Bore roughness and tone

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david_h
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Re: Bore roughness and tone

Post by david_h »

Tunborough wrote: Mon Jun 28, 2021 6:33 pm... the DC flow and the standing wave can each go about their own business without interfering with each other, but in a rough bore the DC flow gets turbulent, with faster-moving spots in the eddies, and it starts to disrupt the standing wave because of non-linear effects. Anyone care to convince me otherwise?
No, but to convince me I need some hard science on the effect of turbulence on sound waves. Sound waves are pretty good at going on their way with a lot of other things happening in the air - in the context of music the other things happening include other sound waves going in different directions and in roughly the same direction. Has anyone detected an effect in the sound of listening through the turbulence downstream of the jet at the embouchure? The school textbook thing that happens with sound in moving air is refraction because of varying speed and direction of the air.

How do we know it is not something to do with the tiny back and forth particle movements making up the pressure wave when they are adjacent to the bore? Those are the movements that figure large in the descriptions of the physics of wind instruments.
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Re: Bore roughness and tone

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Hmmm, so my notoriously devious mind starts musing on "how can we test this thing?" And so I take my playing flute down to the workshop, and cut a piece of fairly coarse abrasive paper about 115mm long (4 & 5/8") to the right width, curl it around a piece of dowel and offer it up into the tuning slide protruding from the head, with the course side facing into the bore. Reassemble and test. Performance really bad, especially on low E and D. Triumphantly remove abrasive paper, recurl it the other way, reinsert and test. Performance just as bad, especially on low E and D. Pull out paper, reassemble and test, just to make sure, performance great. Hurrumph...

So that didn't work. Presumably the bore reduction caused by the paper either way messed too much with the acoustics of the flute. I didn't go on to test the tuning, which would confirm that diagnosis. So, the question remains: "How can we test this thing?"

[Prizes will be awarded to best and neatest qualifying entry. The judges decision will be binding. No correspondence will be entered into....]

The ideal test would be reversible and repeatable. Just in case I'm making this too easy.....
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Re: Bore roughness and tone

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Terry McGee wrote: Tue Jun 29, 2021 10:55 pm... Performance really bad, especially on low E and D. .
If roughness also effects the bell note you don't need a flute with finger holes. If the cylindrical bores also show the effect then you just need a tube. If it's surface texture - so only indirectly related to material - a PVC pipe would do. If lengths of PVC pipe can be tested to sound the same it doesn't need to be reversible.

How about making a head that fits the PVC pipe and try things like coating the insides of lengths of pipe with glue and blowing sawdust and such like up them?

I can't help thinking that this is related to the to the effect, subjective or otherwise, of oiling a flute.
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Re: Bore roughness and tone

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david_h wrote: Wed Jun 30, 2021 3:43 am
Terry McGee wrote: Tue Jun 29, 2021 10:55 pm... Performance really bad, especially on low E and D. .
If roughness also effects the bell note you don't need a flute with finger holes. If the cylindrical bores also show the effect then you just need a tube. If it's surface texture - so only indirectly related to material - a PVC pipe would do. If lengths of PVC pipe can be tested to sound the same it doesn't need to be reversible.

How about making a head that fits the PVC pipe and try things like coating the insides of lengths of pipe with glue and blowing sawdust and such like up them?
So let me test that I understand. You make a single head and two poly bodies that fit the head. You make the two bodies as identical as possible (not hard if from cylindrical stock). You test both to make sure they play identically, and perhaps tweak if needed to ensure identical performance. Once satisfied, one of the bodies becomes "the control" and the other "the test piece". You do mean things to the "test piece" to introduce and ideally measure changes in performance between the control and test pieces. Perhaps at the end, you return the test piece to original condition (dissolve the glue, sand and re-polish the bore, whatever), and confirm that control and test are again interchangeable (or at least document why that couldn't be achieved).
I can't help thinking that this is related to the to the effect, subjective or otherwise, of oiling a flute.
I think we are in the same ball-park. And possibly in the same ball-park as playing your dry flute in the session for a while, then swabbing out the excess moisture (that often builds up firstly as globules demeaning the performance), thus uniformly wetting the bore, not only restoring performance, but perhaps enhancing it? Or is that enhancement just you warming up? Sigh....
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Re: Bore roughness and tone

Post by Terry McGee »

Hmmm, I'm also thinking that one of those weird 19th century embouchure adapters would be convenient in an experiment like this. I'm thinking of the snap-on adapter that effectively turns a flute into a side-blown recorder. Unfortunately, I can't remember what they were called!

Ah, here's a modern variant:
Image

With one of those, one could standardise the embouchure, taking that out of the equation. One could then feed it with a standardised air flow rate, and then measure the output in terms of sound level and harmonic content. The performance levels difference between the control tube and the test tube would then be available for direct comparison.
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Re: Bore roughness and tone

Post by Peter Duggan »

david_h wrote: Wed Jun 30, 2021 3:43 am How about making a head that fits the PVC pipe and try things like coating the insides of lengths of pipe with glue and blowing sawdust and such like up them?
Isn't this effectively still an experiment in reducing bore volume as well as roughening it?
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Re: Bore roughness and tone

Post by Geoffrey Ellis »

Peter Duggan wrote: Wed Jun 30, 2021 7:22 am
david_h wrote: Wed Jun 30, 2021 3:43 am How about making a head that fits the PVC pipe and try things like coating the insides of lengths of pipe with glue and blowing sawdust and such like up them?
Isn't this effectively still an experiment in reducing bore volume as well as roughening it?
I would guess that if you are using a cylindrical bore for the test, then any constriction of the bore (reduction in volume) would not be significant if you didn't use a heavy hand with the coating, especially using at least a medium sized bore (19mm or more). So long as the adhesive material is not too thick. Something like thinned epoxy that could wet the inner bore, then introduce the material that is meant to rough up the bore.

I don't know how precise you'd need to be in the execution of such an experiment, depending upon what you are trying to "test". If you are trying to determine whether or not bore roughness affects tone and response, then to me that's a no-brainer. I've done this experiment in some form many times (and anyone who makes a lot of flutes from different materials is going to have observed something similar), and the results are quite obvious. Not delicate shades of nuance in most cases, but rather a hit-you-over-the-head sort of obviousness. But if you are micro-testing the degree to which said roughness impacts the tone/response, then that's another thing. In the first sort of test, you simply take a headjoint that is well made, cleanly finished, etc. as described above. Use some wood for the body rather than PVC, choosing one that is not naturally smooth when bored out. If I were doing an extreme version of the test I'd use some Pacific Quilited Maple that I have on hand. When you bore it, the inside bore is totally hairy--literally lots of visible fuzz standing proud of the surface. It's a total mess. Make two of these and don't bother with finger holes. Seal the outside of the bore thoroughly to make sure there are no air leaks through the wood (maple sometimes leaks). Then simply sand and finish one of the bores (I'd use my usual epoxy method). The result will be glaring, and the question of variations in the players embouchure and such probably won't come into play. That is to say that the difference is so substantial that you won't be wondering if you are just not blowing consistently--it might instead be a question of "I can barely produce a tone on this thing!". As I say, I've seen this effect so many times with so many different woods that it's not a question for me as to whether (or to what degree) smoothness effects performance. It absolutely does make a difference, and the difference is big.

Of course one can also test nuances by using some wood that is not as awful as the quilted maple. Black walnut would be a good choice, but there are others that will also bore fairly clean but will still show an obvious difference between the finished and unfinished (smooth versus raw) bore.

Determining where the difference starts to vanish is a different sort of experiment and I think that the use of something like PVC would be a good way to test it. Introducing roughness into the bore in small amounts to test if/when you can start to hear it or feel it as resistance. Such a test as that would probably be helped by some type of embouchure control device like Terry suggests. I do think that there is a gray area on the spectrum of smoothness where one will get acceptable performance characteristics and still be able to perceive small differences in timbre, and (as was said earlier in the thread) this is undoubtedly the reason for most makers using a limited number of woods, focusing on those that machine cleanly enough that they will get good results without having to take heroic measures.
Last edited by Geoffrey Ellis on Wed Jun 30, 2021 9:42 am, edited 1 time in total.
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Re: Bore roughness and tone

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Peter Duggan wrote: Wed Jun 30, 2021 7:22 am
david_h wrote: Wed Jun 30, 2021 3:43 am How about making a head that fits the PVC pipe and try things like coating the insides of lengths of pipe with glue and blowing sawdust and such like up them?
Isn't this effectively still an experiment in reducing bore volume as well as roughening it?
Yes, but you could also keep adding layers of glue, without a roughening agent, to a tube and see what effect that had. If there was an added thickness that sounded like a roughened tube it might suggest that the mechanism was related.

One could also say that Geoffrey polishing the bore was increasing the bore volume.

Oiling has the effect of a small decrease in bore volume but it sounds like the effect on tone is similar to that of polishing.

Could this also be related to the "lined heads give a brighter tone that unlined heads" discussions?
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Re: Bore roughness and tone

Post by Geoffrey Ellis »

david_h wrote: Wed Jun 30, 2021 8:58 am
Peter Duggan wrote: Wed Jun 30, 2021 7:22 am
david_h wrote: Wed Jun 30, 2021 3:43 am How about making a head that fits the PVC pipe and try things like coating the insides of lengths of pipe with glue and blowing sawdust and such like up them?
Isn't this effectively still an experiment in reducing bore volume as well as roughening it?
Yes, but you could also keep adding layers of glue, without a roughening agent, to a tube and see what effect that had. If there was an added thickness that sounded like a roughened tube it might suggest that the mechanism was related.

One could also say that Geoffrey polishing the bore was increasing the bore volume.

Oiling has the effect of a small decrease in bore volume but it sounds like the effect on tone is similar to that of polishing.

Could this also be related to the "lined heads give a brighter tone that unlined heads" discussions?
I believe that in the case of bore polishing, since there is a sealing agent involved (epoxy in my case) that they sort of cancel each other out. The epoxy penetrates the wood and might shrink the bore ever so slightly, but the sanding and polishing removes that additional material. In essence, the epoxy fills up any cavities on the surface of the bore, seals the wood, and then the polishing trues up the surface, ideally giving you the bore diameter that you were shooting for. If one adds a final layer of thin epoxy, it will technically shrink the bore, but we are talking about pretty small differences here. A few thousandths of an inch, which while it might (on paper) be thought to have some acoustic impact, in reality it's not enough to notice. If you keep going, building up the finish to where you can reduce the bore by ten thousandths or more, then it starts to be perceivable. In fact, differences in bore diameter of a mere ten to twenty thousandths of an inch can define an optimal bore diameter for intonation. I've run into this a lot with my xiao designs (which have cylindrical bores). I've spent years messing around with bore diameters for given key, trying to find that sweet spot where you get the characteristic tone and response with the added benefit of better intonation in both octaves. It's just like what Theobald Boehm discovered--he preferred his flute design with a 20mm bore instead of a 19mm one. But he constricted it to get better third octave tuning, trading down on sweetness of tone.

A metal liner is smoothness taken to the extreme, hence the really bright tone. But it's possible to get an almost identical level of smoothness using the epxoy/re-bore method on a headjoint as well.
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Re: Bore roughness and tone

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I'm no scientist, but if you want to remove as many variables as possible, you might want to consider doing a test on a tube without fingerholes. At least at for the initial stage.

If you use PVC, you can probably use a single headjoint(as mentioned above) and two bodies. Perhaps you can rough the inside with a stiff wire wheel on a mandrel. Chuck it up in a drill and rough up the bore on the test body. If you can avoid removing a lot of material, the inner volume should be roughly the same as the [smooth] test body. I think it is more important for the bores to be the same length than the bores to be precisely the same diameter.

IMO, if you want to test again with fingerholes, cut them after the initial test. Maybe even one at a time.

I'm also thinking about embouchure. Are we testing the voice potential of the two flutes or are we testing the two flutes with the exact same airstream? Do you want an adaptable player to see what the flutes are capable of? Or are you trying to compare the two flutes with the exact same airstream/embouchure? I would surmise that the two flutes would have different airstream/embouchure needs.
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Re: Bore roughness and tone

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Peter Duggan wrote: Wed Jun 30, 2021 7:22 am
david_h wrote: Wed Jun 30, 2021 3:43 am How about making a head that fits the PVC pipe and try things like coating the insides of lengths of pipe with glue and blowing sawdust and such like up them?
Isn't this effectively still an experiment in reducing bore volume as well as roughening it?
Quite coincidentally, I can put some numbers on this. Turns out that this morning's first job was polishing the bore of a new keyless I'm working on. I like to artificially raise the grain after reaming but before oiling, so that it won't be raised later by playing. Rather like the old cabinet makers would do the same to their furniture before French Polishing. I measured the bore diameter in it's roughened state, at 19.06mm. I chose laterally just at the embouchure hole to make sure I could easily repeat it. Then went through the polishing process and remeasured the same spot. I got 19.09mm, an increase of just 0.03mm (30 microns) or 0.0012", barely over 1 thou! In proportional terms, 0.0016% of the diameter. So we're not talking about a lot of material here. I imagine seasonal movements would easily exceed that.

It would be more interesting perhaps to do the same measurements on a degraded 19th century original, although of course they tend to have lined heads, so you'd have to do it on the tapered section, which then makes repeated measurements more difficult, as variation due to the rate of taper will exceed the amount likely to be taken off in polishing! Grumph. Might have to wait for a French original, with a partially lined head.

Looking at the raised grain I was dealing with, it's really very fine, more like flour than say sawdust. Very obviously degraded to the eye, and to the fingertips, although I suspect I'm sensing friction there, rather than static roughness. I guess if my old fingers can easily detect friction, tiny air particles will get lost in it.

It shines now, and feels silky smooth. Enjoy, air particles, enjoy!
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Re: Bore roughness and tone

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Terry McGee wrote: Wed Jun 30, 2021 7:26 pm... friction, tiny air particles will get lost in it.
It shines now, and feels silky smooth. Enjoy, air particles, enjoy!
I get the impression that no-one in this discussion has serious disagreements with Geoffrey Ellis' observations given in the third post of the thread. Smoother bores give a 'brighter' sound.

If experiments with smooth and rough tubes didn't confirm this I suspect we would look for a defect in the experiment rather than doubt the observations of makers and players. Experiments would, however, add some rigour to the observations. With appropriate instrumentation, they would help quantify things like, for example, 'a darker timbre'. I take Geoffrey's mention of non-linear effects and Loren's to loss of energy to be references to the volume of some harmonics changing more than others. If not what is timbre?.

I suspect if there was some data on the change in the spectrum with different bore surfaces we could approach a physicist with experience of these things (maybe any physicist) and they would turn to their blackboard and chalk up some equations covering what the tiny particles might be doing. So far as I can find no-one who knows about that stuff has put a summary anywhere online. If they had I suspect Terry would know about it an tell us. Maybe it's buried in the online notes for an acoustics course somewhere.

I chimed in earlier because I thought that wavelength was getting too much focus and amplitude not enough. I have given up trying to find online a worked example with a figure for how far the tiny particles are moving backwards and forwards to produce the pressure changes we hear when playing our flutes. I brought up loudspeakers because it has to be in the same ballpark as a moving diaphragm that would produce the same volume - millimetres or less. The sideways movement of the 'air reed' just beyond our lips, or the physical reed of reed instruments is much the same.

Does anyone know of a source for harder information about what the tiny particles are doing? Or how the bulk flow of 'waste air' down the tube interacts with this?
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Re: Bore roughness and tone

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I did ask Joe Wolfe, Emeritus Professor of Physics at UNSW if he was aware of anything on this topic. He reminded me that I made a Prattens in pinus radiata (our local plantation timber) which met the criteria for roughness of bore. You can see my article on it at http://www.mcgee-flutes.com/pine_prattens.htm

Indeed, his former PhD student at the time, Paul Dickens, even carried out acoustic impedance measurements on 5 different pine tubes I also made, testing the effects of moisture and oiling on them. He reports (in his thesis):

Exposing the bores to warm, humid air (for prolonged or minimal duration) raises the grain
significantly. The acoustic effect of this is seen in the impedance spectra of the two un-oiled
but humidity-treated pipes. In Figure 6.4 (pipe 3) both impedances measured after treatment
show a greater attenuation and the maxima and minima are shifted to the left in frequency.
This is presumably due to the extra compliance of the rough surface. In Figure 6.3 (pipe 2) the
attenuation decreases during treatment (red trace) and increases after the pipe has dried out.
This shows that the roughness of the raised grain is ameliorated when the surface is wet since
the water fills the bores of the wood.

The acoustic impedance spectra of the oiled pipes show the same general trend as those
of the un-oiled pipes. Here however, the impedance spectra before and after treatment are
roughly the same. This is despite visible raising of the grain caused by the humidity treatment,
suggesting that the oil fills the pores of the wood and compensates for the extra attenuation
caused by the raised grain. In Figure 6.5, we again see that the attenuation is significantly reduced
when the wood is wet.


He concludes later:
In this study there seem to be three different contributors to the effect of surface conditions on
acoustic impedance spectra. These are the macroscopic roughness (increased by humidity),
how wet the surface is (increased by humidity) and whether the wood is oiled. These contributors
are entangled in this study, and an attempt should be made to look at them in isolation.
In future work it may be useful to repeat this experiment with wood more suited for flute
making. Any changes in acoustic impedance spectra would likely be of much smaller magnitude
than found in this study, but would be more relevant to players and makers of wooden
flutes. Such work may well answer questions about the best way to oil and play in a new instrument,
especially if combined with surface microscopy (perhaps using ESEM).


So, to summarise, yes, we could see from Paul's measurement increased loss and a change in resonant frequency in pine bores that had been really badly treated. And support for the theory that providing the moisture isn't coagulating into blobs, it does help "lubricate" the way for the vibrating air column. But we have to keep in mind that pine is possibly the worst timber for making flutes from (which is why we chose it for this exploratory study). As far as we know for now, a proper study is yet to be done.
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Re: Bore roughness and tone

Post by david_h »

Thanks Terry. That seems clear and I think it would be surprise if the changes were not in the same direction, though smaller, for flute timbers.

So. If our flute bore does not look shiny and we prefer the sound or the way it plays just after we have oiled it is there anything do-it-yourself and not too irreversible that we could do? I am thinking maybe of a good buff up with a cotton or linen cloth, a drying oil rather than the usual for once and another buff up when dried. Wax polish inside? Or do those things risk a gunky mess?
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Re: Bore roughness and tone

Post by Terry McGee »

I think you can go beyond cotton and linen without too much risk to the bore. Remember these woods are extremely dense and hard, they are not easily damaged. I can't see much harm in using some reasonably fine steel wool, perhaps wrapped around a bit of dowel, spun in a power drill. That should subdue reasonable levels of degrade and bring back a finish. If that doesn't restore the finish, use some fine abrasive paper, held in a slot in the dowel. I use 240 grit followed by 400 grit, followed by fine steel wool for well degraded cases.

But consider the potential risks to the flute in general. Imagine it's a keyed flute, and the spinning rod jams and whips the section of flute out of your hand and starts it flailing around, slamming into bits of furniture. Within a few seconds, you've reduced your left-hand section to a cluster of bent silverware and cracked blocks. Nasty.

So, be careful. Don't undertake anything you are not confident you can control.

Back here at Central Command, I have developed a lot of tricks to make this easier and safer. I'd routinely take all the keys off first. I spin the pieces of flute in the lathe, using all sorts of contrivances I've made up over the years to hold the pieces securely. I use slotted dowels with abrasive papers and steel wool as I judge are needed, safe in the knowledge that if they jam, I can just let go, stop the lathe and safely remove the rods. (In practice, it doesn't tend to happen, but it's nice to know there's a way out if needed!)

So don't let me scare you, but do think it through and proceed carefully. Remember always that those fingers would come in handy later playing the flute, so don't put them at risk either!

And keep in mind, that if you take off the top surface of the bore, you need to oil the freshly exposed wood thoroughly and let it dry before the next session*!

*Session, noun, Anglo-Celtic. Refers to a meeting of musicians brought together with the aim of playing music, drinking Guinness(TM) and enjoying "the Craic" (see also). Arguably archaic, pre-Covid.
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