Doppler Effect?

[m3838]

Often I hear that a concertina player can produce Doppler Effect by swinging an instrument. A Doppler effect can be audited by human ear at very high speeds. A train passenger can detect lowering of the pitch from a siren of approaching locomotive. Can simple swinging of concertina produce auditable Doppler Effect? If not, what is it, that people are talking about, calling it Doppler Effect?

[JimLucas]

Often I hear that a concertina player can produce Doppler Effect by swinging an instrument. ... what is it, that people are talking about, calling it Doppler Effect?

"Doppler" has already been done to death. Though mainly wrt. fans, the "swinging" technique has also been mentioned. "Doppler effect" seems an attractive explanation based on the (spurious) argument that the Doppler effect is a weird effect on sound caused by motion, and this is a weird effect caused by motion, therefore they must be one and the same. But the Doppler effect depends on the direction of the motion, and I maintain that the swinging motion is in the wrong direction for the effect to be Doppler, so I don't think the real cause has yet been established.

 

Previous Topics:

Ceiling Fans And Concertinas

Pedestal Fans, and that warbling... .. . .

Doppler Effect, A Final"?" Comment on Ceiling Fans

 

But if you're just asking what the effect sounds like, I suggest you try it. Swing your instrument in a circle as you play a steady note.

[Mark Evans]

["Doppler" has already been done to death.

 

 

...and then some.

[David Barnert]

I agree with Jim: For a performer to attempt to bend the pitch of notes by taking advantage of a "doppler" effect by swinging the concertina toward and away from the audience, s/he will have no way of knowing what it sounds like to the audience because the pitch will bend in a different direction when heard from behind the concertina (the performer's perspective) than from the perspective of the audience.

[Geoffrey Crabb]

I think Jim may have been referring to the swing often used by old time professionals when simulating peals of bells in classic favourite tunes like 'The Bluebells of Scotland' or 'The Bells of St. Mary's' etc.

The instrument is swung usually in circles as in the drawing below. Excuse matchstick person.

 

The effect in this case, I believe, is not due pitch bending but 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.

 

A word of warning. This is more effective if a large circle is encompassed so:

Do make sure there is enough room, watch out for low lamp fittings and above all make sure the instrument straps are securely attached.

 

Geoff

[JimLucas]

A word of warning. This is more effective if a large circle is encompassed...

Hmm. Is that why Alistair Anderson seems to do a better job of it than I do? He's significantly taller and with longer arms.

[David Barnert]

I think Jim may have been referring to the swing often used by old time professionals when simulating peals of bells in classic favourite tunes like 'The Bluebells of Scotland' or 'The Bells of St. Mary's' etc.

I have heard it specifically claimed that when Alistair Anderson moves his concertina around that he is doing so to produce a doppler-related pitch bending effect, and this is what I was referring to in my previous post.

 

Mind you, I have not heard Mr. Anderson make any such claim.

[RatFace]

"Doppler" has already been done to death.

 

Assuming a 1m radius circle, and the poor thing gets swung once every second, the average speed is 6.3m/s. The speed of sound is 330m/s so this will result in a +/- 2% change in frequency of the sound emitted in the direction of motion (and no change at perpendicular to this). One semitone is a frequency change of approx 6%, so this is a substantial pitch change. What's more, the speed is probably much greater at the bottom of the arc, so the pitch change would be maybe up to nearly a full semitone.

 

However... the raised pitch emitted in the direction of travel will be accompanied (ha ha) by a lowered pitch in the other direction. What with all the reflections and refraction and so on, what the listener will hear, should they be unfortunate enough to be in the vicinity, is a periodic "blurring" (probably asymmetric) of the frequency, which is perhaps why it sounds so awful. In addition there will be the volume changes due to the impossibility of keeping the bellows pressure constant, and the positional changes. The relative contributions of all the different mechanisms will depend on the environment (indoors/outdoors/via attached microphone etc), but I expect they're all significant.

[JimLucas]

Assuming a 1m radius circle, and the poor thing gets swung once every second, the average speed is 6.3m/s.

Close. That's the average speed tangent to the circle. The average component of the speed in any fixed direction "to the side" of the circle is 4 m/s (across the diameter and back in 1 sec.). That's still almost 2/3 as much.

 

The speed of sound is 330m/s so this will result in a +/- 2% change in frequency of the sound emitted in the direction of motion (and no change at perpendicular to this).

And therein lies the problem with this as an explanation of the "bells" effect, since the audience is normally situated in front of the performer, where the component of motion toward or away from them is quite small (smaller toward being directly in front of the performer, and smaller toward the back). And the distance from the performer's ears varies only slightly, yet (s)he also hears the effect quite clearly. Reports of the effect do not indicate that it is significantly stronger for those audience members closest to the corners of the stage, which the Doppler geometry suggests that it should be.

 

But your analysis suggests that there should be a Doppler effect, which effect should likely be different from what is heard. Why does nobody report such an effect... an effect which is very strong directly to the sides of the performer, but weak or nonexistent directly in front?

 

However... the raised pitch emitted in the direction of travel will be accompanied (ha ha) by a lowered pitch in the other direction. What with all the reflections and refraction and so on,...

This seems to assume a small, enclosed space. That hasn't been the case on the occasions when I've heard the "bells" effect in performance. Large halls and outdoor stages.

 

...what the listener will hear, should they be unfortunate enough to be in the vicinity, is a periodic "blurring" (probably asymmetric) of the frequency, which is perhaps why it sounds so awful.

Isn't it the "fan" effect that sounds awful, and not the "bells" effect? Your analysis seems to predict an effect which is quite different from what is experienced, and I wonder why.

Edited May 7, 2006 by JimLucas

[ragtimer]

However... the raised pitch emitted in the direction of travel will be accompanied (ha ha) by a lowered pitch in the other direction. What with all the reflections and refraction and so on, what the listener will hear, should they be unfortunate enough to be in the vicinity, is a periodic "blurring" (probably asymmetric) of the frequency, which is perhaps why it sounds so awful.

I haven't attended any performances where this was done, so I can't comment subjectively on how "awful" it sounds. However, the effect of mixing lowered and raised pitched versions of the same tone (possibly including the original unshifted tone) is the so-called "chorus effect" which is deliberately generated (at the player's discretion) in electronic and Hammond organs, and in effects boxes for electric guitars.

 

Done tastefully (your taste may vary!), this can add life and interest to an otherwise static sound. Overdone, it may indeed be "awful", or at least cloying after a few seconds. It's not quite the same thing as "wet" tuning, but listeners' reactions to/for/against it may be similar.

--Mike K.

[RatFace]

Assuming a 1m radius circle, and the poor thing gets swung once every second, the average speed is 6.3m/s.

Close. That's the average speed tangent to the circle. The average component of the speed in any fixed direction "to the side" of the circle is 4 m/s (across the diameter and back in 1 sec.). That's still almost 2/3 as much.

 

Sorry - but you're totally wrong Jim

 

The average speed (i.e. the integral of velocity magnitude over the period, all divided by the period), is 6.3 m/s.

 

The average velocity is zero: (0, 0, 0)

 

The average component of velocity in any one particular direction is zero (0) - there's no such thing as "component of speed".

 

As for the idea that the majority of what you head comes directly from the instrument in a straight line - well if you go to a concert and somebody sits in front of you and their head (which is probably quite a good sound absorber) obscures not only the concertina on stage... but the whole stage - does the sound change? Not noticibly, in my experience*. Similarly, if you sit to one side of a concertina player - say a couple of meters away- when they play notes on different sides of the concertina then yes, one is louder than the other, but it's not a vast difference. If they played two notes a semitone apart at once, one on one side and the other on the other, you could still easily tell it was two. So the sound intensity probably only varies by 10-20% (wild guess... but I would place money on it being less than 50%!) as you vary the orientation of the concertina and listener.

 

* OK to really do this experiment you'd need TWO people in front of you - one in front of each of your ears - but that's not often hard to arrange!

[JimLucas]

As for the idea that the majority of what you head comes directly from the instrument in a straight line...

You're right, it doesn't. But not just straight lines between reflections, either. If that were the case, the line in the Irish song -- "I sat myself down behind an old stone wall, just to hear two lovers talk," -- would be pure fantasy.

 

Without attempting technical explanations, I will simply observe that placing a barrier (e.g., an "old stone wall") between yourself and a sound source in a open field, with no straight-line reflections available (no birds overhead ) does not prevent you from hearing the sound emanating from the source. Whatever the technical explanation, "direct" transmission of sound isn't necessarily "straight line" in the mathematical sense. Though I don't dispute that reflections can be significant where reflecting surfaces are available, they aren't necessary either to hearing in general or to the "bells" effect.

[Robin Madge]

OK. I have just tried playing a single note while swinging the instrument in a 1 metre circle, and I hear the effect. I am not hearing a variation once per revolution but a sort of phase difference "flutter". I still hear it if use an earplug in one ear so it's not to do with having two ears. It could be to do with variations of interference patterns of reflected sound from the somewhat cluttered room I tried it in.

 

Robin Madge

[David Barnert]

Just remember this about the Doppler effect:

 

If a sound emanates from an object (a concertina, say) between two listeners (the performer and the audience) and can accurately be described as a Doppler effect (caused by the object moving closer to or further from the listeners), then it will sound different to the two listeners. The one it is moving toward will hear a higher pitch and the one it is moving away from will hear a lower pitch. If the motion of the object is perpendicular between them, then neither hears a Doppler effect.

 

This breaks down only when the object is sufficiently distant (more than an arm's length) that it can't really be described as "between the two listeners" (an approaching train whistle as perceived by listeners standing on opposite sides of a track, for instance).

 

Whatever you're hearing while you swing a concertina is either not Doppler-related or is very different from what the audience will hear at the same time.