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How Do Concertina Reeds Go Out Of Tune?


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#1 richard

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Posted 28 November 2017 - 03:02 PM

Hello

 

I apologize if this has been discussed before(I couldn't find such a discussion) but what is the mechanics of how concertina reeds go out of tune?

 

The idea doesn't make sense to me. If a reed is in tune when leaving the original shop how does it get to the state where it needs metal (mass) removed from it in order to sound the same frequency it originally did?

 

What is wrong with me?

Richard



#2 Jody Kruskal

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Posted 28 November 2017 - 04:25 PM

Aside from other causes, reeds can get dirty from corrosion. Also dust and contaminants can accumulate where the air passes by them and slowly build up over time. It used to be that tobacco smoke would commonly dirty the reeds and reed shoe passage edges.

 

It's easy to clean some ot this by slipping a peice of paper under the reed and then dragging it out along the reed edge. If that causes a dark mark on the paper then you can see that junk has been removed. I've found that minor tuning problems can often be fixed by this treatment.


Edited by Jody Kruskal, 28 November 2017 - 04:26 PM.


#3 Anglo-Irishman

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Posted 28 November 2017 - 05:12 PM

I agree with Jody. I addressed this phenomenon in an earlier post: Don't panic!

 

I had a reed that dropped dramatically and progressively in pitch over a few days. I took it out, cleaned the crud off the slot, and it was OK.

 

Cheers,

John



#4 Jody Kruskal

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Posted 28 November 2017 - 11:58 PM

I guess that I forgot to mention the obvious... if a slight accumulation of gunk slows the reed down without stopping it entirely, then it will likely be playing flat.

 

On the other hand... If a reed suddenly goes flat, it is probably cracked and needs replacing.

 

Free reeds are remarkably stable in pitch but traditional reed shoes are generally brass which is somewhat flexible. The reed shoe could be too tight or too loose in its slot through temperature or humidity variations. So, simply sliding the reed shoe out of its slot and pushing it back in can also correct many tuning issues.

 

If cleaning the reed and re-seating the reed shoe does not do the job... before getting out your file, try replacing the valve. The valves can shift position over time or lose the springy snap of new leather. A new valve is much simpler to install than tuning a reed. I don't know exactly why the valves effect the pitch, but I'm told that they do. What I do know is that the whole system of reed, slot, valve, chamber and end enclosure all work together to create pitch and timbre.



#5 alex_holden

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Posted 29 November 2017 - 02:23 AM

Another possible issue: the set of the reed (the size of the gap between the reed tongue and frame at rest) has an effect on its pitch, and the set of a new reed can change a little when it first 'breaks in', as the flexing of the tongue cause the stresses in the steel to even out.

The pads seem to have a slight effect too - I'm not saying you should change them just because the instrument has gone out of tune, but the effects of age on the leather and felt of the pads could be another possible contributing factor in the pitch drifting over time.

Leaks between chambers can cause odd effects too.

#6 wayman

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Posted 29 November 2017 - 04:46 AM

I'm not seeing how the pads affects pitch: when a pad is down, there's no note being played. When a pad is lifted, I wouldn't think the pad is having a meaningful effect, being 1/8-1/4 inch above the hole. Sure, the air is flowing around it ... but does the material of what air is flowing around affect the pitch? At most I would think the condition of the leather or felt might have a subtle muting or amplification or timbre-changing effect.

 

But, this is one of those things I've never tested experimentally. And I know well that, with concertinas, much can be demonstrated experimentally that remains difficult to precisely explain!  :wacko: So if you've seen this happen, I don't doubt it, I just don't understand it.



#7 alex_holden

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Posted 29 November 2017 - 05:32 AM

I'm not seeing how the pads affects pitch: when a pad is down, there's no note being played. When a pad is lifted, I wouldn't think the pad is having a meaningful effect, being 1/8-1/4 inch above the hole. Sure, the air is flowing around it ... but does the material of what air is flowing around affect the pitch? At most I would think the condition of the leather or felt might have a subtle muting or amplification or timbre-changing effect.


I could be wrong, but an instrument I was working on seemed to go slightly out of tune when I adjusted the action to make the button heights consistent, thus altering the heights the pads lifted to.

#8 wayman

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Posted 29 November 2017 - 05:52 AM

The height of the pad changing, that makes more intuitive sense to me, as that actually makes a small change to the path the air is traveling. I can see there being a subtle effect on pitch from that.



#9 Chris Ghent

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Posted 29 November 2017 - 07:03 AM

Going back to the OP's question, I have heard a theory the bending action throws off sub-atomic particles at the bending point, which over time adds up to enough lost metal to shift the pitch. The only thing which worries me about this is it would mean lower reeds would drop in pitch, so far so good, but higher reeds would increase in pitch and this has not been my experience. 



#10 Wolf Molkentin

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Posted 29 November 2017 - 07:24 AM

Chris, maybe I'm slow-witted here, but why should this approach not apply to higher reeds in the same way as to lower ones?

Best wishes - Wolf

#11 Rod

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Posted 29 November 2017 - 02:03 PM

I have been playing my 36 button, steel-reeded Anglo virtually daily, solo, for 38 years now, and my ear has yet to detect any necessity for any re-tuning. I am getting the impression that there are those who are less fortunate.

#12 d.elliott

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Posted 29 November 2017 - 03:37 PM

There are several potential causes for reed pitch drift, or just one off changes, some of which have been mentioned above. The pitch is set by the combination of the 'spring' in the belly of the reed in combination with the mass of the moving reed tongue section. It is also influenced by the damping and reinforcement from sound wave reflections resulting from the dimensions of the acoustic chamber. Finally the air flow through the system, from the reed tongue gap, out past the open valve and through the open pad hole. The last variable is the mechanical couple between the reed frame and the reed pan, how tight/ loose it is. I also ought to mention the stresses in the spring area of the reed can affect pitch if they are re-distributed through the reed section

 

We file the belly of the reed to flatten the pitch, and we file the tip to sharpen the pitch, so clearly muck on the tip will flatten the reed, or corrosion pitting in the reed belly near the reed root can weaken (flatten) the reed too. 

 

we know that a reed sound about 5 cents sharper out of the concertina than when sounded in the assembled concertina, this is the effect from the acoustic chamber, I have been told by very respected individual that if the pad lift height changes the reflective characteristic from under the pad and can influence pitch. I can't say that I have ever come across this myself, but I report it for completion's sake.

 

Air flow, or resistance to air flow, however, is very obvious. As valves stiffen with age then they can have the effect of dampening reeds, equally if the tongue set or gap is wrong the reed pitch as well as response can be affected.

 

When you file a reed you take off a surface layer, that surface layer held stresses which maintained the shape of the reed in the reed frame. File the reed and it curls away from the surface you just changed. We can talk about the neutral axis shift but please just accept that it happens. The curl closes up the reed gap so you need to carefully bend the tongue to re-gap the reed. This action takes the steel through it's elastic limit and re-sets the stress distribution in the reed. by so doing you have now slightly flattened the red again. 

 

However the most common cause over time, is the phenomena of work hardening and age hardening. With continual vibration and springing the crystal structure boundaries of the reed tongue start to be more fragmented causing the metal to harden a little. this makes the reed 'spring' stronger and more difficult to move under the influence air flow. A situation which can flatten the reed pitch. Work Hardening can also initiate micro cracks and hence metal fatigue. Brass is more susceptible to age hardening but steels can change in time too.

 

So yes, reeds can change pitch with time, a long time. This drift is usually through muck or corrosion, It can also be metallurgical but in steel reeds this is usually minimal. Most of the pitch influences are set by the instrument build design, acoustic chambers, materials used, pad hole diameters etc. so they are constants anyway. Then there are the things which we do to reeds that can deliberately or inadvertently change pitch, but that is not pitch drift. Although the playing in after a re-tune can cause pitches to drift a little as they settle down

 

In my work flow I always clean the reeds first, then replace the valves. Only then do I measure tuning. New valves and clean reeds can reduce the amount of filing needed. I always resist tuning an instrument with old or obviously not too new valves. However, that is a client decision in the end. Finally I usually play the instrument a bit before admitting that it is complete, especially really nice ones, and really, really very nice instruments take longer to play in. A phenomenon yet to be understood.

 

Dave



#13 Chris Ghent

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Posted 29 November 2017 - 05:03 PM

Chris, maybe I'm slow-witted here, but why should this approach not apply to higher reeds in the same way as to lower ones?
Best wishes - Wolf

If the particles were thrown off at the point where the reed bends the most then this would weaken the belly in lower reeds and they would drop. In higher reeds the tip would be lightened, as that is where they bend the most, leading to the reeds raising in pitch, which is not my experience.

#14 Wolf Molkentin

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Posted 29 November 2017 - 05:25 PM

Chris, maybe I'm slow-witted here, but why should this approach not apply to higher reeds in the same way as to lower ones?

as that is where they bend the most
Never would have figured that - got the concept now - thank you for your reply Chris!

Edited by Wolf Molkentin, 29 November 2017 - 05:26 PM.


#15 gcoover

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Posted 29 November 2017 - 07:03 PM

And I can attest to the importance of doing the valves first - several years ago I retuned a nice Aeola and then afterwards decided to go ahead and replace the valves and that knocked the entire tuning out by about 5 cents or so.

 

What say the cnet Brain Trust about reeds that go sharp? I've got a hybrid Anglo with a low G (on the pull) that after nearly 20 years of perfect pitch has decided to go about 10 cents sharp (ouch!). I've opened it up and nothing about the reed looks out of the ordinary. No crud, no cracks, no valve weirdness. All ideas and suggestions welcome!

 

Gary






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