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Mook Johnson
science forum beginner

Joined: 21 Jul 2006
Posts: 1

Posted: Fri Jul 21, 2006 11:46 am    Post subject: sound waves in wind (doppler like shift?)

We all know the classic doppler effect of when there is relative motion
between the acoustic source and a observer.

If an observer is standing upwind of an acoustic source would there be a
small but measureable change in the pitch of the sound due the the fact that
the medium that the sound wave is moving through is also moving?

I'm visualizing a fixed observer and speaker in a wind tunnel.

The speaker is a piston creating say 1KHz continuous tone(1 compression wave
per mS). The air speed is 0.5 the speed of sound (ignore winds noises). When
the comression wave is made by the speaker, it moves through the air, upwind
and downwind, at the speed of sound. since the mass of air is moving
relative to the observer upwind, the the compression wave will only extend
towards the observer 1/2 the distance it normally wound in still air before
the next compression wave is created by the speaker.

This continues to happen until the wave finally reaches and the comression
waves are spaced more closely together which is an increase in pitch.

The opposite happens downwind of the speaker.

Is my thinking valid?
N:dlzc D:aol T:com (dlzc)
science forum Guru

Joined: 25 Mar 2005
Posts: 2835

Posted: Fri Jul 21, 2006 1:07 pm    Post subject: Re: sound waves in wind (doppler like shift?)

Dear Mook Johnson:

"Mook Johnson" <mook@mook.net> wrote in message
 Quote: We all know the classic doppler effect of when there is relative motion between the acoustic source and a observer. If an observer is standing upwind of an acoustic source would there be a small but measureable change in the pitch of the sound due the the fact that the medium that the sound wave is moving through is also moving?

The pitch is written into the moving medium at one Doppler value
(Vsource - Vmedium), and received at another (Vmedium -
Vreceiver). Mostly what changes in what you imagine is arrival
time of the sound.

 Quote: I'm visualizing a fixed observer and speaker in a wind tunnel. .... Is my thinking valid?

No, but it will be good for an experiment. You went from "also
moving" to "only fluid moving".

David A. Smith
Gordon
science forum Guru Wannabe

Joined: 12 May 2005
Posts: 115

Posted: Fri Jul 21, 2006 1:16 pm    Post subject: Re: sound waves in wind (doppler like shift?)

On Fri, 21 Jul 2006 11:46:06 GMT, "Mook Johnson" <mook@mook.net>
wrote:

 Quote: We all know the classic doppler effect of when there is relative motion between the acoustic source and a observer. If an observer is standing upwind of an acoustic source would there be a small but measureable change in the pitch of the sound due the the fact that the medium that the sound wave is moving through is also moving? I'm visualizing a fixed observer and speaker in a wind tunnel. The speaker is a piston creating say 1KHz continuous tone(1 compression wave per mS). The air speed is 0.5 the speed of sound (ignore winds noises). When the comression wave is made by the speaker, it moves through the air, upwind and downwind, at the speed of sound. since the mass of air is moving relative to the observer upwind, the the compression wave will only extend towards the observer 1/2 the distance it normally wound in still air before the next compression wave is created by the speaker. This continues to happen until the wave finally reaches and the comression waves are spaced more closely together which is an increase in pitch. The opposite happens downwind of the speaker. Is my thinking valid? Mook, there would be a change in transfer time, but no Doppler

frequency shift. Since both the source and the observer are
stationary, relative to each other, there will be no Doppler
shift in frequency. But, sound moving "with the wind" will take
less time, and sound moving "against the wind" will take more
time to travel from the source to the observer. Doppler shift can
occur only when the path length is changing.

Gordon

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