Bore roughness and tone

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Re: Bore roughness and tone

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I believe horsetail was also used for scrubbing pots back in the day.
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Re: Bore roughness and tone

Post by skap »

Some time ago I asked here about bubinga wood. Despite uniformly discouraging answers,
I still bought the baroque one-keyed flute I was looking at (mainly because
it was a bargain, but also since I already trusted the maker). When the
instrument arrived and I saw (and felt with my finger) how rough the bore was,
my heart sank. I still oiled it and left it like that for many hours, I
wasn't inpatient to try it anymore. When I finally tried it, though, I rejoiced:
it was responding beautifully. Daily oiling the first 3 weeks and vigorous wiping
after playing made the bore much smoother, then I even used some extra fine/soft
scouring metal sponges that I had available, to get rid of that fluffy feeling
that the bore had in some places. It still can be characterized as "moon surface"
today, I guess. If it caused some improvement in response, it is imperceptible to
me, but I was fully satisfied from the start, so... I have the exactly same model
in Delrin, made by the exactly same maker: I don't feel any difference in response,
but the tone is entirely different, and the bubinga's tone was really a relief for
my ears after Delrin: mellow and rich, a tone you never get tired of (which I
cannot say about Delrin, unfortunately).

By the way my Delrin flute's bore is not particularly smooth, probably on purpose,
but I doubt it has any impact on its tone in reality. As to the response, it doesn't
seem to be significant either : my other flutes (blackwood and ebony) with perfectly
smooth bores feel very comparable (different model and makers, though, so comparison
is not fair, of course).

Terry McGee's bore roughness evolution theory sounds very interesting.
Is boxwood really impossible to be made as smooth as blackwood ?
I didn't know that. I thought the difference in tone was due to different densities.
Also, Delrin could be made sound more like wood by simply roughening the flute's bore
(with medium sandpaper ?), in this case? Too good to be true, in my opinion.

I had a different theory to explain the drastic difference between wood and plastics
(acoustical properties-wise). Delrin has an elasticity modulus of 3.5 GPa, and this
is as rigid as it gets for polymers (well, unless it is charged with glass particles,
but that would hardly suit any flute maker), while any dry tonewood is at least 10 GPa,
but more likely 12 GPa. Imagine a mass suspended on a spring. When set in motion, its
frequency is determined by the square root of the spring's stiffness divided by the mass.
The flute's material can be simplified as an arrangement of masses joined by springs in all
directions: the elasticity modulus is the stiffness of those springs. Delrin may have
a density close to that of blackwood, but the same mass suspended on a spring three times
less stiff will have a totally different frequency response.
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Re: Bore roughness and tone

Post by Loren »

Is boxwood really impossible to be made as smooth as blackwood ?
That depends on how you define smooth, but that’s a rabbit hole.

That said, Boxwood can be made crazy “smooth”. As or even more smooth than blackwood, but it depends on the specific cuts of wood and how you define smooth, and what your reaming and finishing processes are. I think for acoustical purposes the two woods are similar enough, in finish smoothness capability, for that to not play a significant role in acoustical properties, however I cannot prove this scientifically.

Also, while both woods’ surfaces can be made quite smooth to the touch, I suspect the finished surfaces look rather different under a microscope, and therefore potentially reflect sound somewhat differently.

Hard to know what the specific impacts of microscopic surface differences, material density differences, EM differences, etc. are. At least not without multiple PhD level degree’s in things like acoustical engineering, materials engineering, fluid dynamics, and who knows what else. (Insert “Ain’t nobody got time for that” meme here)
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Re: Bore roughness and tone

Post by Geoffrey Ellis »

Loren wrote: Sat Jun 19, 2021 3:36 pm
Is boxwood really impossible to be made as smooth as blackwood ?
That depends on how you define smooth, but that’s a rabbit hole.

That said, Boxwood can be made crazy “smooth”.
Definitely. I made some boxwood headjoints for Boehm flutes and I was experimenting a bit, so I finished the inside with a few coats of boiled linseed oil and cyanoacrylate. Messy business. But I had some special swabs I made, and I'd apply a thin coat of oil, then a coat of thin CA glue, let it cure, polish it and repeat. Did a few coats and the result was astounding. Smooth as glass without any substantial build up of material in the bore. I can't imagine a smoother bore unless it literally IS glass. However, I have found that boxwood makes a damn smooth bore with no treatment at all apart from polishing. I bored some out then polished the bore with some fine steel wool on a rod chucked in my drill, and it was smooth and shiny in it's raw state. But admittedly I didn't have a microscope handy :-)
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Re: Bore roughness and tone

Post by Loren »

Yeah, we used to coat the bores with epoxy and then re-ream. Kind of a pain but works well, IF you have extremely well made and hardened final reamers.

But like you said, even without that boxwood can be made nearly glass smooth with proper sanding, assuming the reamers used didn’t suck. I’ve seen too many modern flutes with horrible bore gouges from poorly made and/or poorly used reamers. And those gouges don’t polish out, even when the rest of the bore is sanded smooth.
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Re: Bore roughness and tone

Post by Geoffrey Ellis »

Loren wrote: Sun Jun 20, 2021 10:54 am Yeah, we used to coat the bores with epoxy and then re-ream. Kind of a pain but works well, IF you have extremely well made and hardened final reamers.

But like you said, even without that boxwood can be made nearly glass smooth with proper sanding, assuming the reamers used didn’t suck. I’ve seen too many modern flutes with horrible bore gouges from poorly made and/or poorly used reamers. And those gouges don’t polish out, even when the rest of the bore is sanded smooth.
Yes, I've made a couple of poor reamers along the way that got scrapped because they left some marks. All part of the learning curve.

But the epoxy/re-reaming trick has proven useful with a lot of woods that don't lend themselves to reaming. I've made shakuhachi flutes (with tapered bores) from paulownia wood, which is the softest and lightest wood I've ever met, with the possible exception of balsa wood. I only use it for players who have serious arthritis or other hand issues and need the lightest flute possible. But it bores and reams horribly! However, doing a couple of passes with the epoxy/re-reaming approach and the finished bore is totally solid. Takes about three reaming sessions to get it to an acceptable level.
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Re: Bore roughness and tone

Post by an seanduine »

A very long time ago, I made some cylindrical ´folk´ flutes. The wood I used was Koa Wood. Today, it is crazy expensive, but at that time (in another galaxy, it seems), I was getting it for next to nothing. It had relatively poor machining characteristics, and I got rough interiors and sanded them, then oiled. Probably would have profited from the epoxy sealing treatment. They were nearly as light as balsa, but easily as resonant as any bamboo.

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Re: Bore roughness and tone

Post by skap »

The thing is, sound waves are pretty big guys (A = 440Hz => wavelength = 78 cm), even the highest
audible sound frequency is still about 15 mm long (wavelength = distance between two subsequent
pressure peaks). They are not liable to be perturbed by microscopic defects of the bore surface
(even the highest audible harmonics). The propagation of a sound wave has little to do with the
movement of individual air molecules. Where it could be critical (and very probably is) is where
you have air flow (windway of a recorder, blowing hole of a transverse flute).

I remember a blackwood baroque flute I had on trial some time ago. You know those longitudinal
microgrooves that can be seen on the surface at some angles? There was one of those just at the
blowing edge, sort of merging with it and going a bit into the blowing hole's wall (it was tiny,
but I have sharp eyes). The instrument had some other marks of bad craftsmanship, including rough
bore, but I really suspected this tiny defect (and the form of the embouchure overall) to be
responsible for this flute being an absolutely awful thing. It needed considerably more air than
normally, no sweet spot, difficult response, third octave F and A impossible, and an unpleasant
hiss coming from the embouchure hole very audible to me while playing it. As to the aforementioned
bubinga flute (which is excellent), its bore is even more rough, but the embouchure seems flawless
and smooth (and I hope it will stay this way) because the maker certainly knew what really matters.

So maybe the question we should ask is "what is the influence of embouchure hole roughness on tone
and response"? I feel that it is this tiny spot on the flute that makes all the difference between
good and ... hum... not so good flute makers. Which is relieving in a way, because you can make
a great flute even with crappy reamers (did they have good reamers in the 18th century? I doubt this).

By the way, when coating the bore with epoxy, what happens to the embouchure hole wall? Is it left
uncoated ? And I wonder why such treatments are so uncommon. I don't know of any baroque flute maker
mentioning this, and I think you would want to mention this, because it's a selling point. Even
the Von Huene site doesn't mention it, so I suppose they don't do it anymore.
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Re: Bore roughness and tone

Post by Geoffrey Ellis »

skap wrote: Mon Jun 21, 2021 7:38 am The thing is, sound waves are pretty big guys (A = 440Hz => wavelength = 78 cm), even the highest
audible sound frequency is still about 15 mm long (wavelength = distance between two subsequent
pressure peaks). They are not liable to be perturbed by microscopic defects of the bore surface
(even the highest audible harmonics). The propagation of a sound wave has little to do with the
movement of individual air molecules. Where it could be critical (and very probably is) is where
you have air flow (windway of a recorder, blowing hole of a transverse flute).
One of the truisms I've found making flutes over the last twenty-four years is that there is an inexplicable gap between the science of acoustics and the reality of acoustics as it relates to flutes. It's quite possible that (theoretically) sound wave propagation should not be perturbed by microscopic defects in the bore surface...but it is. You can have a very smooth embouchure cut, and if the bore is rough, the flute's response is impacted. Why this is so I can't tell you (though I speculated above). I know very little about physics, being more of a trial-and-error maker who has learned by observation, but my observation has been that quite often something that should be "ideal" in terms of flute design does not actually present itself that way in real world results. One of the things that I like about flute making (and at times it's one of the things that can be frustrating or baffling) is that a strictly scientific approach doesn't seem to answer, in terms of making a perfect flute every time. There seems to be an unquantifiable "art" aspect as well that gets in there and exerts an influence. I value both aspects, and the fact that I've continued to hurt my brain by trying to understand the science over the years is testimony to that :-). An interesting anecdote about bore smoothness: at one point, Jem Hammond was testing out one of my ebonite Pratten flutes (when I was still making them), and he took it along to Chris Wilke's place and Chris tried it out. According to Jem, he said positive things about it (thank you, Chris!) but he did look down the bore as any good flute maker will do, and commented that it would be improved by some polishing. Ebonite does not ream really smooth, being inclined to exhibit micro-tears in the surface. I take pains to minimize this (keeping my reamers sharp, freezing the ebonite first and trying to avoid too much friction during reaming) but it's unavoidable. However, I was not bothering to smooth and polish the bore as much, mostly for fear of messing with the bore profile. So there is no leakage in the bore, and there are no "hairs" of wood fiber sticking up, but there were a lot of tiny, shallow pock-marks throughout the bore. Science may say that these shouldn't matter, but Chris seemed to agree that they make a difference.
maybe the question we should ask is "what is the influence of embouchure hole roughness on tone
and response"? I feel that it is this tiny spot on the flute that makes all the difference between
good and ... hum... not so good flute makers. Which is relieving in a way, because you can make
a great flute even with crappy reamers (did they have good reamers in the 18th century? I doubt this).
I am inclined to think that 18th century makers were perfectly capable of making a good reamer. It may not have been as easy or convenient for them to manufacture them, but the technology of metalworking was certainly advanced enough that they could figure out how to make a smooth-cutting reamer. Think of some of the swords and knives that were manufactured even centuries ago that were so amazing (Japanese katana, for example). I do think a rough embouchure cut is going to give you problems, however.
the way, when coating the bore with epoxy, what happens to the embouchure hole wall? Is it left
uncoated ? And I wonder why such treatments are so uncommon. I don't know of any baroque flute maker
mentioning this, and I think you would want to mention this, because it's a selling point. Even
the Von Huene site doesn't mention it, so I suppose they don't do it anymore.
When I use the epoxy method, it coats the embouchure cut as well (if it's the type of flute that has that sort of cut that can be coated). Once the cut is made, if I use a very thin, clear coat of epoxy (thinned with alcohol), it puts a nice, smooth finish on the cut. I take pains to make sure it does not get thick enough to impact the shape of the cut or it's size (it would take a fair few coats to really build up to any extent). I'm not surprised that Baroque makers don't speak of epoxy because depending upon how traditional their approach is they wouldn't be using a modern material like that. I've only had contact with a few makers of early flutes, but they all seemed very inclined to do things "the old way" in terms of materials and methods, trying to be true to the originals. This involves selecting historically appropriate timbers and finishing them in a way that is consistent with what the 18th century makers did. As for the epoxy method being a selling point, I think that depends upon your audience. Early flute enthusiasts might not take kindly to such an approach any more than they'd choose a Baroque flute made from Delrin (though Aulos does have some plastic Baroque flute replicas). But players who are hep to alternative materials and who are not too hung up on "authenticity" might think it to be a selling point. I think it's a selling point for my own flutes because it makes for a more stable and maintenance free flute. But all of my flutes are contemporary interpretations of folk instruments and I don't attempt any sort of old-school approach to them, so my customers don't seem troubled by it. But if I were to take a stab at making a Baroque flute (and I've seriously considered it, even going so far as to acquire blueprints of museum pieces) I would try to stick to the old methods as well.
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Re: Bore roughness and tone

Post by david_h »

skap wrote: Mon Jun 21, 2021 7:38 am ... The propagation of a sound wave has little to do with the movement of individual air molecules.
Are you sure about that? What's moving it's not molecules ?
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Re: Bore roughness and tone

Post by skap »

david_h wrote: Mon Jun 21, 2021 3:09 pm
skap wrote: Mon Jun 21, 2021 7:38 am ... The propagation of a sound wave has little to do with the movement of individual air molecules.
Are you sure about that? What's moving it's not molecules ?
My formulation was sloppy. What I meant is there is no considerable displacement of air molecules from one place to another, but some short-distance movement is involved (but air molecules move chaotically in all directions all the time anyway). The sound wave in the flute is just a zone of higher air pressure/density moving back and forward from the blowing hole to the next open hole (at 300 m/s !). This can be decomposed into a superposition of standing waves (which don't move anywhere at all) which represent the harmonics. Also think of sound waves propagating in solids (metal, wood), where particles really can't move but at microscopic distances.
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Re: Bore roughness and tone

Post by Tunborough »

skap wrote: Tue Jun 22, 2021 4:46 amWhat I meant is there is no considerable displacement of air molecules from one place to another, but some short-distance movement is involved (but air molecules move chaotically in all directions all the time anyway). The sound wave in the flute is just a zone of higher air pressure/density moving back and forward from the blowing hole to the next open hole (at 300 m/s !). This can be decomposed into a superposition of standing waves (which don't move anywhere at all) which represent the harmonics. Also think of sound waves propagating in solids (metal, wood), where particles really can't move but at microscopic distances.
With bore roughness, I think we are dealing with a situation where the superposition principle doesn't apply. Either because of friction or because of turbulence, I don't think we can split apart the standing waves at different harmonics and the DC static flow of air through the flute. Those components are interacting, and that's what messes up the tone in ways that are hard to predict with linear acoustics.
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Re: Bore roughness and tone

Post by Loren »

The discussion has gone way beyond my education and expertise level regarding acoustics at this point, so I’ll just say this: The issue of degraded performance that comes with a rough bore seems to me to be primarily one of energy loss, and from an observational standpoint I find that soft, significantly less dense woods, even with a well polished bore, tend to yield a very similar fall off in performance when compared with a harder/denser wood with a rough bore. My take is that the better the instrument’s internal reflectivity of energy, the better the tone and response. Hard/dense wood with mirror smooth finish is best (although can be too “bright” sounding for some), while the worst results are had from soft/low density woods with rough bores. Increase the reflectivity by improving density, hardness, or smoothness of the bore, and improvement follows. That’s been my experience.

Now perhaps that’s what the more educated on the subject of acoustics have been saying or debating, but I don’t know what I don’t know so :oops:
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Re: Bore roughness and tone

Post by Geoffrey Ellis »

Loren wrote: Tue Jun 22, 2021 10:10 am Hard/dense wood with mirror smooth finish is best (although can be too “bright” sounding for some), while the worst results are had from soft/low density woods with rough bores. Increase the reflectivity by improving density, hardness, or smoothness of the bore, and improvement follows. That’s been my experience.
Same here. Brightness seems to be the price the player pays for that really excellent response. Luckily, there is a spectrum where a flute can have really good response without the tone being too bright. One of the mysteries of bamboo (for me) has been the strange place it occupies in the world of flute materials. I've never found it to be too bright sounding, yet even in it's raw state (a bore that is clean and polished but not treated with anything) it can still have excellent response. Maybe not quite as lively as a smooth hardwood bore with an epoxy finish, but still really good.
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Re: Bore roughness and tone

Post by david_h »

Loren wrote: Tue Jun 22, 2021 10:10 am... level regarding acoustics at this point, so I’ll just say this: The issue of degraded performance that comes with a rough bore seems to me to be primarily one of energy loss
That view may be more pertinent than talk of the wavelength and speed of sound or the flow of blown air down the flute. Another perspective is that the sound we hear is a series of compressions and rarefactions caused by molecules moving closer together and further apart.

Something like this: Image

The amplitude is very large in that animation - the red particles would just jiggle a little in our context. The scale of movement being in the same ballpark as the that of the diaphragm of a domestic HiFi speaker or the skin on a banjo. Sub-millimetre for our A 440 at sensible volumes. A twang of A on the banjo doesn't propel molecules a distance near to the sound wavelength, just pushes them together a little then moves back for them to spread out again, 440 times a second.

That scale of movement, parallel to the bore, adjacent to our hairy flute wall (or next to a leaky pad or joint) is what we are concerned about.
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