Resonance

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Resonance

Postby Barry Say » Tue Nov 08, 2011 11:46 pm

This may well be the first posting which will be moved to a new forum called: 'Science Stuff: Dont Bother if 'You're Not Interested''

If your eyes glaze over, don't worry, you don't need to know any of this stuff to be a good piper. On the other hand, if you want to be a good pipemaker or 'fettler' (I hate that word) a nodding acquaintance with some of this 'stuff' is far from unhelpful.

Vibration and resonance are different. When you stand in front of a disco speaker, you are experiencing vibrations. If you blow across the top of a Guinness bottle you are stimulating resonance.

When we wind our pipes, the chanter reed vibrates at many (or all) frequencies, which stimulates resonance in the bore of the chanter at a particular set of frequencies (harmonics). Which set of frequencies is determined predominately by the highest open hole on the chanter. So we have a vibrating reed causing vibrations in an air column and some of this leaks out to cause vibrations in the surrounding air which we hear as a note.

As well as the pulses of air coming out of the tone holes, some are going down the bore to the foot, hence the cotton wool insert, ball bearings, flea holes and all the other arcanae of chanter tuning that have come down to us from former generations.

At the same time, the fact that the chanter reed allows little packets of air through when it is open and then shuts like a valve, causes pressure variations in the stock which propagate as a wave back into the bag. No problem. If at some point up the bag there is a restriction which is sufficient to reflect the pulse of air back towards the reed , we have a problem! This forms another resonant cavity which can disturb the reed. This is what I call neck resonance.

Francis Wood wrote:Neck resonance in itself is not a problem and indeed is very much part of the sound of the instrument, together with the vibrations emanating from the bag. If you touch the bag, you will feel it vibrating; it is therefore radiating sound. This aspect of the sound is very noticeable when the listener is standing beside or behind the player, a sound lacking in the upper harmonics enriching the sound emanating directly from the tone-holes in front.
Francis
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Re: Resonance

Postby Julia Say » Wed Nov 09, 2011 10:01 am

If you want people to try and understand this stuff, you're going to have to explain it more clearly. In particular the difference between vibration and resonance.

You wrote:
"When you stand in front of a disco speaker, you are experiencing vibrations. If you blow across the top of a Guinness bottle you are stimulating resonance."

Possibly, but the disco speaker itself is resonating (I think), and the air in the Guinness bottle is vibrating (?) to produce that resonance. Confusing, that.

Having said that, the rest of the post makes sense, and the interesting bit - to me at any rate - is the final paragraph. I will wade through a certain amount of brain cell glazing information to get to the bits I can absorb, but not unless they are clearly explained.

Julia (playing devils advocate on behalf of other interested but science-challenged subscribers)
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Re: Resonance

Postby Barry Say » Wed Nov 09, 2011 9:06 pm

OK try some more.

The term resonance is used with two slightly different meanings.

One case would be represented by the face of a fiddle or guitar. The vibrations from the strings are transmitted through the bridge to the face of the instrument which is then said to 'resonate' in sympathy with the strings, amplifying the sound. In this case we would like to see the instrument respond nearly equally to all frequencies, so that no particular notes are favoured or inhibited. A converse to this is that the body of an electric bass guitar, is made far heavier than is mechanically necessary in order to avoid any resonance at all as this would inevitably favour some notes.

The other meaning refers to something like the guinness bottle, where the air column in the bottle has a natural frequency. Blowing over the top of the bottle produces all frequencies, but one of them matches the natural frequency of the bottle and this stimulates the air column and that frequency is amplified in preference to others and that is the sound we hear.

In the case of a smallpipe chanter, we have the air column between the reed and the soundhole containing a standing wave which is the sound we want. We have a second air column to the bottom of the chanter and we could have a third air column between the reed and some point in the bag neck where there is a constriction. If either of these last two have a natural frequency close to the note being played, then we could well encounter some difficulties.

For an illustration of resonance, try

http://www.youtube.com/watch?v=3mclp9QmCGs

Barry
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Re: Resonance

Postby John Gibbons » Wed Nov 09, 2011 11:11 pm

A loudspeaker is designed to respond uniformly at the frequency it's driven, across a wide range of frequencies.
This is what Barry means by (forced) vibration. The sound depends on the forcing rather than on the design of the speaker.

A Guinness bottle (why not Brown Ale?), when you blow across it, only responds at isolated narrow frequency bands.
This is what Barry means by resonance, and what instrument makers aim for in designing oscillators, of any kind, that play a well defined note.

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Re: Resonance

Postby Francis Wood » Wed Nov 09, 2011 11:48 pm

I'd have to have emptied quite a few Guinness bottles before I'd be found in front of a disco speaker. At this point I would expect to be encountering dissonance.

Barry's description accords with my own understanding of what's going on with the reed/bore association. I'd want to emphasise the fact that when pressure waves reach an open tone hole, some of the energy passes through to the outside and some is reflected backwards from that hole, causing the reed to open. The open-and-shut cycle begins all over again, the speed (and therefore pitch) of this depending on the dimensions, shape and quality of the bore, and more importantly, the density of the air it contains. This, of course, is why the Himalayan Piping Group play in B flat.

This is, admittedly, an over-simplification. Almost every acoustic explanation however sophisticated is a kind of metaphor because acoustical science is vastly complex and far more than I can cope with.

Francis
Last edited by Francis Wood on Fri Nov 11, 2011 7:46 am, edited 1 time in total.
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Re: Resonance

Postby Barry Say » Thu Nov 10, 2011 9:23 am

Francis Wood wrote: I'd want to emphasise the fact that when pressure waves reach an open tone hole, some of the energy passes through to the outside and some is reflected backwards from that hole, causing the reed to open.

Francis' description also fits with my understanding of the usual explanation for a resonant pipe. I would like to emphasise that when waves are being transmitted, any change in the environment along the direction of propagation can cause reflections. The open tone hole is one such change in the environment but the irregularities in the bore caused by the tone holes which are closed can also cause reflections, and in the case of drones, the cavity caused by lengthening the drone when tuning will also cause reflections. The reflection from the tone hole causes constructive interference, enhancing the note. Other reflections will probably cause destructive interference which could weaken the note. This destructive interference can affect the higher harmonics changing the tone quality of the note. I don't think it is possible to do any meaningful calculations on this system, but a general understanding of the processes can be useful.

When Francis says some of the energy passes through to the outside, he is echoing the general understanding of musical acoustics, but in the case of a chanter with a closed end, what actually happens is that a small volume of air comes out of the tone hole and creates a localised area of high pressure which travels out into the universe like a miniscule pneumatic tsunami. This is repeated at the frequency of the sound we hear. This is where the air from the bag goes and why we need bellows to keep the bag full.

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Re: Resonance

Postby John Gibbons » Thu Nov 10, 2011 10:54 pm

Francis Wood wrote:I'd have to have emptied quite a few Guinness bottles before I'd be found in front of a disco speaker. At this point I would expect to be encountering dissonance.

Barry's description accords with my own understanding of what's going on with the reed/bore association. I'd want to emphasise the fact that when pressure waves reach an open tone hole, some of the energy passes through to the outside and some is reflected backwards from that hole, causing the reed to open. The open-and-shut cycle begins all over again, the speed (and therefore pitch) of this depending on the dimensions, shape and quality of the bore, and more importantly, the density of the air it contains. This, of course, is why the Hymalayan Piping Group play in B flat.

This is, admittedly, an over-simplification. Almost every acoustic explanation however sophisticated is a kind of metaphor because acoustical science is vastly complex and far more than I can cope with.

Francis


Building (and tuning) a 1-octave set of Guinness pan-pipes is a skill which can only be acquired by long and assiduous practice! Mess it up and you have to start all over again....

The speed of sound depends on the ratio of the pressure to the density - so in effect only on the temperature. Equatorial pipe bands will play sharper.

Though the science is fiercely complex, the nice thing is that grossly over-simplified models are so effective and reasonably accurate.

John
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Re: Resonance

Postby Barry Say » Fri Nov 11, 2011 8:41 am

As a moderator of the list, I have moved the two posts on Cotton Wool Into another topic because I think It could form a discussion in its own right, and some of those who might not be interested in the science in this topic, would be interested in the cotton wool thread.

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Re: Resonance

Postby GrahamRB » Fri Nov 11, 2011 9:21 am

Barry could you include a link, please, to 'elsewhere' when threads are moved there? ;)
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