Cotton Wool Plug at the foot of the Chanter

Pipemaking, reedmaking & maintanence. Expert pipemakers are eager to answer your questions

Sliver of cane

Postby GrahamRB » Sat Nov 12, 2011 9:48 am

Barry, I can only tell you what I saw ...I watched the maker cut and trim the sliver until it achieved his desired effect. That effect was not discernible to my ears; he did mention that it could be trimmed further to flatten or sharpen but I had only the one memory cell functioning on the day :lol:

I've read, with little understanding, about pipe fettlers using a 'rush' and wondered if the sliver is just one such example.

(...) downside is that if pipers will not make the effort to understand what makes their pipes so good when they are working well, they wont have a clue what to do when they fall below par.


With so many interactive variables I'd rather leave it to the cognoscenti than meddle. I wonder how many pipes have succumbed to fix it 'til it's broke :P
Graham
N. Cambs/S. Lincs
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Re: Cotton Wool Plug at the foot of the Chanter

Postby John Gibbons » Sat Nov 12, 2011 12:30 pm

An idealised plug, the one you want, is a perfect absorber. Its acoustic impedance should match the bore of the chanter.
A real plug, a physical lump of cotton wool will always reflect something.

John
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Re: Cotton Wool Plug at the foot of the Chanter

Postby John Gibbons » Tue Nov 15, 2011 8:36 pm

The impedance you need for the plug is the same as an infinite length of chanter -
if I did the calculation right, and you believe Wikipedia's numbers, that is 28 MPa s^3 m^{-3} at 20 degrees C.
That's based on a 2.15mm chanter radius.

A cotton wool plug is often very dense packed - particularly if it is a vandalised cotton bud.
Its impedance will be a lot higher than the chanter's.
It will be a lot more like a perfect reflector - though fluffing the cotton wool up will get its impedance,
and helpfully will smear out a sharp reflection.

However the smearing will be over only a few mm,
and for the incoherent reflection that Barry wants,
the reflection needs to be smeared over a large fraction of a wavelength -
tens of centimetres at least, more for the low notes.

Perhaps a few mm length of open-cell foam would be a better absorber than cotton wool?

Over to the experimentalists, I think!

John
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Cotton Wool insert

Postby Barry Say » Tue Nov 15, 2011 10:21 pm

Apologies one and all. I have been guilty of terminological inexactitude, or perhaps I have been bringing a chemists outlook to this physics problem.

John is perfectly correct in thinking of a cotton wool plug as a tightly packed mass of cotton.
John Gibbons wrote:A cotton wool plug is often very dense packed - particularly if it is a vandalised cotton bud.

I have never used a vandalised cotton bud in this fashion and wouldn't consider doing so. I only used the term plug because that was the word Colin Ross was using when I started pipemaking. As soon as I understood that the purpose was to damp out the reflections it was obvious that a tightly packed mass of cotton wool would be the least effective configuration, so I ensured that the part heading into the chanter was less dense and tapered.

I realise that this is far from the ideal John describes but it will certainly lower the Q-factor of the lower resonant chamber.

In Chemistry, we often used glass wool plugs in glassware. These were intended to catch solids, slow down liquids but allow gasses to pass. These were not tightly packed, hence my different usage.

Using inserts as I have described, I have never encountered problems on any pipes, providing the cotton wool is dry and not too tightly packed.

Barry Say MSc (Chomical Physics)

------
Physicist predicting the outcome of the Derby, "Consider a horse as a perfect sphere .....................
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Re: Cotton Wool Plug at the foot of the Chanter

Postby John Gibbons » Wed Nov 16, 2011 8:56 pm

Even a wispy cotton plug might well block off 50% of the cross-section of the bore - doubling the impedance in a space of a few mm.
That would give a massive reflection. I should be able to calculate the reflection coefficient in my head but am long out of practice.
The plug would have to be wispy, or we would get total reflection, and these reflections, if resonant, could affect the tuning of some notes.

John
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Re: Cotton Wool Plug at the foot of the Chanter

Postby John Gibbons » Wed Nov 16, 2011 11:44 pm

If the impedance doubles suddenly, say at a permeable cotton wool plug,
then the reflected (upward) wave would have 1/3 the amplitude of the incident (downward) wave.
This could easily be enough to mess up the tuning of a note,
and a change in the impedance of the plug, say if it became oily and less permeable to air,
would alter the reflected wave, increasing its amplitude.

John
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Re: Cotton Wool Plug at the foot of the Chanter

Postby Sqweezer » Fri May 25, 2012 9:09 am

John Gibbons wrote:This could easily be enough to mess up the tuning of a note,
John


That's exactly what happened with my chanter. D was miles out of tune and the rest of a scale was not too good as well. It works ok now as I removed the plug.

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