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.
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.
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).
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.
The following are the three bold statements that I make.
Out of the speeds of the source and the observer/receiver, the wavelength of a sound depends only upon the speed of the source.
Out of the speeds of the source and the observer/receiver, the frequency of a sound depends upon the speeds of both of them.
Out of the speeds of the source and the observer/receiver, the "apparent speed" of a sound (i.e. speed wrt a observer) depends only upon the speed of the observer/receiver.
Here is perhaps one reason why the speed of sound, the speed of light is not added to the speed of the source that emits it. While you add the speed of a rock to the speed of the source.
You can only apply force to an object that has mass. You can not apply force on something with mass of 0. Imagine pushing again space.
Sound has no mass, it does have momentum... but that is not mass.
Light has no mass. Light has momentum.
When you throw a rock. The rock resists you because it has mass. If it had no mass you are applying force on nothing so to speak.
That is what it means when a photon does not respond or is affected by the sources force. The source can not throw a photon like it is a rock. You can put a rock in a sling shot, if you put a photon in a slingshot, it is the same as putting nothing in the sling shot.
It is because they have no mass. Anything that does not have mass, its speed can not be added to the source that eminates it.
