Doppler Effect

[PeterT]

Chris,

 

Some interesting points in this discussion thread. Two questions:

 

(1) Did you get any response from "New Scientist"?

(2) Going back to absolute basics (i.e. no fan), does a different effect (or perceived effect) happen depending on whether you swing clockwise, or anti-clockwise? I'm thinking more from an audience perspective, for those sitting extreme right, or extreme left.

 

Regards,

Peter.

[ragtimer]

OK, I've done a little impromptu experimenting, and while I wasn't able to do all the experiments I previously envisioned, what I was able to do convinced me more than ever that my original guess was correct and the Doppler effect has *nothing* to do with the fan effect.

 

There was a standing (pedestal?) fan, about 18 inches across and 4 feet off the floor at Morris Dance practice this evening. I had with me my concertina and a low F tabor pipe. No digital recorders.

 

I stood facing of the fan at a distance of two feet and played the low note (F above middle C) on my pipe (case A). No distortion. Nice smooth tone. Then I played the same F on the concertina (case . The sound was very choppy, the classic fan effect, and I distinctly heard it from the direction of the concertina, not the fan. Then I turned my back on the fan and held the concertina in front of me, hidden from the fan by my torso. I played the note again, louder, loud enough to be heard behind me, at the fan (case C). No distortion.

 

Whatever is happening is happening at the location of the sound production (the reeds) and must rely on the presence of the air pressure effects of the fan, unimpeded by my rather substantial midsection.

 

It is still only at the guess level on my part that the explanation is that rapidly varying air pressure at the reeds is causing rapid changes in pitch, but other explanations are disappearing quickly.

Did you know that American pedal-pumped reed organs -- at one time the major usage of free reeds in the USA -- almost all have a "Vox Humana" tremolo gadget? This is a small horizontal pair of fan-like blades mounted behind the reed banks and driven by a small enclosed air turbine when the stop knob is pulled on. Its blades shove little puffs of air at regular intervals over the reed openings, which modulates the sounds, producing a vibrato or tremolo or whatever. A deliberate use of airflow to produce an effect on the reeds.

 

This is consistent with David's observation that shielding the 'tina from the fan with his body, killed the effect. It also fits the other person's experiment with mikes mounted inside the 'tina, whereby he could swing his instrument and get the effect -- as he swooshed his 'tina thru the air, the airflow over the reeds would affect the pitch and tone.

 

So, we have two observations in favor of airflow affecting the reeds, versus Doppler.

 

But -- I have sung and whistled into fan blades, and it does produce an effect much like the Chorus switch on a vintage Hammond Organ -- not suprising, given how that chorus generator works (by adding interference from varying delay times). I like the effect, done in moderation, but that's a matter of taste.

 

I would still maintain that swinging your box, especially when standing close to reflective walls (in a corner would be best), adds some Doppler chorus to the sound. My handbell coach insisted that we not "park" our bells during long tones, but "gyrate" them about to create a warmer tone.

 

So both Doppler and direct reed effects are valid, under the right conditions.

I apologize for resurrecting this "dead" topic, but nobody had cited the free-reed organ example.

--Mike K.

Edited May 6, 2006 by ragtimer

[JimLucas]

I apologize for resurrecting this "dead" topic, but nobody had cited the free-reed organ example.

Well, the equivalent -- Hammond organs and Leslie speakers -- has been mentioned in more than one Topic.

 

I have sung and whistled into fan blades, and it does produce an effect much like the Chorus switch on a vintage Hammond Organ...

I would expect variation in the external pressure against which your lungs are working would have an effect on the sounds -- whether voice or whistling -- that they produce. I would expect rapid variation of the external pressure to result in some sort of matching variation in the resulting sound.

 

I would still maintain that swinging your box, especially when standing close to reflective walls (in a corner would be best), adds some Doppler chorus to the sound.

Three comments:

... 1) Whatever "Doppler" effect might be produced in such a situation is not the effect usually observed from swinging the instrument, since that has also been observed in large halls and even on open-air stages.

... 2) Assuming that there is some Doppler effect in the situation you describe, I don't think it's been demonstrated that it's large enough to be noticeable. If it were, I would expect it to produce a noticeable change from the effect noticed without reflective walls. Maybe you could try that and let us know what change you hear in the sound.

... 3) One reason I doubt a perceptible Doppler effect is that I suspect the velocities are too low. The usual example of a Doppler effect is the whistle of a moving train. Another is the horn of a fast-moving car. Note that "fast-moving". You may expect to hear the effect if you're standing still and the car is moving at highway speeds, but have you ever noticed a Doppler shift in the pitch of a car horn on city streets? I haven't. I suspect the shift (for cars travelling at legal speeds ) is too small to notice. Now, how fast is your concertina moving when you swing it in a circle?

[Chris Timson]

(1) Did you get any response from "New Scientist"?

Nope

 

I think my question must have been too esoteric.

 

Chris

[David Barnert]

Did you know that American pedal-pumped reed organs -- at one time the major usage of free reeds in the USA -- almost all have a "Vox Humana" tremolo gadget?

No.

I would still maintain that swinging your box, especially when standing close to reflective walls (in a corner would be best), adds some Doppler chorus to the sound.

What does "Doppler chorus" mean in this context? I have seen the expression used to signify a synthesizer effect, but have never seen it defined (I just googled it) and never saw it used to signify a sound that could be produced naturally.

[ragtimer]

Did you know that American pedal-pumped reed organs -- at one time the major usage of free reeds in the USA -- almost all have a "Vox Humana" tremolo gadget?

No.

I would still maintain that swinging your box, especially when standing close to reflective walls (in a corner would be best), adds some Doppler chorus to the sound.

What does "Doppler chorus" mean in this context? I have seen the expression used to signify a synthesizer effect, but have never seen it defined (I just googled it) and never saw it used to signify a sound that could be produced naturally.

OK. Well, the Hammond organ, and the Schober kit-built electronic organ, used different technical means to create a "chorus" effect (meaning simulated multiple tone sources just slightly out of tune, as in "wet" tuning), given a single tone source. The idea is to have audio delay lines, which are clocked at a varying rate, so a single tone gets mixed with the outputs of these delay lines, which are slowly varying sharp or flat relative to the original sound. The effect is logically identical to having the sound source alternately moving away from and back towards your ear, while at the same time you also hear the source standing still. Imagine moving walls behind it, or moving fan blades (except that fan blades move much faster).

 

An older way of doing this, easier to undertand, is a tape delay, but with the tape speed slowly varying. This is close to the "flanging" effects in early rock studios.

 

Unlike a wet-tuned melodeon, the effect varies with time.

 

Another exampe is the Leslie SPeaker used in vintage Hammonds -- the speakers, or a reflecting vane next to the speaker, rotates at a slow or fast (player selectable) speed, giving genuine Doppler effects. Allen Organ Co. had a sort of Ferris wheel of speakers, called the "gyrophonic" effect. Done slowly, these are very warm and tasteful.

 

These are methods of taking a rather dull, fixed, "perfect" tone (such as from an electronic organ) and giving it some life and interest. Swinging a concertina (or handbell) around is a physical way of getting this effect, especially given reflective walls to create and combine different "delay lines".

 

Back to the "Vox Humana" stop in reed organs -- this uses the "other" effect, that of air flow over reeds modulating the tone. Austin Organ Co. used a similar trick in pipe organs.

 

So both effects seem to be valid, at least in my experience. Even Google can't turn up everything :-)

 

Edited to add: Besides consulting scientists or the Net, it's useful to look at existing musical instruments and their effects, such as handbells, reed organs, and electronic organs.

 

Interesting question" Few musicians or scientists (after Wheatstone :-) knwo much about 'tinas. What could we squeezers teach them about acoustics?

--Mike K.

Edited May 7, 2006 by ragtimer

[David Barnert]

...The idea is to have audio delay lines, which are clocked at a varying rate, so a single tone gets mixed with the outputs of these delay lines, which are slowly varying sharp or flat relative to the original sound.

OK, so this is a third and unrelated invocation of the doppler effect in concertina playing, and the only one that sounds believable to me.

 

Case 1: Doppler effect to explain the fan blade/tremolo effect (I'm not convinced).

 

Case 2: Doppler effect to create pitch bending by swinging the concertina while playing (I'm not convinced).

 

Case 3: Doppler effect to simulate chorus by creating a reflected sound at slightly different frequency than original sound and relying on the changing interference between the two to create the effect.

 

Note that in the creation of interference patterns, miniscule differences in frequency can have profound effects on the interference pattern, so even if the doppler effect is inaudible its effect on the interference pattern might be dramatic.

 

I can believe this. But it does not lend any credence to the first two cases listed above.

[ragtimer]

I apologize for resurrecting this "dead" topic, but nobody had cited the free-reed organ example.

Well, the equivalent -- Hammond organs and Leslie speakers -- has been mentioned in more than one Topic.

But the free-reed organ is quite the opposite effect from the Doppler -- I cited it as an example of external air flow affecting the tone production by the reeds.

I have sung and whistled into fan blades, and it does produce an effect much like the Chorus switch on a vintage Hammond Organ...

I would expect variation in the external pressure against which your lungs are working would have an effect on the sounds -- whether voice or whistling -- that they produce. I would expect rapid variation of the external pressure to result in some sort of matching variation in the resulting sound.

I understand, but I can tell that what I've heard is a reflection effect. I trust my intuition on this (from making just a "shhh" sound and hearing waht the fan blades do to it).

I would still maintain that swinging your box, especially when standing close to reflective walls (in a corner would be best), adds some Doppler chorus to the sound.

Three comments:

... 1) Whatever "Doppler" effect might be produced in such a situation is not the effect usually observed from swinging the instrument, since that has also been observed in large halls and even on open-air stages.

OK. Let me propose something NEW! An acoustic instrument (handbell, concertina, etc.) does not radiate sound in all directions equally, in terms of volume and frequencies. When a player gyrates a handbell, or swings a concertina, he is rotating the radiation pattern around, and treating the listener to a rapidly varying position relative to the instrument. Thus the listener hears a time-varying quality of sound as the instument moves. Not a variation in pitch a la Doppler, but a variation in intensity and tone quality or timbre.

 

This adds life and variety to the sound. It doesn't require any velocities or reflections.

 

It also has nothing to do with fan blades, but we still have the reed organ tremolo effect to help explain that.

--Mike K.

PS: If only this bull board had a spelling checker/correcter!

[JimLucas]

These are methods of taking a rather dull, fixed, "perfect" tone (such as from an electronic organ) and giving it some life and interest.

Ooh!

Contentious claim regarding musical taste!

Potentially classifiable as "troll", even if not intended as such.

 

Besides, it's a radical digression from the main subject of this thread. Therefore, I will note it, but not respond with declarations regarding my own taste. (If anyone else wishes to debate this particular subject, please start a new Topic.)

[JimLucas]

I apologize for resurrecting this "dead" topic, but nobody had cited the free-reed organ example.

Well, the equivalent -- Hammond organs and Leslie speakers -- has been mentioned in more than one Topic.

But the free-reed organ is quite the opposite effect from the Doppler -- I cited it as an example of external air flow affecting the tone production by the reeds.

Are you suggesting that a fan does not affect the air flow?

 

You seem to be ignoring the whole point of the prior debate, which is whether the fan effect is a "Doppler" effect. If the Leslie effect is not Doppler, then "opposite from the Doppler" does not imply "opposite from Leslie".

 

I have sung and whistled into fan blades, and it does produce an effect much like the Chorus switch on a vintage Hammond Organ...

I would expect variation in the external pressure against which your lungs are working would have an effect on the sounds -- whether voice or whistling -- that they produce. I would expect rapid variation of the external pressure to result in some sort of matching variation in the resulting sound.

...I can tell that what I've heard is a reflection effect.

But the Doppler effect is not a reflection effect. It's an effect of relative motion. And while it's true that a Doppler effect may be observed in reflection from a moving object, there are other significant reflection-induced effects that have nothing to do with motion. Among other things, the acoustic design of concert halls is based on this fact.

 

Standing-wave resonances -- e.g., the principal tones and harmonics of vibrating strings -- are also a consequence of reflections between "walls". Moving the source of the sound-producing energy relative to these "walls" -- e.g., changing the position of a violin bow relative to the bridge -- can have a profound effect on the quality of the resulting sound, but it has nothing to do with the Doppler effect.

 

I trust my intuition on this....

I often "trust" my own intuition by acting on it. But I am careful to distinguish it from experimentally established fact.

 

Let me propose something NEW! An acoustic instrument (handbell, concertina, etc.) does not radiate sound in all directions equally, in terms of volume and frequencies. When a player gyrates a handbell, or swings a concertina, he is rotating the radiation pattern around, and treating the listener to a rapidly varying position relative to the instrument. Thus the listener hears a time-varying quality of sound as the instument moves. Not a variation in pitch a la Doppler, but a variation in intensity and tone quality or timbre.

A nice insight, though Geoff Crabb's report indicates that that alone is not the cause of the swinging-concertina effect:

The effect in this case, I believe, is [due] to the sound source actually moving, the sound being alternately predominate in one ear then the other of a listener. My belief of this is based on the fact that a friend, who is profoundly deaf in one ear, cannot detect this effect but is quite aware of doppler effects, trains, police car sirens etc.

I suspect that you each have this half right, and that the effect is due to variations in the radiated sound, but is more directly the effect of variations in the differences in sound detected by the two ears. I.e., no effect is observed if there's no variation, but neither is one observed if there's no difference, which there can't be with only one ear.

 

An important question -- which I hope Geoff and his friend would answer for us -- is whether the friend can hear the "fan effect".

 

A comment of David B.'s is also relevant here:

Note that in the creation of interference patterns, miniscule differences in frequency can have profound effects on the interference pattern, so even if the doppler effect is inaudible its effect on the interference pattern might be dramatic.

Moiré patterns are an excellent visual example of this principle.

 

The fact that interference is far more sensitive to differences than is independent measurement is fundamental to the way human perception works, and is not limited to frequency differences. Differences in timing and intensity that we can't consciously distinguish can still be detected and interpreted by our brains into an awareness of the direction and motion of a sound source. Similarly, differences in the relative frequency of the "formants" of human speech are interpreted as the different vowel sounds, rather than as independent "melody" and "harmony" lines.

[ragtimer]

These are methods of taking a rather dull, fixed, "perfect" tone (such as from an electronic organ) and giving it some life and interest.

Ooh!

Contentious claim regarding musical taste!

Potentially classifiable as "troll", even if not intended as such.

Certainly not a troll. Nor was it intended to imply that a single sustained note on a concertina was "dull", though most experienced players would expect to add expression to a long note by varying bellows pressure and/or moving their instrument about.

Besides, it's a radical digression from the main subject of this thread. Therefore, I will note it, but not respond with declarations regarding my own taste. (If anyone else wishes to debate this particular subject, please start a new Topic.)

Maybe a radical digression from the topic of fans affecting concertinas, but quite relevant to the effects of Doppler and other phenomena on musical tones.

 

I know there are wide variations in taste regarding "wet" (celeste) tunings, and electronically added chorus and flanging effects to miked tones, and I agree such preferences belong in a separate thread.

 

I see this thread as identifying physical explanations for generally observed effects, where citing examples from other musical instruments and technologies may shed light on the questions.

--Mike K.

[ragtimer]

Are you suggesting that a fan does not affect the air flow?

Certainly not. In fact, I'm proposing that the fan effect may consist entirely of air-flow over the reeds, as opposed to anything Doppler. This is why I cited the deliberate usage of air flow tremolo effects in parlor reed organs.

You seem to be ignoring the whole point of the prior debate, which is whether the fan effect is a "Doppler" effect. If the Leslie effect is not Doppler, then "opposite from the Doppler" does not imply "opposite from Leslie".

Oops, by "opposite from Doppler" I meant "the other effect", namely, air flow over the reeds -- an effect confirmed by its successful application in a free-reed instrument family. Sorry about the sloppy English. And of course the Leslie and Gyrophonic effects are Doppler, or more precisely, Doppler against a "reference" or "dry" signal.

But the Doppler effect is not a reflection effect. It's an effect of relative motion. And while it's true that a Doppler effect may be observed in reflection from a moving object, there are other significant reflection-induced effects that have nothing to do with motion. Among other things, the acoustic design of concert halls is based on this fact.

I mention reflection only because it can enhance the Doppler effect. If a sound source is moving away from you, while it's also reflecting from a wall behind the source, then you hear two Doppler-shifted sounds, one sharp, one flat, which "beat" together in your ear to produce a "chorus" effect. You will hear this intereference or beat effect even though the pitch shift (in cents, or Hz) is not normally detectable by your ear.

Let me propose something NEW! An acoustic instrument (handbell, concertina, etc.) does not radiate sound in all directions equally, in terms of volume and frequencies. When a player gyrates a handbell, or swings a concertina, he is rotating the radiation pattern around, and treating the listener to a rapidly varying position relative to the instrument. Thus the listener hears a time-varying quality of sound as the instument moves. Not a variation in pitch a la Doppler, but a variation in intensity and tone quality or timbre.

A nice insight, though Geoff Crabb's report indicates that that alone is not the cause of the swinging-concertina effect:

The effect in this case, I believe, is [due] to the sound source actually moving, the sound being alternately predominate in one ear then the other of a listener. My belief of this is based on the fact that a friend, who is profoundly deaf in one ear, cannot detect this effect but is quite aware of doppler effects, trains, police car sirens etc.

I suspect that you each have this half right, and that the effect is due to variations in the radiated sound, but is more directly the effect of variations in the differences in sound detected by the two ears. I.e., no effect is observed if there's no variation, but neither is one observed if there's no difference, which there can't be with only one ear.

One way to test this would be to make mono recordings of alleged effects, as well as stereo recordings, and see if the effect is lost in the mono recording, which simulates listening with only one ear.

I agree that changes in the sounds relative to each ear are important, but that changes in timbre, as well as beat-frequency effects, can be heard with one ear.

An important question -- which I hope Geoff and his friend would answer for us -- is whether the friend can hear the "fan effect".

 

A comment of David B.'s is also relevant here:

Note that in the creation of interference patterns, miniscule differences in frequency can have profound effects on the interference pattern, so even if the doppler effect is inaudible its effect on the interference pattern might be dramatic.

Moiré patterns are an excellent visual example of this principle.

 

The fact that interference is far more sensitive to differences than is independent measurement is fundamental to the way human perception works, and is not limited to frequency differences.

Now we have something to agree on. I was going to point out that when one hears a tone directly, and also hears that tone phase-shifted by a Doppler or other effect, the "beat" or interference between those two tones is readily audible even if only one Hz. This is how the Leslie speaker achieves its effects at slow "city traffic" velocities, and how swinging a 'tina could work if there are any reflective surfaces around.

Differences in timing and intensity that we can't consciously distinguish can still be detected and interpreted by our brains into an awareness of the direction and motion of a sound source. Similarly, differences in the relative frequency of the "formants" of human speech are interpreted as the different vowel sounds, rather than as independent "melody" and "harmony" lines.

All quite true, especially when the formants frequencies are moving rapidly.

So we agree there are all sots of subtle effects that human ears pick up on. Now we just need to decide which ones are relevant to swinging 'tinas, 'tinas played near fans, etc.

 

Edited a half-dozen times to make the stinking "quotes" balance out!

--Mike K.

Edited May 8, 2006 by ragtimer

[JimLucas]

And of course the Leslie and Gyrophonic effects are Doppler, or more precisely, Doppler against a "reference" or "dry" signal.

There's the point of our difference. Regardless of how the manufacturers describe them, I'm not convinced that these are true Doppler effects. I suspect that they are interference effects and that the interference might have a Doppler component, but I don't know how significant it is.

 

In fact, I'm not at all sure that the variations involved in the various effects we're discussing -- Leslie, floor/ceiling fan, swinging the instrument -- are primarily pitch variations. I'd be interested in seeing the time-varying frequency spectrum as confirmation.

... 1) It's been quite a while since I've heard Leslie, and I won't trust my memory. I recall a variation in quality, but my memory doesn't tell me whether that quality was pitch, or volume, or something more complex.

... 2) When I swing my concertina, I don't perceive any change in pitch; it's a variation in apparent volume and possibly a subtle change in tonality.

... 3) The effect from a floor or ceiling fan is a raucous "fluttering", which is so complex that I can't tell how much of it -- if any -- comprises a variation in pitch.

 

In the case of a standard, bladed fan, there are reflections from a continuous range of distances travelling at a continuous range of speeds, interfering with non-reflected sound and usually also with sound reflecting from a fixed surface (wall or ceiling) behind, though the directions of the fixed reflections will vary as the blades (which block that reflection) move. (Even a minor component from multiple reflections between the wall/ceiling and the blades, but I suspect that's insignificant.)

 

An interesting experiment would be to observe what differences -- both in qualitative perception and in measured spectrum -- there are in the fan effect between a standard bladed fan and a cylinder fan, since in the latter case the spectra of distance and velocity of the reflecting surfaces will be quite different from the former.

 

Edited a half-dozen times to make the stinking "quotes" balance out!

Thanks. I wish everyone would take the trouble.

[ragtimer]

And of course the Leslie and Gyrophonic effects are Doppler, or more precisely, Doppler against a "reference" or "dry" signal.

There's the point of our difference. Regardless of how the manufacturers describe them, I'm not convinced that these are true Doppler effects. I suspect that they are interference effects and that the interference might have a Doppler component, but I don't know how significant it is.

I've never seen nor heard any makers' sales pitches for organ speakers. But I think we can agree these are Doppler effects if we extend "Doppler" to cover not jsut frequency (pitch) changes, but phase variations. Strictly, you can't have one without the other, but phase changes can occur slowly and subtly. Certainly the mechanical organ speakers and variable delay line flangers are doing phase changes.

In fact, I'm not at all sure that the variations involved in the various effects we're discussing -- Leslie, floor/ceiling fan, swinging the instrument -- are primarily pitch variations. I'd be interested in seeing the time-varying frequency spectrum as confirmation.

I bet there are phase changes involved, though that would require frequency shifts as well.

... 1) It's been quite a while since I've heard Leslie, and I won't trust my memory. I recall a variation in quality, but my memory doesn't tell me whether that quality was pitch, or volume, or something more complex.

It's quality, volume, and pitch, all wrapped up together. Rather appealing at a slow rate, rather apalling at the top speed ;-)

... 2) When I swing my concertina, I don't perceive any change in pitch; it's a variation in apparent volume and possibly a subtle change in tonality.

Well, gee, I finally tried swinging my big Stagi Hayden, and it really does have a major effect! Quality mostly, and a "rippling" in the sound, that remind me of "Doppler chorus" but I'm sure can't be that. I'd vote for this being an effect of turbulent airflow over the reed pad openings. If so, it should sound the same to you as to a listeneer in any position.

... 3) The effect from a floor or ceiling fan is a raucous "fluttering", which is so complex that I can't tell how much of it -- if any -- comprises a variation in pitch.

That could be a Doppler chorus, but I'd bet on it being the airflow over the reeds again, possibly modulated by the fan's delivering the air in puffs from each fan blade, not a steady stream.

In the case of a standard, bladed fan, there are reflections from a continuous range of distances travelling at a continuous range of speeds, interfering with non-reflected sound and usually also with sound reflecting from a fixed surface (wall or ceiling) behind, though the directions of the fixed reflections will vary as the blades (which block that reflection) move. (Even a minor component from multiple reflections between the wall/ceiling and the blades, but I suspect that's insignificant.)

How close is the concertina held to the fan, to get these effects? If very close (less than a meter), I'd say reflections are part of it. If farther away, I'd guess it is mostly airflow over the reeds.

An interesting experiment would be to observe what differences -- both in qualitative perception and in measured spectrum -- there are in the fan effect between a standard bladed fan and a cylinder fan, since in the latter case the spectra of distance and velocity of the reflecting surfaces will be quite different from the former.

Another experiment would be to use an enclosed rotary blower, which delivers a fairly steady airstream but shield the internal blades from any reflection effects. Move your 'tina in and out of the airstream, see how the sound varies. Hmm, I have a fan right here in the basement, a blade fan, not a rotary, but it's time to see what happens.

 

Ugh! When I hold the tina right in front of the fan, on a slow speed, I get a rapid modulation of the pitch and volume, like a radio that needs a new filter capacitor to remove the power line hum. Awful! When I mvoe the tina back away from the fan, still in a strong but now more continuous air stream, the rapid modulation goes away. I think it's the air puffs from individual fan blades. More later, though I'm leaving soon on vacation -- alas, sans 'tina and Internet. --Mike K.

[JimLucas]

...I think we can agree these are Doppler effects if we extend "Doppler" to cover not just frequency (pitch) changes, but phase variations.

No, we can't. And we shouldn't. The "Doppler" effect is specifically an observation of changes in pitch. If you want to talk about "phase-variation effects", go ahead, but please don't call them "Doppler", because such a general concept was not (as far as I know) described by or even known to Christian Doppler, for whom the "Doppler effect" is named. Einstein's laws of relativistic mechanics are generalizations of Newton's laws of mechanics, but we don't call them "Newtonian", for good reason.

 

Strictly, you can't have one without the other, but phase changes can occur slowly and subtly. Certainly the mechanical organ speakers and variable delay line flangers are doing phase changes.

You can have a phase change without a change in pitch, and a delay line is a perfect example. A delay line is supposed to shift an entire waveform in time (a phase shift), without changing its frequency spectrum.

 

Well, gee, I finally tried swinging my big Stagi Hayden, and it really does have a major effect! Quality mostly, and a "rippling" in the sound, that remind me of "Doppler chorus" but I'm sure can't be that. I'd vote for this being an effect of turbulent airflow over the reed pad openings. If so, it should sound the same to you as to a listener in any position.

But does it? If a person with hearing in only one ear can't detect it, that implies that it depends on differences in the sound received by the two ears, which would have to depend on the relative positions of the ears and the sound source. But maybe what Geoff reported is just a coincidence, and there's some other reason why that particular person couldn't hear the effect? Needs more research.

 

How close is the concertina held to the fan, to get these effects? If very close (less than a meter), I'd say reflections are part of it. If farther away, I'd guess it is mostly airflow over the reeds.

Except that Chris Timson has reported in this post: "At home I get a clear vibrato when the ceiling fan is on in the neighbouring room." I suspect that rather than affecting the air flow, what's affected -- at least in that example -- is the air pressure, but as I say, that's a suspicion, not an established "fact" or even a claim. And I don't think we should arbitrarily rule out the possibility that there may be multiple effects that we perceive as similar, but which have different causes, with different ones being dominant under different spatial relationships.

[Robin Madge]

A quick question for consideration. Will varying air pressure have a different effect on different frequencies?

I'm just wondering if we are hearing a variation in the magnitudes(volumes) of the harmonics rather than the basic pitch of the note. Nothing to back this up, just a thought.

 

Robin Madge

[Theodore Kloba]

You can have a phase change without a change in pitch, and a delay line is a perfect example. A delay line is supposed to shift an entire waveform in time (a phase shift), without changing its frequency spectrum.

Then you combine the delayed signal with the original signal and you get comb filtering. I believe the "ceiling fan effect" is one of varying comb filtering. Should be easy to test.

I'm just wondering if we are hearing a variation in the magnitudes(volumes) of the harmonics rather than the basic pitch of the note. Nothing to back this up, just a thought.

That's exactly what I think, and the comb filtering is what I think is causing it.

HAS ANYONE TRIED THE AFORESAID EXPERIMENTS IN EXTREME WEATHER CONDITIONS EG AT O DEGREES CELCIUS OR 80 DEGREES CELCIUS TO SEEIF THERE WERE ANY DIFFERRENCES IN THE SOUNDS OFTHE DIFFERENT EFFECTS.

I tried it at 80°C; I'll give you the full results once I'm released from intensive care.

[JimHarvey]

I must assume you are kidding about intensive care as well as about reaching 80 degrees centigrade!