Effects of Thread Wrapping series:



Well, about now, we need to sit back, absorb all we've found and wonder what it all means!

Not just the top tenon...

I've been concentrating in this article on the top tenons of all these flutes, probably because it's that tenon that has the most profound effects.  A constriction at this point of the flute will effect all the notes, but in different directions and to differing degrees.  Constrictions in the lower tenons will mostly impact on notes there and lower.

But it needs to be remembered that most of these flutes are also constricted where the LH meets the right, and where the body meets the foot.  So we can expect more disruption to tuning and, in the more strangled cases, to performance.

Compression at the lower tenons is actually easier to detect, as it forms a single point of inflection, rather than two.  There is also no chamber formed, until you plug in the mating section.

Serial Strangulation?

Now, let's just imagine that the test tenon on our make-believe flute had suffered the kind of distortions we've seen above.  It had started out with a thread wrap of 20.6mm, which would have been exactly what it needed for a nice snug fit in the socket.  But that wrap has now reduced to 20.25mm in diameter, 0.35mm too small for a snug wrap.  Indeed, well before this stage, the joint would be sliding about uncontrollably, and even leaking.  So what would we do about that?  Put on 0.35mm more thread of course, or perhaps even remove the old thread and start again.  Either way, we now have a tight grip on the tenon again, and an even stronger band of thread.  The game starts over!

Remember my Rudall Carte we met right at the top?  Black thread covered by some additional white thread.  Only mild compression so far, but clearly we're on the way...

Isn't it one of those cruel ironies that, the more you look after your flute, lovingly changing or augmenting the lapping when it gets a bit loose, the more likely you are to kill it!  I'm reminded of Oscar Wilde's terrifying poem the Ballad of Reading Goal.  "Yet each man kills the thing he loves..."  Indeed, the very benefit touted for strung flutes, that seasonal variation can be taken up by the owner, might in fact be the invitation into the strangulation cycle.

The Good Old Days?

Was it better in the Good Old Days?  Some have suggested that perhaps threads were softer then.  Glancing at the graph at the top though seems to dispel that hope.  The most damaged bores were flutes from the 18th and 19th centuries, and the baroque instruments in particular would not have seen much if any use until the Early Music Revival in the 1970's.  By then the Schuchart was in museum hands, yet it it one of the most damaged.  The Richard Potter is missing several keys, and has probably not been played for 100 years or more.  There was nothing soft about the thread I took off it.

Duplicated Distortion Doubled

Imagine this ghastly scenario.  A well-intentioned modern flute maker faithfully copies a period flute without making allowance for the thread-induced bore compression.  So the copy is also distorted.  But then the maker also uses thread to wrap the copy's tenons.  After some time, it starts to work its ugly magic, and the copy is now more distorted than the original!  I'm told it happens regularly in the early flute field.  Perhaps it happens in our field too?

Clearly, we must understand bore compression and take steps to guard against it.

What about re-reaming?

Some have suggested that a flute displaying signs of constriction should simply be re-reamed to remove the offending in-growing tumour.  A moment's thought though should ring warning bells.  When you ream off the protrusion into the bore, you're further weakening the tenon wall.  Carry on like that - putting more thread on the outside and reaming off the wood on the inside - and the two will finally meet.  Probably with a bang!

A cause for instability?

Sometimes you come across flutes (and not just old ones) where one or more notes are unstable.  This can take several forms, but a common one is a warble, probably caused by the vibrating air column vacillating between two competing resonances.  It strikes me that, given the nature and degree of tuning change the computer modelling suggests is likely, that strangulation could easily shift the modes enough to create instability problems where none previously existed.  Could be a good thing to keep in the back of the mind to check for the next time one comes across such a case.

Hard D rehardened?

One of the noticeable improvements on the performance of the strangled cocus flute was a very significant improvement to bottom D.  Where it had been really quite wuzzy, it was now crisp, focussed and strong. Yet the computer model predicts very little effect on the D notes (see graph reproduced below).  How could this be?

Bottom D is a note rich in harmonics, as we confirmed in Flute Tone Investigations - Analysing an existing recording, and you can prove easily to yourself.  Finger xxx xxx, you should be able to blow the harmonic series D4 (low D), D5, A5, and D6 quite easily.  By comparison, how many harmonics can you blow on xxo ooo? 

Further, in the Irish style of playing, as we discussed in Getting the hard, dark tone, we purposefully shift the energy away from the weak fundamental note (low D) and into its harmonics to give the illusion of the piper's "hard" D.  But that's only going to work if all the harmonics are in good tune.  Indeed, if they are not, they are not harmonics, by definition, but rogue resonances.  If not in tune, they fail to coordinate the return of their contribution to the pressure wave back to the embouchure hole, and jet switching efficiency drops.

The modelling predicts that the D notes would be unchanged by the strangled bore under the tenon, but the third harmonic, A5 is near the peak of the affected notes.  Twenty-odd cents flat is probably enough to pull the contribution of the third harmonic out of the mix (we have more work to do to define the acceptable window, but 20 cents is probably close, and that really means within +/-10 cents, depending on how accurately the window was installed in the first place).

To confirm the importance of the third harmonic in the mix, check out the analysis below of notes played by the late Paul Davies in the Lament of the Three Maries.  D4 is the navy blue trace.  You'll see that the fundamental (D4) in the H1 column is down at about -17dB.  Very weak.  But the 2nd and 3rd harmonics (D5 and A5) are up at -4dB.  They are shouldering the bulk of the load.  The 4th harmonic (D6 - our high D), is down at -10dB and it's all downhill after that.  So, if strangulation can dramatically diminish the support of the equal front runner, no wonder our low D is in some trouble!

Let me say that this hypothesis needs to be further tested before we can rely on it.  I did test it on Prof. Neville Fletcher, who responded that it sounded "exactly right" to him.  Encouraging!  It does seem to fit the facts, and to explain the prima facie paradox between the modelling and reality.  The current version of the computer model does not allow for viewing harmonic coincidence, but it's high on my wishlist, for obvious reasons.  It could offer a far better glimpse into the invisible workings of the flute.

There are important ramifications of this observation:

  • it may explain why many old flutes still do not have a satisfactory low D even after ruling out all the other possible culprits - leaking pads, misplaced stoppers, badly cut embouchures, etc.  Testing for bore compression, and being able to do something about it, become part of the restorer's job description.

  • What applies to low D will probably apply to notes nearby.

  • We need to keep in mind the contributions made by the harmonics when applying computer modelling to performance issues.  This will be made easier, indeed semi-automatic, when a means for viewing the harmonic alignment is built into the modelling.

  • What applies to the top tenon strangulation may apply to strangulation of the other tenons, but will have its influence elsewhere in the flute range.  We can use computer modelling to identify the likely places to look, then before-and-after RTTA to confirm.

  • Strangulation might be an appropriate term for a murderous constriction just below the head, but is less apt for constrictions around the middle and just above the foot.  I need some new terminology! 

  • This issue could apply to new flutes too, where the new design is based on an original with bore compression. 

  • it might mean that a better home-test for strangulation might be acoustic rather than measurement-based.  Identify those notes most likely to be affected by compression under each tenon and test them.  Computer modelling will suggest them to us, but we need to confirm the results in the real world.

Aha, a mystery explained?

Every now and then, the restorer or repairer is confronted by an intriguing mystery.  An old flute with two or more sections joined together inseparably.  The joined sections are free to rotate, but will not pull apart.  What clearly has happened is that the entire thread-band has "glued" itself to the inside of the socket, but separated itself from the bottom of the thread trough it was originally wrapped around.  The tenon can rotate inside the thread-band, sometimes very freely, but cannot be extricated. 

It's not hard to imagine why two parts of a flute may get jammed together, but it hasn't been clear why one of them is able to rotate freely inside the other.  Now we can see why. 

Imagine a flute has been played for some good time, is thoroughly wet inside, and the owner has tumbled into bed without pulling it apart and mopping out.  (Obviously, the flute had to be left together for the parts to become so firmly attached.)  During the night, the moisture continues to soak into the tenon, swelling it further and further, expanding the thread band as much as its limited elasticity will allow and pressing it all the more firmly to the inside of the socket.  Perhaps the wood shoulders at the sides of the thread band are also swollen enough to jam them against the inside of the socket.  Whatever, in the morning, the flute owner finds the flute thoroughly jammed and has little choice other than to leave it some days for the swelling to abate. 

Unfortunately, the outer layer of thread really adheres firmly to the inside of the socket, possibly for a number of reasons.  Perhaps breath condensate is acting as a glue.  Or whatever oil or finish has been applied to inside the socket is the adhesive.  Or something that has been applied to the thread to improve its water resistance or to make assembly easier.  Or an unfortunate combination.  Whatever, the thread forms a permanent attachment. 

But now here's the interesting bit.  Finally, the flute dries out and the tenon shrinks.  The crushed tenon wood now releases its attachment to the inside of the thread band, but the thread-band remains adhered to the socket.  The joint with the tenon is now free to rotate within the thread band, but is not free to be withdrawn.  The same action that crushes our tenon, and causes strangulation of the bore, is what separates the thread band from the bottom of the thread trough.  It will be interesting to check the next jammed-but-rotating flute for bore compression!

Follow up: I've just removed the thread from the lower end of the Richard Potter flute.  The outside of the very old thread was very compressed - indeed polished and shiny.  But, as I started to remove it, I became aware that the rest of the thread was loose in the trough - I could rotate it very easily.  This confirms the mechanism above - the tenon wood had expanded as far as the thread would let it, when wet, and then retreated as it dried out, leaving the thread pressed outwards.  Most interesting!


To summarise, let me advance this analysis.  For tenon-wrapping (cork or thread) to work it must just firmly fill the void between the socket and the tenon.  Any less and it will leak or not hold the flute together satisfactorily.  Any more and it will put unacceptable outward pressure on the thin wood of the socket.  But the tenon wood is going to get wet during playing, and will want to expand.  If the tenon wrap is thick-enough cork, it will have that room, provided by the resiliency of the cork.  But if it is thread it will not, and the wood of the tenon will be compressed.  Even if the thread wrapping is applied loosely, it will be compacted by the same expansion-contraction wetting-drying cycle, until more thread is needed to secure and seal.  Once enough thread is applied, the pressure goes onto the wood. How can it be any different?

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  Created: 23 January 2011; last updated 13 April 2011.