Doppler Effect or Not?

[s/v Jedi]

Sooooo, when I speak into the wind, towards somebody standing 10' away, and it's blowing hard enough, he might hear my voice coming from the opposite direction, and changed in pitch like mickey mouse! Powerfull stuff that Doppler

[Paul Elliott]

Originally Posted by s/v Jedi

Sooooo, when I speak into the wind, towards somebody standing 10' away, and it's blowing hard enough, he might hear my voice coming from the opposite direction, and changed in pitch like mickey mouse! Powerfull stuff that Doppler

Kinda like when we were playing the Beatles' White Album backwards ("turn me on, dead man."). Or Hotel California.

Yep, powerful stuff, and not to be tampered with by the untrained. Kids, don't try this at home! Closed course, trained professionals, etc.

[jongleur]

Thanks, everyone, for playing along.

[2Wind]

From where I sit (is that a Doppler shift concept?) the issue with the "ticker tape" is that in the model the tape conceptually stretches, so to the observer the holes appear farther apart.
Whether the source is moving or the medium is moving is irrelevant. Shift is about relativities. Yes, in the OP's case, neither the the source nor observer are moving, but as soon as the sound waves exit the source, they are immediately influenced by the medium and in this case those waves propogating down wind "see" the same physics as they would if the source was moving up wind. Hence the downwind observer will perceive a downward shift in frequency. So, if source, medium and receiver all move together - no shift. If only the source moves - shift. If only the receiver moves - shift. If the source and medium move relative to the receiver the shift becomes the sum of vectors in the direction of the source.

I've been wrong in the past (more often at home) but this is my reckoning.

[Albro359]

wrong !!!

It has NOTHING TO DO WITH THE MEDIUM

It has EVERYTHING to do with the movement of the source and/or receiver.

IT IS ONLY the relative velocities of the source and receiver that cause the Dopper effect.

This is BASIC physics.

[Paul Elliott]

Originally Posted by 2Wind

From where I sit (is that a Doppler shift concept?) the issue with the "ticker tape" is that in the model the tape conceptually stretches, so to the observer the holes appear farther apart.
Whether the source is moving or the medium is moving is irrelevant. Shift is about relativities. Yes, in the OP's case, neither the the source nor observer are moving, but as soon as the sound waves exit the source, they are immediately influenced by the medium and in this case those waves propogating down wind "see" the same physics as they would if the source was moving up wind. Hence the downwind observer will perceive a downward shift in frequency. So, if source, medium and receiver all move together - no shift. If only the source moves - shift. If only the receiver moves - shift. If the source and medium move relative to the receiver the shift becomes the sum of vectors in the direction of the source.

I've been wrong in the past (more often at home) but this is my reckoning.

2Wind, why do you figure that the Ticker Tape is stretching? It doesn't in the analogy I have been discussing, and I don't think the "stretch" applies in the real world. Think of pulses per second (frequency) being imposed upon a moving medium. As long as both ends (source and receiver) are stationary, or moving at the same speed/direction, the frequency at the receiver will be the same as the frequency at the source.

Originally Posted by Albro359

wrong !!!

It has NOTHING TO DO WITH THE MEDIUM

It has EVERYTHING to do with the movement of the source and/or receiver.

IT IS ONLY the relative velocities of the source and receiver that cause the Dopper effect.

This is BASIC physics.

Albro, you don't need to shout, we can hear you just fine.

First, I agree with you that there is no Doppler (or Doppler-like) shift when the source and receiver are stationary relatice to each other, regardless of the density or motion of the intervening medium. By the way, this has been my position from my first post on this topic.

But when there *is* relative motion, and so Doppler shift, the density of the medium has a profound effect on the amount of shift. For a given geometry, media with faster Vprop (air moving towards the receiver, water, steel) will cause there to be less of a shift than you would get with a slower Vprop (air, air moving towards the source). Someone presented an equation that showed this, and the basic geometry of the situation makes this pretty obvious.

[Matt sachs]

Someone should do a simple experiment to settle this. All you need is something to measure the pitch of a sound, like the piano tuning thingy. Get two cars next to each other. Blow the horn on one of the cars and record the pitch of the horn from within the other car. Repeat this at highway speeds and see if the pitch changes. Could be fun.

[jongleur]

Aw, heck. One more?

Remember, this is about sound. (and theoretical)

How about this: With the emitter and receiver stationary with regard to each other and the air still. Then the emitter emits waves and when they are half-way to the receiver, the wind blows toward the receiver.

Would the receiver perceive a higher pitch?

[Paul Elliott]

Originally Posted by jongleur

Aw, heck. One more?

Remember, this is about sound. (and theoretical)

How about this: With the emitter and receiver stationary with regard to each other and the air still. Then the emitter emits waves and when they are half-way to the receiver, the wind blows toward the receiver.

Would the receiver perceive a higher pitch?

This is kind of like the shop-vac experiment I may get around to doing, where there is a constriction in the pipe, and so an increase in air velocity. It is tempting to think there will be a pitch change, but I think not. Consider my radio example (signal path includes coax and air, each with significantly different Vprops). The received frequency still equals the transmitted frequency. There's got to be some sort of "conservation of cycles" principle to help us think about this stuff.

OK, pondering the wind-change scenario, the pitch *would* change if somehow a chunk of air, with the sound waves propagating, were to accelerate as a uniform block. The "embedded" sound waves would indeed be at a higher frequency as they reached the listener. There would also be a gap, or lower-pitch section in the sound following the accelerated section, thus conserving cycles. This is not a steady-state situation.

I can't imagine that this is a realistic scenario though. Any real change in wind speed would have a transition region where the pressure waves would be coupled, and where the wavelength (not the frequency) would be changed.

I obviously have way too much spare time today...

[Andrew Troup]

The changing windspeed during propagation scenario is certainly harder to come up with simple thought experiments which are satisfyingly conclusive.

However I have to say that it's contrary to experience: I don't recall ever hearing a constant pitch sound from far away (a ship's siren, a diesel locomotive horn, a cathedral bell) undergoing 'pitch bend' in gusty winds. I think it would be very noticeable.

The Goons used to fool around with recordings of Big Ben, inducing 'wow' to alter the pitch, and it sounded extremely unnatural and highly remarkable.

[Andrew Troup]

The consideration of a wave travelling through a moving medium is highly relevant to anyone who sails in areas with strong winds and strong currents. The extent of the effect on waveform is much more than simplistic notions would suggest. (Not frequency, I hasten to add: as far as I know there are no UFOs hovering around to hoover up unused waves, or magic them up out of nowhere)

I once lost an argument where, unknown to me, my shipmates were operating on the assumption that seas driven by winds of 30 knots into a contrary tidal current of 5 knots would be similar in steepness and character to seas driven by winds of 35 knots with no current, in other words, not much change. It was only their conclusions, rather than the basis for them, that they reported to me, and it was two against one.

I knew they were wrong, but I didn't understand why well enough to argue persuasively (an early opportunity to realise that assertions are no subtitute for explanations)

As a result of me losing the argument, the three of us were lucky not to lose our lives.

[Paul Elliott]

Originally Posted by Andrew Troup

[...] the [wrong] assumption that seas driven by winds of 30 knots into a contrary tidal current of 5 knots would be similar in steepness and character to seas driven by winds of 35 knots with no current [...]

It does seem to me that relative wind over sea should be the critical factor in wave height, regardless of the current. (Ground wind + current = sea wind.) I've been told that this is not the case, and I suppose I believe it, but I have no idea why. I've not heard a satisfactory explanation.

[Dockhead]

Originally Posted by Andrew Troup

The changing windspeed during propagation scenario is certainly harder to come up with simple thought experiments which are satisfyingly conclusive.

However I have to say that it's contrary to experience: I don't recall ever hearing a constant pitch sound from far away (a ship's siren, a diesel locomotive horn, a cathedral bell) undergoing 'pitch bend' in gusty winds. I think it would be very noticeable.

The Goons used to fool around with recordings of Big Ben, inducing 'wow' to alter the pitch, and it sounded extremely unnatural and highly remarkable.

That's a really good one - I think it's like wind against tide. Is there a physicist in the house?

[Dockhead]

Originally Posted by Paul Elliott

It does seem to me that relative wind over sea should be the critical factor in wave height, regardless of the current. (Ground wind + current = sea wind.) I've been told that this is not the case, and I suppose I believe it, but I have no idea why. I've not heard a satisfactory explanation.

It's because of the relative motion of the water, compared to the water across whose fetch the waves were generated. Indeed changes the frequency. I don't know whether sound in air behaves the same way or not.

[Fortytwo]

"For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium."

Doppler effect - Wikipedia, the free encyclopedia