John W Coltman, físico e investigador ejecutivo de la Westinghouse Electric Corporation, ha dedicado mucho de su tiempo libre al estudio de la flauta en sus aspectos musicales, acústicos e históricos. El es un activo participante de grupos musicales amateur. Sus investigaciones en acústica musical, llevadas a cabo en un pequeño laboratorio en su propia casa, han contribuído significativamente a lo que hoy se sabe sobre el comportamiento de las flautas y tubos de órgano.
En su carrera profesional, el dr Coltman ha recibido muchos honores por su invención y desarrollo de amplificadores de imagen de rayos x, hoy mundialmente usados en fluoroscopía médica. Es miembro de la Academia Nacional de Ingeniería de los Estados Unidos, teniendo en su haber 22 patentes y publicado 70 trabajos.

A continuación, parte de nuestro dialogo

Mis preguntas están en negro, y sus respuestas en blanco

Dear Mister
John Coltman:

Allow me to introduce myself. I'm an Argentine flutemaker. I make traditional, folk instruments from the Andes as well as other projective instruments as bamboo saxophones and clarinets. (www.unmundodebambu.cjb.net)

Guided by my own experience and by readings about woodwind acoustics, I conclude that the influence of material of the body is no direct, contrarily as lots of musicians and flutemakers guess.

Dear Angel,

I looked at your web site, and found the material there very interesting. The workmanship is beautiful, and my guess is that the sound of the instruments is similarly appealing. I will try to answer your questions below.

Have been them repeated by other physicians or laboratories, independently of yours?
Are there other articles available about this item?
A number of similar listening tests have been made, though none that I know of have been as carefully designed to be free of errors as the tests I describe in the Journal of the Acoustical Society of America, 49, p.520-523, August 1971. No evidence for material effects has been brought out by these. The most careful theortical and laboratory work has been reported by John Backus, Effect of Wall Material on the Steady State Tone Quality of Woodwind Instruments, JASA 36, (1964), 1881-1887.

Also, I'm sure that in most cases the material doesn't resonate toghether with the air column. But, Is it true in flutes or horns made of very, very thin materials?
Is the air molecules energy enough to move thin metal, or goat skin, or cardboard walls?

My question is inspired in the Chinese flute D'tzu, which have a silk-paper attached to its body. This membrane actually vibrates.
Material effects are unimportant only as long as three conditions are met:
1. The tube must be rigid
2. The interior surface must be smooth.
3. It must be impervious - leak-free and not spongy.

A cylindrical tube can be sufficiently rigid even with quite thin materials. However, a square tube must have thicker walls to resist deformation, as Miller found in an experiment with a square organ pipe made of thin sheet copper. You are quite correct about the vibration of the membrane on some Chinese flutes. On a Boehm flute, the pads are sufficiently deformable that one can feel the keys vibrate under the fingers, though the contribution to the radiated sound is negligible.

I found that wood polished with very fine sandpaper and finished with 0000 steel wool has acoustical losses only very slightly greater than a polished silver tube.

Many woods leak sufficiently to affect the tone quality if they are not oiled. Maple is quite unsuitable in its dry state - soaking in linseed oil will make it as good as any other. Bamboo is quite impermeable - I would guess that you do not need to treat your instruments.

I hope you will find these answers helpful. Best of luck in your work.

John Coltman
Dear Angel,
Some answers below. Best regards, John

----- Original Message -----
From: John Coltman
To: Angel Sampedro del Río
Sent: Wednesday, June 27, 2001 6:43 PM
Subject: Re: Material influence on flutes

Many woods leak sufficiently to affect the tone quality if they are not oiled.
But, I underestand that is more due to small air cells or wall irregularities than to vibrational properties of the material itself.?

That is correct.

What happen if you use a roughly finished metal wall?

A rough surface will increase losses whether it is wood or metal.

Although internal wall of bamboo is quite impermeable, I find (more correctly, I think that I find) differences in sound when I seal or laquer it. Am I adding any "reflective property", or I'm just making it smoother?

I believe that it is primarily making it smoother, though it is also possible that you are sealing small leaks.

Siguiendo con preguntas, ahora acerca de la influencia del material y, en la flauta traversa, el espacio entre el corcho y la embocadura y la placa del resonador:


David Slomka mentioned me his axiom, "the more expensive the material, the better the sound"!)

Yes, it's like jewelry. Natural emeralds are poor gems, full of flaws, but are more highly prized than perfect artificial ones, because they are more expensive.

Appart of that anecdote, we realize that the SHAPE of this resonator (...in Sankyo Flutes) is quite particular, concave center with a convex rim -if I remember well.
Do you have any information about the action of this type of resonators?

The dimensions of the cavity beyond the mouth hole to the cork are small compared to the wavelength, so the shape of this cavity is unimportant - only its volume counts. One must not change the shape very close to the mouth hole, where the jet might be affected, but otherwise it doesn't matter if the cork face is concave, convex or flat. The cork-cavity and the mouth hole form a resonant system. At frequencies below its resonance ( which is well above the flute range) the impedance of the combination acts like that of an extra length of tubing, keeping the octaves in tune. Without the cavity the octaves are stretched. The effect of the cork cavity is mostly in the third octave.

Seems that it is another false belief...
The question was more related to the movement of air molecules, preciselly close to a zone of maximum air movement. If I underestand well, only a small ammount of energy of the flutist breath is converted into sound waves. Is well known the belief among flutists that the cork front must be metalic (and concave following Sankyo Flutes) in order to reflect energized molecules to the mouthpiece system. Does it happen or not?
Or the ammount of energy converted into sound is only related with the aerodinamics of the mouthole?

No. there is no "reflection" in the usual sense. The air between the mouthhole and the cork face is being sqeezed and then expands each cycle, acting like a little spring. This affects the resonance frequency, and therefore the intonation, but the sound is produced by the vibration of air particles flowing in and out of the mouthhole ,and the first open finger hole.

The impedance is a resistance which varies with frequency (?-please correct me if is wrong). So, if I underestand well, this air cell between the cork and the mouthole has a different resistance at different frequencies. Does it make the air slower at low frequencies than at high frequencies?

One should use the word "impedance" here rather than "resistance". Resitance is one type of impedance, but it involves loss of energy, converting some of the motion into heat. The impedance of the cork cavity is like that of a capacitor, where pressure is required to squeeze the air in, but when the pressure subsides or reverses, the air flows back out.

I thought that this correction of harmonical modes has been made by the tapered head joint.

That is true, but the cork cavity also plays a part, especially at the highest frequencies

A physician gave us an speech, and when we asked about the influence of materials in Boehm flute, he mentioned some experiments about the influence of molecular structure in high frequencies. Do you know something about this?

The only studies I am aware of concerning molecular structure as affecting the tone quality of instruments, are those dealing with those cases where the material is part of the sounding mechanism, as in reeds, violin bodies, piano strings, etc.

Another example is a parallel traced between some Yamaha and Selmer saxophones, where they seem to be equal in dimensions and metal recipie, but the difference seems to be in the construction method. I have not enough vocabulary in English, but very roughly, the Selmer is "hammered" while the Yamaha is shaped with a press. Selmer is supposed to be better for this reason. Do you have any idea about this?

Unless the final shape is different, I would not expect to find any change that depended on how the material is formed.

In the North of Argentina and Bolivia are a myriad of panflutes, and traditionaly blown in quite different ways as the European ones. The bass ones - called toyo or hacha-siku- are about 1.6m long, 2,5cm diam longer tube. Correctly blown, the sound is very characteristic. The musician also put the mouthole of the tube several centimeters below the lips, and somehow exitates various harmonics, giving a tremendous sound. These panflutes are made from a very very thin bamboo (less than 1 mm).
Every siku-player is absolutely convinced that this sound is due to this bamboo. Also, when you slightly touch the far end of the tube (closed, maximum pression variation) can feel the bamboo wall vibrating. If this vibration affect the sound or is negligible is another question, but is difficult to convince others that it hasn't influence on sound. What's your opinion of this phenomenon?

I can believe that if the tube wall is thin, and if the tube is not quite round, that it might vibrate appreciably. I think you can appreciate that a tube of elliptical cross-section would tend to expand, under pressure, to more nearly approach a circle, and that a periodically alternating pressure could cause a mode of vibration where one axis of the ellipse expands as the other shrinks. A circular tube, however, already has a cross section of maximum area for a given circumference, and is very resistant to expansion. So a thin tube, not quite round, could vibrate much more than is usual in a commercially made instrument. Backus treats this case in one of his articles, if I remember. It is certainly conceivable that the the sound could be affected.
It would be interesting to make an instrument like this out of plastic plumbing pipe, which is quite round and thick-walled, and see if one can get the same sound quality. I believe in controlled experiments, like the one performed many years ago by a curve-ball pitcher who hurled a ball past three posts set on a line, missing the first and last posts on their left sides, and the middle one on its right side. This sent the physicists back to the lab to ponder the forces of air on a spinning sphere!

Entramos en las diferencias acústicas entre quenas y flautas dulces y flautas traveseras de concierto

So, Why the notched end flutes like quenas and shakuhachi -without trans-mouthpiece cavity- plays octaves in tune?

The answer is - they don't. Here are the frequencies I measured on a quena as I blew the first, second, third, and fourth modes. Also shown are the ratios, which should be 1, 2 ,3, and 4 for perfect intonation.

385.04 1.000
787.2 2.044
1208.4 3. 138
1657.4 4.304
The last note is sharp by 1.25 semitones, which would be intolerable in an orchestral flute.

Agreeing that 1,25 semitones is intolerable not only for an orchestral flute but for all music, I make quenas for 15 years. The standard quena is tuned in G4, 6 holes at front and one back. Either the G and A6 -fourth mode- could be played in a good quality instrument by blowing harder or by uncovering a bit the back hole. Believe me that if I found one of my quenas playing 1,25 semitones apart (say, A#6+25 cents) , I burn it in my charcoal barbacue.
In fact, in almost cylindical bamboo quenas, the tendency of this 4th mode is to be flat instead of sharp. In some of these cases, we use for the A6 an optional fingering (closer to the D fingering, 3rd mode)
Maybee player's embouchure plays a rol here. Maybee is due to the inverse conical shape (as the renaissance flute), although I dont believe in that posibility.
Sometimes -really almost always- quenas are done with a partially covered far end, which makes the instrument as it were longer than it actually is. It is an ancient tradition that I don't use. I noticed that in that cases detunings as you mention are quite common for the G

You are correct about the restricted end hole. It adds to the stetch just like the restricted mouth hole does. The quena I measured has such an end. I don't have a quena without this feature, but I measured a Mexican tarahumara which is cylindrical and has a whistle type mouth piece. Results are:
Mode Sharpness relative to 1, cents (hundredths of a semitone}
1 0
2 39
3 68
4 95
Here the sharpening is still very evident, though not as great as for the quena. A row of finger holes rather than a single large opening gives an octave shrink, so that for many notes the lack of a cork cavity may compensate. The conical bore may help too. Have you actually measured the intonation of your quenas with a tuner? The ear likes a stretched octave, particularly in the high frequencies. A true octave sounds flat.

I underestand from this table that mode 2 is 39 cents sharper than the correct value (so, 1239 cents instead of 1200). It is a lot!!!
Yes I always tune my quenas with a Chromatic tuner, Korg DTM-12. I also noticed that sometimes flutemakers commit self-hoax when tune their own instruments, by blowing harder or softer, either using a tuner or not.

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