Pedestal Fans

[Gerry]

I know this subject has been done to death on ceiling fans. In my mind this was just down to the reflections from the fan blades and the sound passing through the blades and reflecting off the ceiling. For me there was no real mystery. Now I'm not so sure. Last night I was playing at a rather hot and humid session, and parked myself in the cooling breeze of a pedestal fan. The first tune, and the most awfull warbling came out of the box.

 

Now the fan was in a position where there is no wall, just open space behind it, which rather throws my blade/ceiling theory out of the window.

 

It's certainly not just doppler, that would produce a constant pitch shift, but I thought that if the rotating blade produced a local "pulse" of air as it passes a point, then that with doppler might account for it.

 

Any further thoughts on this over-done subject?

 

Chris, Did New Scientist ever come back with answers?

[A.D. Homan]

It's certainly not just doppler, that would produce a constant pitch shift, but I thought that if the rotating blade produced a local "pulse" of air as it passes a point, then that with doppler might account for it.

 

Recently, I asked a local fiddler who also happens to be a physics professor, who also just so happens to teach physics of sound, and his response was: "Simple: Doppler effect." He didn't think that there was anything else to it.

[Gerry]

It's certainly not just doppler, that would produce a constant pitch shift, but I thought that if the rotating blade produced a local "pulse" of air as it passes a point, then that with doppler might account for it.

 

Recently, I asked a local fiddler who also happens to be a physics professor, who also just so happens to teach physics of sound, and his response was: "Simple: Doppler effect." He didn't think that there was anything else to it.

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I don't think just putting it down to doppler is enough. A shift in pitch, yes, but not the warble. I'm reluctant to call it vibrato, which is mostly quite pleasent. What I heard in the air stream was very unpleasant. Doppler may be part of it, but not the whole story. Could your professor friend have given a dismisive reply because he didn't want to talk shop when he was out to play music? In anycase, I would welcome a more detailed explanation.

[JimLucas]

It's certainly not just doppler, that would produce a constant pitch shift, but I thought that if the rotating blade produced a local "pulse" of air as it passes a point, then that with doppler might account for it.

Recently, I asked a local fiddler who also happens to be a physics professor, who also just so happens to teach physics of sound, and his response was: "Simple: Doppler effect." He didn't think that there was anything else to it.

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I'm getting tired of these Doppler-gangers, and tired of repeating this most basic fact about the Doppler effect:

...The Doppler effect is a consequence of relative motion of the sound source and the target toward or away from each other. (Or in the case where the sound is reflected back and the source is the same as the target, the relative motion of the source-target and the reflector.) But the motion of the fan blades is almost entirely sideways to the direction between it and the concertina. In fact, the effect is most severe if you're sitting directly under (or in front of) the fan, where any motion in the Doppler direction would be at a minimum.

 

Please tell your "physics of sound" teacher what I just said and ask him if I'm right.

 

Edited to add: I overlooked what Gerry said about a "pulse" of air. If a "puff" of air is pushed toward the player by the fan blade, I could imagine that partial reflection from the moving puff would result in a Doppler effect. However, my guess is that the magnitude of the effect would be less than whatever might result from the movement of the nearby fiddler's bowing arm. So I'd still be looking for a different explanation.

Edited July 15, 2005 by JimLucas

[Chris Timson]

Chris, Did New Scientist ever come back with answers?

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Looks like NS decided the subject was too esoteric for theor hallowed pages . Still, at least Jim will be pleased . I also agree with Jim, your teacher wallah was copping out, indulging in a bit of "impress the natives", I would guess . Next time you see him, just say .

 

Chris

[A.D. Homan]

Could your professor friend have given a dismisive reply because he didn't want to talk shop when he was out to play music? In anycase, I would welcome a more detailed explanation.

 

Uh, no, this is a person who ONLY talks shop, whether physics or fiddles.

Gerry, the "warble" is essentially what you could get on an accordion with two reeds tunes to slightly different pitches, but in this case it's an unpleasantly tuned accordion. I don't think that the sound reflected by the fan is itself warbling, but the interaction between it and the instrument's sound results in the socalled "wet" tuning.

 

Jim, I'll ask him about your comment. (BTW I didn't comment on the pulse, Gerry did.) I do think that the direction of movement might have something to do with the tilt of the fanblades and direction of airflow (in pulses). But I'll ask him for more detail.

Edited July 15, 2005 by A.D. Homan

[JimLucas]

Gerry, the "warble" is essentially what you could get on an accordion with two reeds tunes to slightly different pitches, but in this case it's an unpleasantly tuned accordion.  I don't think that the sound reflected by the fan is itself warbling, but the interaction between it and the instrument's sound results in the socalled "wet" tuning.

While it may have a similar frequency of repetition, it is (in my experience) a far more complex "vibration" than anything I think you could get from having two reeds not quite in tune with each other. I suspect it also has a less simple cause.

 

I do think that the direction of movement might have something to do with the tilt of the fanblades and direction of airflow (in pulses).

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Is this something you think you've observed? It seems reasonable that there should be some effect, but I myself don't have enough experience with different fans, blade angles and (relative) locations to even hazard a guess as to what it might be.

[E Stander]

Folks:

I just tried something simple that's worth doing. Stand under a ceiling fan with the blades off. Turn on a chromatic tuner, or similar device that emits a pure tone. Now turn on the fan. You'll hear the beat frequencies immediately, and they will follow the speed of the fan. As the fan rotates faster, the frequencies will increase.

 

It's subtle, but easy to hear once you tune your ear to it.

 

So what is happening? My guess is that the reflected sound is interfering with the emmited sound. This typically does two things to the note. First, it changes the volume of the note, depending on whether the reflected sound is constructively or destructively interferered with ( a frequency effect - easily explained on a blackboard - hard on a discussion board - look up destructive interference on the web). Secondly, it produces secondary tones due, once again, to frequency differences arising between the note and the reflective surface. These tones are the lesser effect, but are still present if you listen closely.

 

Neither has anything to do with Doppler effect. They have to do with the fact that the reflecting surface is changing with respect to the sound source. One second it's the fan blade that reflects the sound, the next it's the ceiling. Since the fan/concertina distance is different than the ceiling/concertina distance, the interference is different.

 

That's my 2$ worth. Cheers - Ed

[Chris Timson]

So what is happening?  My guess is that the reflected sound is interfering with the emitted sound.

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Now that's an explanation I can live with. It feels right to me, from what I know of interference phenomena, and I shall adopt it until someone proposes a better.

 

Nice one!

 

Chris

[A.D. Homan]

So what is happening?  My guess is that the reflected sound is interfering with the emitted sound.

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Now that's an explanation I can live with. It feels right to me, from what I know of interference phenomena, and I shall adopt it until someone proposes a better.

 

That's what I was trying to say above when I said:

"I don't think that the sound reflected by the fan is itself warbling, but the interaction between it and the instrument's sound results in the socalled "wet" tuning."

However, my physicist friend would disagree.

 

In the meantime, I've been playing under a ceiling fan quite a bit lately...

[spindizzy]

That's what I was trying to say above when I said:

"I don't think that the sound reflected by the fan is itself warbling, but the interaction between it and the instrument's sound results in the socalled "wet" tuning."

However, my physicist friend would disagree.

 

In the meantime, I've been playing under a ceiling fan quite a bit lately...

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Sounds good to me ... (as a physicist .. and I have the papers to say so!)

I was going to start up a "shouting into a high wind" thread branch, but I'm not up to the research involved at the mo!

 

Chris J

(phys/astro)

[JimLucas]

So what is happening?  My guess is that the reflected sound is interfering with the emitted sound.

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The problem with that theory is that the effect should then be observed equally with any and all instruments, yet most people (myself included) have not noticed it with fiddles, whistles, etc. at the same time that the concertina is sounding severely distorted. If it's only free reeds, then something affecting the production of the sound must be the cause.

 

The trouble with that theory is that in this earlier Topic Jim Harvey mentions that a cello player he knows does experience the effect. So are many of us so "tuned" to our concertinas that we are greatly disturbed by a distortion of their sound that we don't even notice when the same effect is produced in other instruments? I think that before we discuss any further the cause, we first need to settle this point of information about the effect. (I'd be happy to do it, but although we're having an unusually warm summer here in Denmark, I myself don't have access to such a fan.)

[Clive Thorne]

Doppler could be a contributary factor irrespecive of any air flow caused by the fan blades (infact, as Jim states, doppler is only caused by relative movement of source and listener. It is not caused by movement of the carrier, i.e. the air). The fan blades are inclined, so as they rotate the reflecting surface at one point moves towards (or away from) the listenener at a speed determined by the pitch of the blade and the RPM. The reflected sound is thus coming from a source that is thus moving relative to the listener, hence the potential Doppler affect. This slightly shifted pitch will interfere with the original, causing the tremelo, as ADH suggests.

 

As Jim says the blades are not at a great angle, but we are only looking for a shift of 5 or so hertz to make a pretty horrendous tremolo affect (tastes may vary), and this is a only very small percentage shift. If I had the time I'd do the calculations.

 

Add to this the fact that, as already mentioned, you may be getting some reflections from the blades and some from the ceiling, you end up with a complicated situation.

 

A test that might be worth trying is to play behind some distance behind the fan so you'd still get reflected sound, but you wouldn't get any significant air movement (steady or pulsing).

 

Clive

Edited July 15, 2005 by Clive Thorne

[Henk van Aalten]

My guess is that the reflected sound is interfering with the emmited sound.

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Ed,

This is a very plausible explanation. As far as I understand interference, the distance between the sound source and the "reflector" should have an audible effect. Can anyone confirm this?

[Clive Thorne]

I'm getting tired of these Doppler-gangers, and tired of repeating this most basic fact about the Doppler effect:

 

You could always choose not to contribute, Jim, if it's becoming so tiresome.

Other people people obviously still think its a subject worth musing over.

 

Clive

Edited July 15, 2005 by Clive Thorne

[JimLucas]

I'm getting tired of these Doppler-gangers, and tired of repeating this most basic fact about the Doppler effect:

You could always choose not to contribute, Jim, if it's becoming so tiresome.

Other people people obviously still think its a subject worth musing over.

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Clive, I think you misunderstood me. I think it's a very interesting subject. What I find tiresome is the repetitive misstatements of what the Doppler effect is/does.

 

If people kept saying that concertina reeds are made of cardboard, that would also get tiresome, but I would still feel obliged to point out that it's not true.

 

The fact is that there is no Doppler effect unless the distance between the listener and the sound source (which could be a fan blade or wall from which sound is reflected) is changing. Sideways motion that doesn't change the relative distance produces no Doppler effect, and motion that has a negligible "toward-away" component should have negligible effect on the sound.

 

I'll admit that if you're not sitting directly under the center of the fan, then the distance to the individual blades will vary slightly. But then there should be almost no "rasping" if you do sit directly under the fan, a strong increase as you move out from the center, and then a gradual decrease as you move away and the effect of reflections from the fan gets drowned out by the effect of reflections from objects nearer to the concertina. But as I recall, that is not what happens, so I conclude that any such effect must be negligible, and therefor what we hear must have a different cause.

[E Stander]

Folks:

To continue...

Sonic Interference is very frequency and spatially sensitive. You can prove this yourself with your concertina and ceiling fan. Put the fan on medium speed, and play a high note. You'll immediately hear the beats On the other hand, the beats wind up very indistinct if you play the lower register. This, I believe, is the reason most instruments don't have the same problem with fans - the have a lower register which is more or less unaffected by the fan blades.

By the way - in passing - I play wineglasses professionally, and have always had problems with ceiling fans due to the near total absence of bass register.

So there.

Best of all to all - Ed

 

(professor of Astronomy and Geology in my spare time)

[Stephen Chambers]

I'm getting tired of these Doppler-gangers ...

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The Doppelganger effect ?