# The Naked Scientists Forum

### Author Topic: Does the speed of sound depend on the speed of the emitting source?  (Read 5686 times)

#### flr

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##### Does the speed of sound depend on the speed of the emitting source?
« on: 06/06/2012 08:44:10 »
The speed of sound does not depend on the speed of the source, is that actually right?

For example: suppose that B and C moving at different speed relative to frame A.
The speed of sound measured from the frame A does not depend on the velocity of B and C relative to A. Am i right ? I think yes...
« Last Edit: 06/06/2012 08:47:51 by chris »

#### chris

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##### Re: Does the speed of sound depend on the speed of the emitting source?
« Reply #1 on: 06/06/2012 09:07:00 »
Sound - which is a compression wave - travels at a specific speed in a given medium; as a general rule, stiffer materials convey sounds more rapidly than floppier media.

So in air, for example, sound travels at about 340 ms-1; in water, which is a bit stiffer, it's 1430 ms-1 and in steel, which is stiffer still, the waves propagate at over 6000 ms-1.

This is a simplification, because "sounds" can consist of many different frequencies and, under certain circumstances, some of these will actually travel in air at slightly different speeds.

But the bottom line is that the measured speed of sound in a substance is related to the stiffness and density of the substance it is moving through and not the velocity of the object producing the sound.

So Concorde flying at twice the speed of sound does not produce sound waves that move through air at twice the speed of sound. Once the waves are initiated in the air they are no longer connected to the aircraft (the emitting source) and hence travel at the speed of sound in air predicted by the Newton-LaPlace equation.

Chris

#### CliffordK

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##### Re: Does the speed of sound depend on the speed of the emitting source?
« Reply #2 on: 06/06/2012 09:08:16 »
The speed of sound is dependent on the medium that it is travelling through.  In air, the speed of sound will be dependent on the density of the air, and varies with altitude.  It also would vary with the wind.

The frequency of the sound will be dependent on the speed of the source due to doppler shift.

If you are inside of a jet travelling at Mach 0.9, the air within the cabin is all moving at the same speed as the jet, and thus the speed of the sound inside the jet is unaffected by the speed of the aircraft.

#### syhprum

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##### Re: Does the speed of sound depend on the speed of the emitting source?
« Reply #3 on: 06/06/2012 14:08:58 »
Would that electromagnetic radiation would behave in a nice simple way like this.

#### flr

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##### Re: Does the speed of sound depend on the speed of the emitting source?
« Reply #4 on: 06/06/2012 19:54:03 »
Just like the speed of sound does not depend on the the velocity of the emitting source [as it is a property of the environment (i.e. air)], I can easily picture how the speed of electromagnetic wave (i.e. or photon) does not depend on the speed of the emitting source, for the sitting observer "A".

The issue is when we add an additional observer "X" moving relative to "A" with a certain speed:  the observer "X" will disagree with the observer "A" what the speed of sound is, and the observer "X" will see a speed of the sound equal with the speed of sound from frame "A" minus the speed of "X" relative to "A".

If they are to observe an electromagnetic wave, both "X" and "A" will observe the same speed of the light.

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I used to think at the invariance of "c" as a result of the inherent property of the vacuum, from the relation:
c^2 = 1/(epsilon_0*miu_0), hence an observer "A" will see two travelers emitting light at the same speed (equal to c), regardless of the speed of the travelers.
Just like in the analogy with the sound wave: the speed of sound does not depend on the velocity of the emitting source for the observer "A" (because it is a inherent property of how air can vibrate).

However, there is a difference: another observer "X" will measure from its frame a different speed of sound, but exactly the same speed of light as the observer "A".

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The limiting speed as "inherent property of the propagating environment" does not ensure invariance of the speed of sound wave for any observer.

Similarly, I can understand the electromagnetic wave speed as a "inherent property of something (vacuum?)"  but that should only ensure constant c only for the observer "A" no matter what the speed of emitting source is.
What makes "c" constant for any other observer "X"?

#### Bored chemist

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##### Re: Does the speed of sound depend on the speed of the emitting source?
« Reply #5 on: 06/06/2012 20:16:33 »
An answer, in the terms you are using, would be to say that all sources and observers are at rest with respect to any vacuum.
How can you say how fast you are moving compared to something that's just "nothing"?

#### flr

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##### Re: Does the speed of sound depend on the speed of the emitting source?
« Reply #6 on: 06/06/2012 20:40:59 »
An answer, in the terms you are using, would be to say that all sources and observers are at rest with respect to any vacuum.

OK, indeed, my formulation of sitting observer "A" should be replaced by observer "A".
If I say "sitting" I introduced another frame of reference relative to what "A" is sitting, and we don't need this additional frame as it is redundant to "A".

With this change, probably  I no longer have anything that (implicitly?) could be assumed as "at rest with respect to vacuum".

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##### Re: Does the speed of sound depend on the speed of the emitting source?
« Reply #6 on: 06/06/2012 20:40:59 »