that. One a more basic level, if you are standing downwind from a sound source, that sound will reach you more quickly than if you were standing the same distance upwind - because the pressure disturbance travels with the bulk of the medium.

It follows that the speed of sound depends on the observer's velocity relative to the medium.

  • $\begingroup$ So I was just wrong saying it doesn't pick the speed of the source, right? $\endgroup$
    – maha
    Feb 16 '16 at 17:48
  • 1
    $\begingroup$ Yes, sound does not "pick up the speed of the source". However, if a source of a certain frequency is moving towards you, the frequency you will observe is different than the frequency of the source. In that way, there is an interplay between observed frequency and source velocity - called the Doppler effect. But I don't think that is what you were asking about. See also this earlier answer or this one $\endgroup$
    – Floris
    Feb 16 '16 at 17:55
  • $\begingroup$ Well.. I shall check the links you sent me, thanks. But firstly I want to make these two statements: 1. The speed of sound would change if the observer is moving, 2. The speed of sound doesn't change if the source is moving. I kind of find this a little bit confusing :\ $\endgroup$
    – maha
    Feb 16 '16 at 18:12
  • $\begingroup$ No - the speed of sound is constant relative to the medium. It is not measured relative to the observer, or the source. Although you can measure the time it takes sound to get from A to B, that time doesn't depend on whether A or B is moving - assuming that you know the position of A when the sound is transmitted, and the position of B when it is received. However, you still need to know whether the medium is stationary in the frame in which you determined the position of A and B. $\endgroup$
    – Floris
    Feb 16 '16 at 19:13
  • $\begingroup$ I just would disagree with the example; since if B is moving, then we can assume that the medium is moving relative to B. I think I now understand, thanks :) $\endgroup$
    – maha
    Feb 16 '16 at 19:38

When you are quoted a value for the speed of sound it is implicit that it is the speed of sound relative to the medium which in air is about 330 m/s.

The significant difference with electromagnetic waves is the necessity for a medium for a sound wave to exist.
It means that if there is a wind of speed 30 m/s (relative to the ground) blowing from the source towards you (standing still on the ground) wounld measure the speed of sound as 330 + 30 = 360 m/s. You would get the same value for the speed of sound if the source of sound was still (not moving relative to the ground) and you were moving towards the source at 30 m/s.

In answer to you question, when the sound wave is in the air it will travel at 330 m/s and that speed has nothing to do with the speed of the source.

However a moving source will have an effect on the sound wave.
If source and you are not moving you can imagine the sound wave as a succession of compressions and rarefactions with the separation of the compression being the wavelength of the sound wave.
If the source now start moving towards you it will send out a compression and then before it sends out the next compression the source would have moved towards you.
This means that the distance between compression (wavelength) decreases and the frequency of the sound wave increases.
This is an example of the Doppler effect.

  • $\begingroup$ I understood from the first paragraphs that you agree that the sound would pick the velocity of the source. However I find " when the sound wave is in the air it will travel at 330 m/s and that speed has nothing to do with the speed of the source." a little bit contradictory since it shall be this speed w.r.t the source for example. $\endgroup$
    – maha
    Feb 16 '16 at 18:01
  • $\begingroup$ I know also that there will be an effect resulting from the moving source on the acoustic wave but I'm not sure if this has something to do with the its speed. $\endgroup$
    – maha
    Feb 16 '16 at 18:03

The sound wave is generated due to the vibration of the source. The source movement pushes/pulls the medium creating pressure vibrations on the medium. Once there, they propagate according to the medium's physical properties, in this case, the speed of sound.

What happens when the source (or the listener) is moving is that the frequency at which the waves are generated or received change. This is known as the Doppler effect (think F1 cars passing by you).

  • $\begingroup$ By the way, that's why breaking the sound barrier causes the sonic boom. If the source is "generating more sound" in the location where it's previous wave is, the waves add up creating the "boom". $\endgroup$ Feb 16 '16 at 18:24
  • $\begingroup$ I want to get this fully through these questions: Does the vibrations (which generates the sound waves) will be affected by the speed of the source ? Or is it the medium that according to the source movement will be affected (at that instant) which will cause a difference in the response to the vibrations? – maha 41 mins ago $\endgroup$
    – maha
    Feb 16 '16 at 19:15
  • $\begingroup$ Sound waves are actually pressure waves propagating through a medium. Vibration is nothing but movement of the source in a sinusoidal manner. When the source moves, it pushes the medium it is immersed in (usually air). If not on vacuum, the "pushing" of the medium propagates as pressure waves according to the medium's physical properties (speed of sound in the medium). If those waves are in hearing range (from 300 to ~22000Hz) we call them sound. $\endgroup$ Feb 18 '16 at 16:54

Once the sound leavs the source, it has to travel through the medium. Basically sound source is nothing but some sort of vibrations. Whether the vibration happens while the source is moving, or not, does not depend at what speed the sound is going to travel through air. However, the pitch of sound will change depending upon which direction you listen in. Speed of sound is not a constant in context of relativity. Anyway, it is too slow for those effects.

  • $\begingroup$ Well..I know that sound is some sort of vibration in the medium yet to have some speed.I can't see how it wouldn't (or would actually) pick the speed of the source. It's like I'm asking about the mechanism of calculating "the speed of sound" in general.And I don't think I agree with you about the relativity thing;since if its not constant according to relativity, then you implicitly admitted the relativistic law of velocity addition which means you agree that the non-relativistic velocity addition rule is applied in this case which will make the speed of sound picks up the speed of its source. $\endgroup$
    – maha
    Feb 16 '16 at 17:32
  • $\begingroup$ The way that air molecules communicate is via collisions. The only way that you can make the communication speed faster is to make the air molecules move faster. The source might well do this but it would not be an affect which extends very far in front of the source because the collisions between the faster air molecules and slower air molecules would distribute this extra speed (kinetic energy) amongst a lot more molecules. $\endgroup$
    – Farcher
    Feb 16 '16 at 18:11
  • $\begingroup$ @Farcher again I think this is against that the sound picks up the speed of the source, lol! Rather you were clear from your answer that it will change :) $\endgroup$
    – maha
    Feb 16 '16 at 18:24
  • $\begingroup$ @kpv many thanks dude; you're answer directed me to the way. But I do disagree, until discussed, with that the speed of sound will be changed due to relativity. $\endgroup$
    – maha
    Feb 17 '16 at 12:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.