Doppler Effect or Not?

[kthoennes]

Well - I actually got preoccupied with the pitch of ocean liner horns on the two queen marys, the 1936 edition and the 2004 version. I won't go into details on hertz or piano keys, but I took a video of the old queen Mary horn that is now mounted on the QM2 - and carefully pitched the same - in a 40 knot wind blowing bow to stern, recorded on the stern. That's probably 500 feet from the horn. Then I played the video on Queen Mary in Long Beach when they blew the old (other) horn as they do at regular times each day. I was standing on the bridge deck, painfully close to the horn. Sounded exactly the same to me. Of course at such a low frequency (no longer allowed by modern regs) it makes me wonder if that attenuates the effects of the medium through which the sound passes.

[kthoennes]

Oh, and one other thing. I used to be a commercial diver for a few years, and I know we have some others on here too. The medium definitely does change the character or quality of sound (i know, not technical terms), but from my observation it did not change the pitch. Furthermore, we used to use an ultrasound probe to take readings of pipeline thickness to test for deterioration. We made no adjustments for use of that equipment above or below water.

[Richard_W]

Good question ... can wind (air speed) modify the sound frequency the same way the motion of the source of sound does (Doppler Effect)?

The source of sound or receiver needs to be moving to affect the frequency of sound heard. It is time compression, not medium compression.


The change in sound characteristic due to changes of/in surrounding medium is another matter, I think.

[Andrew Troup]

Here's another thought experiment:

Say the sound source is vibrating at 100 Hz. In one second, one hundred pressure pulses will leave the source.

In a steady breeze, regardles of strength or direction, those pressure pulses must reach our ear at the same rate they left the source. Otherwise, over time, where would the excess or the deficiency be buffered?

This tallies with the persuasive thought experiment in post #14

It's a bit more mindbending thinking about a gusty breeze with my experiment, whereas it seems to me that jongleur's gives a pretty satisfactory explanation as to why there would still be no pitch variation.

[Albro359]

The frequency(pitch) is purely due to the vibrations produced by the emitter...the speed of motion of the medium into which those vibrations are emitted has no effect on the frequency (pitch)

The movement of the source of the sound does not affect its frequency. It is only if the source and the receiver are moving RELATIVE to one another that the receiver hears a change of pitch.

If both the source and receiver are moving at the same velocity, no matter what velocity, the receiver hears the same frequency as the source is emitting.

I'm not looking at this thread anymore...its getting silly !

[s/v Jedi]

Originally Posted by Albro359

The frequency(pitch) is purely due to the vibrations produced by the emitter...the speed of motion of the medium into which those vibrations are emitted has no effect on the frequency (pitch)

The movement of the source of the sound does not affect its frequency. It is only if the source and the receiver are moving RELATIVE to one another that the receiver hears a change of pitch.

If both the source and receiver are moving at the same velocity, no matter what velocity, the receiver hears the same frequency as the source is emitting.

I'm not looking at this thread anymore...its getting silly !

I knew it was gonna be about denying basic scientific principles so I checked myself to not post and am glad I did now This is common on CF.

[Richard_W]

Originally Posted by s/v Jedi

I knew it was gonna be about denying basic scientific principles so I checked myself to not post and am glad I did now This is common on CF.

Really, I would never thought ... ... although, I had a suspicion.

[Paul Elliott]

Originally Posted by Albro359

The answer IS NO !
The definition does preclude that possibility...the moving medium has no effect on pitch.
It is ONLY the relative motion of the source and receiver that produces the Doppler effect.....by definition...that is what the Doppler effect is.

And whatever you make like to "think"..that's it...no argument

OK, how about a "Doppler-like" effect? You know, where the frequency is changed? That was the O.P.'s original question, regardless of whether it was technically the Doppler effect, or something else that resulted in the same perceived phenomenon.

You seem to be hung up on the definition of the term. That's OK, and it is often a good thing to be precise in that way. At times I myself am an annoying precisionist. But in this case, the phenomenon is perhaps more interesting than the particular name we give it.

With source and receiver stationary relative to each other, the speed of the medium has no effect on the frequency of the sound. Neither does any air pressure difference. The speed and density will effect propagation time.

[Smokingator]

Come on guys (or gals). It has nothing to do with your opinion. I told you the Doppler effect is due to the relative movement of the emitter, the receiver, and the medium. Here is the equation.
f=((c+vr)/(c+vs))fo where
f = frequency from the emitter
c = velocity of waves in the medium
vr = velocity of receiver relative to the medium
vs = velocity of source relative to the medium
fo = observed frequency

You can see that movement of air (or water) is absolutely important. If wind is blowing toward you from emitter, the pitch is higher. If emitter is moving away from you, or if you are moving away from emitter, the same speed as wind is moving toward you, pitch is not changed. It's all relative. It does not matter what you think. This is the equation

[Richard_W]

I know ... it's not about Doppler Effect anymore.

Let's assume, if wind generates sound, somehow, the wind sound wave can interfere with the source sound wave ---> wave interference:



The example above shows amplitude change, but I am pretty sure that the interference could affect the wave frequency in more complex scenario ... maybe something like this?

[Paul Elliott]

Originally Posted by Smokingator

Come on guys (or gals). It has nothing to do with your opinion. I told you the Doppler effect is due to the relative movement of the emitter, the receiver, and the medium. Here is the equation.
f=((c+vr)/(c+vs))fo where
f = frequency from the emitter
c = velocity of waves in the medium
vr = velocity of receiver relative to the medium
vs = velocity of source relative to the medium
fo = observed frequency

You can see that movement of air (or water) is absolutely important. If wind is blowing toward you from emitter, the pitch is higher. If emitter is moving away from you, or if you are moving away from emitter, the same speed as wind is moving toward you, pitch is not changed. It's all relative. It does not matter what you think. This is the equation

What I see from the equation is that the velocity of the waves in the medium (c) indeed has an effect on the frequency shift when there is a difference between vr and vs. But, when vr and vs are equal, we get a frequency multiplier of 1 (no change in frequency), regardless of the value of c.

The movement of air is absolutely unimportant when the source and receiver are not in motion relative to each other. This is what I (and many others) have been saying all along.

Have you ever listened to music underwater? The speed of sound in water is over four times faster than the speed in air. Yet, the music is not shifted in pitch (unless you or the source is moving).

[Paul Elliott]

Originally Posted by Richard_W

I know ... it's not about Doppler Effect anymore.

Let's assume, if wind generates sound, somehow, the wind sound wave can interfere with the source sound wave ---> wave interference:



The example above shows amplitude change, but I am pretty sure that the interference could affect the wave frequency in more complex scenario ... maybe something like this?

In this type of additive mixing, there is absolutely no frequency change, regardless of that the waveforms might look like. If there were some nonlinearity involved you would get multiplicative mixing, which would yield sum and difference products, and (depending on the nature of the nonlinearity) various forms of harmonic products. I don't think any of the factors we are considering would create nonlinear mixing, but even if they did this would still not be the type of Doppler-like frequency shift we are discussing.

[markpierce]

What relevance does this discussion have with operating a boat? The important thing is to have a loud horn, and to use it (not a common practice for pleasure boaters in reduced visibility or in maneuvering), and to keep a watchful ear.

[Dockhead]

Here's the answer:

Special Case III: Moving Medium (with Stationary Source and Observer)
Here
′
=
ff
because the observer is at rest relative to the source. The frequency of a wave is
determined purely by the source properties (relative to the observer), quite independent of what
the medium may be doing.
Consider the case where the medium moves from the source toward the observer. In this
case, the wavefronts are being blown to the observer, so that we expect
′
>
υυ
. In fact, the wave
speed is simply increased by the speed of the medium, as we can see by jumping into the
medium’s frame of reference. Thus,
′
=+ = +
υυυ υ υυ
mm
(/)
1
.
Finally, the wavelength must increase by exactly the same factor as the wave speed
increased, in order to ensure that
′
=⇒
′′
=
ff
υλ υλ
//
.
We thus have
′
=+
′
=
′
=+
λλ υυ
υυυ
(/)
1
m
m
ff
MEDIUM MOVING
TOWARD OBSERVER
As a check, imagine a receiver moving along with the medium which intercepts what the source
emits, resulting in a downshifted frequency of
′′
f
, and then immediately rebroadcasts it to the
observer, who receives it with an upshifted frequency of
′
f
. Now
′′
=+
ff
m
/( / )
1
υυ
from case
I, because in the interceptor’s frame of reference the medium is at rest and the source is moving
away from it at speed
υ
m
. But
′
=
′′
+
ff
m
(/)
1
υυ
from case II, since in the interceptor’s frame of
reference the medium is at rest and the observer is moving toward it at at speed
υ
m
. Substituting
the first expression into the second proves that
′
=
ff
so that there is no Doppler shift

http://www.usna.edu/Users/physics/mu...plerEffect.pdf

Sorry the formulae are not copying; you will have to go to the link.

It's a fascinating question. Absolutely not a silly question. When I read it -- my instinct was that no, there's no Doppler shift from a moving medium; it's all determined by relative motion of the source and receiver. But it's not at all obvious. And in fact when you read what real scientists write about it, you see that it is even much less obvious then you might think.

[Richard_W]

Interesting, and simple explanation even without the formulas ... the frequency shifts of the source and the receiver created by moving medium cancel each other.

Right ...