Pedestal Fans

[Chris Timson]

Has anyone yet considered that the fan may also be acting as an amplifier? The intermitant reflection from the passing blades will bounce the sound back louder than when the sound passes through between blades. The will give a warble of volume rather than a warble of pitch.

<{POST_SNAPBACK}>

I don't think that can be quite right. After all, although the blades are rotating the actual area of blade available to reflect sound is unchanging, so the amount of sound reflected should not change.

 

Chris

[Robin Madge]

But the blades are angled and there will be a suble difference in the direction that the sound is reflected in between the blade being in one position, and then moving to a position that is halfway before it reaches the place the preceding blade was in. (I hope that makes sense!). The fewer the blades the more obvious it would be.

 

Anyone tried fans with different numbers of blades?

 

Robin Madge

[Chris Timson]

But the blades are angled and there will be a suble difference in the direction that the sound is reflected in between the blade being in one position, and then moving to a position that is halfway before it reaches the place the preceding blade was in. (I hope that makes sense!).

<{POST_SNAPBACK}>

Yes, and it's precisely that difference that lies behind the interference hypothesis. It is the reflected sound that causes the interference. But I shouldn't have thought (of course, I don't know) that the variation in reflected sound in itself would have any effect at all, because the ear just sums all the sound that reaches it, and that remains constant (excluding the interference effect) whatever the position of the blades.

 

It's a simple test. Stop the fan completely. Play. Now move the fan blades to a new position by hand. Play. Acording to your hypothesis the sound levels in each playing session should be different.

 

It must be Monday. I never could get the hang of Mondays.

 

Chris

[Gerry]

But the blades are angled and there will be a suble difference in the direction that the sound is reflected in between the blade being in one position, and then moving to a position that is halfway before it reaches the place the preceding blade was in. (I hope that makes sense!). The fewer the blades the more obvious it would be.

I don't believe thats true with the wide angle sound sources we're dealing with here. The sound from the concertina will spread out in all directions and will hit the fan blades wherever they are, and be reflected back. I haven't tried this, but I'm willing to bet that if a "fan" blade was made from ply, MDF or somthing else flat and bolted to the fan motor, there would be no warbling effect at all, even if it was placed in the wind to simulate a real fan. Sound can reflect off objects like light off a mirror, but only from a narrow beam source. An ultrasonic 40KHz beam from a 55 degree source will scatter in all directions, but a narrow 15 degree 235KHz will be like light of a mirror. At 600Hz and upwards (C above middle is 557Hz) the sound has too long a wavelength to bounce so selectively off the blades (about 62cm).

 

I'm still happy with doppler being a predominant influence here, but not to the exclusion of other effects, such as the obvious pulsing of the air movement.

[tony]

I've been following this thread and couldn't resist getting involved so I decided to try it out for myself using one of our three speed fans.

 

These are my observations:

 

Whatever note I play the frequency of the 'interruptions' seems to be constant and at the same pitch as the note.

 

Whatever speed I run the fan at the frequency of the 'interruptions' seems to be constant and at the same pitch as the note.

 

When I play a two note interval (or a triad) the 'interruptions' seem to coincide and are in sympathy.

 

I did not measure the frequency of the 'interruptions' but I'm guessing at about 25 c/s. The mains frequency in this country is maintained at 50 Hertz.

 

What a mystery this one is!

 

Sorry if I'm repeating or duplicating anything anyone else has said or done.

[Clive Thorne]

I've just caught up on this thread having been away all last week (in Ireland, but not on a musical trip unfortunatley).

 

Some of the evidence emerging from tests that people have conducted (for example moving fan blades by hand) seems to be leaning away from the Doppler effect as the cause.

 

Another affect that could be contributing is the spatial interference pattern between two sound sources.

 

If you have two sources a distance apart the interference between the two will create bands of cancellation and re-enforcement, which you will be audible as you walk around at a distance from the two sources. THe effect is rather like throwing two stones into a pond; If you watch carefully you will see a similar pattern of cancellation and re-enforcement. This effect is not dependent on any movement of the sources or the listener, and, as someone's already pointed out, this sort of effect is far more noticeable at higher frequencies.

 

If two fan blades act as two separate reflective sources then you may get the same sort of effect (ie the blades act as a diffraction grating), but as the blades are moving, the bands of sound will also be moving through space, so the listener will hear a regular variation in volume.

 

This is then complicated by the fact that the bands will be a different distance apart depending on the frequency; and by the fact that the fan blades get closer together towards the centre of the fan. At the same time the frequency at which the bands pass the listeners ear is dependent on the fan speed.

 

 

Anyone got any thoughts on this hypothysis?

 

 

Clive

[Baxter]

Aah, I have to weigh in. I have heard this effect (or some effect) from all sorts of fans on a number of instruments. I just tried out my 20-inch window fan with concertina, whistle, and obnoxious nasal electronic tuner. For all of them, I heard a pulsing whose frequency depended entirely on the speed of the fan blades, so I side with those who attribute it to the subtle changes in reflected sound that occur as the blades move. In fact, even differently-positioned but still fan blades gave slightly different tones to the instruments.

 

</geeky scientist behavior>

 

I don't find it a very appealing addition to my practice, but I'm willing to live with it in the summer...

[A.D. Homan]

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.

 

That's what I did. He said that you are not right.

[Chris Timson]

That's what I did.  He said that you are not right.

<{POST_SNAPBACK}>

Don't keep us in suspense! Why did your physics teacher say that Jim was not right? Jim's statement certainly accords with my understanding of the Doppler effect. It seems your physics teacher knows better than NASA:-

The apparent change in frequency of sound or light waves, varying with the relative velocity of the source and the observer. If the source and observer draw closer together, the frequency is increased. Named for Christian Doppler, Austrian mathematician and physicist (1803-1853).

I don't know who wrote the excellent article in Wikipedia, but maybe your physics teacher would care to correct it since he/she clearly knows better.

 

Chris

[Frank Edgley]

I thought "fans" were supposed to put "you" on a pedestal?

[Frank Edgley]

Sorry, I couldn't help myself.

[JimLucas]

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.

That's what I did. He said that you are not right.

<{POST_SNAPBACK}>

My above contains multiple assertions. Which does he claim are incorrect?

... 1) The Doppler effect is due only to motion that changes the distance between source and destination. Any motion perpendicular to that direction has no Doppler effect.

... 2) The motion of the fan blades is mostly perpendicular to the effective direction.

... 3) The effect is most noticeable directly in front of (under)

the fan.

... 4) Directly in front (under) is where motion in the Doppler direction would be least.

 

Caveats on the above:

... 2) I am not claiming (though I think I did earlier imply) that there is no motion in the Doppler direction. But such "motion" would be a result of the fan's rotation shifting between the nearer and farther edges of the canted blade at any given point, during the time it takes the blade to pass that point. On the fans I've seen, that distance is at most a few inches. I'm questioning -- not denying, but questioning -- that this is enough to be a significant contributor to the effect we hear.

... 3) I'm taking this from memory. I could be wrong.

... 4) I was thinking that this motion is "least", because the front-to-back distance between the edges of the blade is far less than the side-to-side travel of the blade as it travels in a circle. I forgot to take into account the fact that the side-to-side motion occurs over a full cycle, while the blade is crossed in much less than a full cycle. Though my by-eye impression is still that the front-back distance travelled in a given time is less than the side to side, I think I should consider that undecided until I (or someone else) check it by taking actual measurments of one or more fans.

 

In case anyone hasn't noticed, I'm admitting to certain uncertainties where previously I felt more certain. I still suspect my earlier conclusions are right, but I admit that more rigorous evidence is needed. However, the same need for rigor is also a requirement for any competing theory, including "it's a Doppler effect".

 

And no caveat on point 1) above. No uncertainty. That is fact.

[JimLucas]

Some of the evidence emerging from tests that people have conducted (for example moving fan blades by hand) seems to be leaning away from the Doppler effect as the cause.

 

Another affect that could be contributing is the spatial interference pattern between two sound sources.

[...]

Anyone got any thoughts on this hypothysis?

<{POST_SNAPBACK}>

Clive, I definitely think what you suggest is part of the answer, and also points to the dynamics being rather complex. E.g., you mention "two" sources, but most fans have more than two blades. Furthermore, most fans have gaps between the blades, and for the average ceiling fan these are much larger than the blades themselves. So while part of the sound is being reflected from the moving blades, much is not, but if the fan is in a room, it's being reflected from the unmoving ceiling or wall behind. In fact, a small (how small?) amount of the sound may be reflected multiple times (e.g., from source to wall to back of fan blade and back to wall before returning to the player/listener).

[A.D. Homan]

That's what I did.  He said that you are not right.

<{POST_SNAPBACK}>

Don't keep us in suspense! Why did your physics teacher say that Jim was not right? Jim's statement certainly accords with my understanding of the Doppler effect. It seems your physics teacher knows better than NASA:-

The apparent change in frequency of sound or light waves, varying with the relative velocity of the source and the observer. If the source and observer draw closer together, the frequency is increased. Named for Christian Doppler, Austrian mathematician and physicist (1803-1853).

I don't know who wrote the excellent article in Wikipedia, but maybe your physics teacher would care to correct it since he/she clearly knows better.

 

<{POST_SNAPBACK}>

 

Not at all -- he confirmed just that definition. He -- a physics _professor_ -- also stated that it was exactly what was happening with the ceiling fan -- it's just that we have to consider the various distances of the fan blades as multiple sound "sources," even though we know they are not sources, but rather, points from which the sound is reflected. He said that Jim's explanation was incorrect because Jim incorrectly stated that effect observed with the instrument and fan was not related to the Doppler effect. So my friend was not attemping to re-write physics -- rather, it is Jim who is contradicting standard knowledge of the phenomenon.

 

For those who are slow on the uptake or prone to uncharitable readings of posts made by those of us who have written fewer than 500 posts on concertina.net: The definition of the Doppler effect is correct; the application of the definition, to prove that it is not at play in the fan phenomenon, is not correct.

 

The warbling itself is cause by the interference of multiple pitches, in an oscillating rhythm. The shifting of the pitch is caused by Doppler.

 

(Rather than being so hostile to my answer, I suggest looking at the hostility with which Jim has approached this topic, which is apparently no longer open to discussion.)

 

I've included below a quotation from a previous discussion on "rec.music.makers.squeezebox":

 

Chris Timson  Oct 12 2000, 3:00 am    show options

DavBarnert <davbarn...@aol.com> writes

 

>I'm not so sure it's a Doppler effect. If so, why does it only

>seem to happen with free reeds?

 

I reckon it is, and it is a version of the Leslie effect on Hammond

organs. I have noticed it with Anne's hammer dulcimer, so it's not just

free reeds, but it is more noticeable with them, I agree.

Chris

 

As a reminder, Dave had explained that it had to do with differences in air pressure. Here is Dave's explanation from that forum:

I'm not so sure it's a Doppler effect. If so, why does it only

seem to happen with free reeds? My guess is that the spinning fan

blades create rapidly alternating puffs of relatively higher and

lower air pressure at the location of the vibrating reed, causing

periodic variations in the frequency of its vibrations.

[JimLucas]

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

That's what I did. He said that you are not right.

<{POST_SNAPBACK}>

He said that Jim's explanation was incorrect because Jim incorrectly stated that effect observed with the instrument and fan was not related to the Doppler effect.

<{POST_SNAPBACK}>

But that wasn't "what I just said". So it appears to me that your physics teacher friend is disagreeing not with what I said, but with a misinterpretation of what I said.

 

In fact, I don't believe I said anything so definite in any of my posts. (If you can provide an exact quote which demonstrates that I did, I might eat my words. It wouldn't be the first time.) My position all along has been 1) that the perceived phonemenon is too complex for a simple explanation like "Doppler effect" to be either adequate or proof, and 2) that I doubt -- not deny -- that any Doppler-induced effect is a major contributor to the phenomenon we hear.

Edited August 25, 2005 by JimLucas

[JimLucas]

He -- a physics _professor_ -- also stated that it was exactly what was happening with the ceiling fan -- it's just that we have to consider the various distances of the fan blades as multiple sound "sources," even though we know they are not sources, but rather, points from which the sound is reflected.

<{POST_SNAPBACK}>

But "various distances" don't produce a Doppler effect, varying distances do. My question is how much the various distances are changing and whether those changes are enough to produce the perceived effect.

 

Also, I'm not the only person who has questioned the Doppler explanation, and some have conducted experiments to test various hypotheses. Some of the results as reported seem to demonstrate that there are significant non-Doppler factors involved.

 

So I'm wondering what experimental research results your physics-teacher friend's conclusions are based on.

[David Barnert]

As a reminder, Dave had explained that it had to do with differences in air pressure.  Here is Dave's explanation from that forum:

I'm not so sure it's a Doppler effect. If so, why does it only

seem to happen with free reeds? My guess is that the spinning fan

blades create rapidly alternating puffs of relatively higher and

lower air pressure at the location of the vibrating reed, causing

periodic variations in the frequency of its vibrations.

<{POST_SNAPBACK}>

Thank you for dredging this up and saving me the trouble of reformulating the same thoughts, which I was considering doing.

 

You use the words "explained" and "explanation" to describe my words, but I used the word "guess" right there in the quote. Nothing I said was meant to be taken as an explanation.

 

The words are no less true today than they were five years ago. I remain struck by the fact that the effect is considerably more pronounced with free reed instruments and am led by this to suspect a mechanism that is specific to the way free reeds produce sound.

 

I have no answers or explanations. I do not know that the Doppler answer is wrong or that my guessing will lead to a correct answer. But for me, nothing I have seen here satisfies me that the question is answered. I have seen scientists guess wrong before in the absence of properly performed experimentation.

 

What is needed is scientific study, not speculation. There are no ceiling fans in my house, and whenever I find myself in a hotel room with one, I am without my concertina. My laptop computer can serve as a digital recorder and I have software for sound analysis. One of these days, it will all come together.

[Ken_Coles]

I have just spent a considerable amount of time editing this thread, because there was no other choice besides deleting it, and it does contain information I would like to see on Concertina.net. This is very slow, and I have better things to do. Anyone posting further comments on this thread needs to stick to the topic, and not to characterizations of other persons OR their arguments OR how they state their arguments. Any of those things should be discussed off Concertina.net, using some forum or means of communication of your own, and not this one that Paul and I have maintained with our effort and paid for with funds that Paul provided or collected.

 

I work as a scientist. We have many disagreements, some more vehement than this one, if you can imagine that. Yet we keep it civil and agree when more evidence is needed, or we agree to leave off the topic completely until another time. If you are a part of Concertina.net those are the choices open to you.

 

Ken Coles