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Updated
A collection of various bits of information and factoids applying to shakuhachi and flutes in general.
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Boundary Layer
There is a thin layer of air on the surface of the bore which tends to stick to the bore and as such isn't acoustically active. This layer's thickness (about 0.1 mm or 0.004") is the result of the viscosity of air and is present regardless of the bore material or it's smoothness. To give an idea of this thickness, a sheet of paper for your printer is about 0.003" thick. The effects of bore roughness become significant only when the roughness itself is significant on the scale of the boundary layer. Should you want to finish the bore in a way which doesn't encourage a greater boundary layer, the finish should either be finer or rougher than about 150-120 grit. Either way takes you out of the range of the layer and doesn't contribute substantially to it. Conversely, if you want to mess around with increasing the boundary layer then sand or line the bore with 150-120 grit material.
The upshoot of all this? It probably makes no acoustical difference whether the bore is finished to a slick, reflective surface. Whatever the final surface, there will be a few thousands of an inch of air sitting on top. A thin boundary layer of air is the final finish (air varnish?) regardless of whatever else you may do to the bore. A smooth shiny bore looks nice--but that's about it.
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Flute Material
There is a common belief that the material from which a flute is constructed defines the character of its sound. This misconception has been put to rest by countless studies and experiments. The quality of any particular flute sound is the result of the geometry of its air column--including the holes and blowing mechanism. The sound of a flute comes from the shape of its air, not the material from which the flute is constructed.
The large tone holes of a silver Boehm flute give it a much brighter tone than that of a Baroque wooden flute with its necessarily rather small finger holes. In the case of these two flute examples, it's hole size (see Cutoff Frequency below) which accounts for different timbre rather than material. Its common to ascribe this tonal difference to the difference between silver and wood as bore materials, but this is erroneous.
There are those who will persist in the "material equals sound" belief as it's the result of associative thinking. Mistaking the menu for the meal, the map for the territory. Bamboo flutes don't have a 'bamboo' sound, they have 'bamboo' geometry. Change the geometry of a bamboo flute and you can make it sound like any 'material' you want. As one example, in shorter shakuhachi hole size begins to catch up with bore and so they are often characterized as 'brilliant'. (See Wall Vibrations below)
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Cutoff Frequency
In woodwinds, there is a correlation between relative hole size (and hole spacing--the lattice) and the highest frequencies of which the instrument is capable. There is a definable cutoff frequency above which most sound production is suppressed. For clarinets, oboes and Boehm flutes it's between 1200 and 2000 hz. The cutoff frequency limits the highest note that is readily playable on the instrument.
Because of the traditional way in which shakuhachi are constructed (fingered holes, etc.) they will forever retain a diminished cutoff frequency. The longer the shak the more diminished as the ratio of hole size to bore size steadily declines. Employing bigger hole sizes helps the situation slightly but at the expense of other acoustical characteristics. If you want a higher cutoff frequency go with shorter shaks.
The cut-off effect has it impact primarily on the high overtones. Large holes and smaller spacing increase the cut-off frequency and harmonic content.
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Acoustical Length
Every flute has two lengths: its physical length and the acoustical length. And they are not the same. A 1.8 D shak usually plays a base tone somewhere near 294 hz., depending on tuning standards, etc. The unencumbered tube length require to support such a frequency is Length = Speed of Sound / (2*Frequency), in this case about 564mm. 1.8 shaku is 545mm, so acoustical length is 19mm longer than physical length. Where is this extra length? Conceptually, you can think of it as sticking out the bottom and top of the flute. About 0.6*Bore sticks out the bottom, the rest out the top.
For a properly proportioned shak, acoustical length always runs 3-4% longer than physical length. With varied blowing (airstream speed and thickness, etc.) and playing methods (kari-meri, etc.) the acoustical length is varied, thus the acoustical properties of the flute are modified. The particular charm of the shakuhachi exists because of this extra length and what one can do with it.
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Other Length Stuff
A one percent change in length will change the pitch by 17 cents. To change a full semitone change the length 5.9%.
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Flute Efficiency
Flutes convert pneumatic energy into acoustical energy with an overall efficiency on the order of 1%. That's a big fat ONE percent. On engineering grounds, one would be hard pressed to find a more inefficient, indulgent device. The 'Zen' of this situation isn't immediately apparent.
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Runway Time
How long does it take a flute to settle into a particular note? How long on the runway before taking to flight? Something like 30 periods of the fundamental, which for a D flute is about 1/10 of a second. Longer flutes take longer--shorter, shorter.
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Bending Bamboo
Surprisingly, bamboo bends (with heat) more readily at the nodes than in between.
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Tapered Bores
Acoustical science can find no compelling acoustical reason for the reverse tapered bore. There is nothing of consequence which can be done with a tapered bore that can't be done with a straight bore. It's now generally felt that the shift to tapered bores in Europe during the Sixteenth century had more to do with construction methods than anything else. The shakuhachi probably never had a straight-bore period so, in Japan, the question is moot.
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Hole size/placement
Roughly speaking, a 10% change in hole diameter, or a 1% positional change relative to the (acoustical) top of the tube, causes a frequency shift of 10 cents.
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ToneVolume
The loudness of flutes is most directly related to hole size. The bigger, the louder.
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Wall Vibrations
There is strong evidence that wall vibrations (if any) do not contribute perceptibly to the total of radiated sound; the material of the wall therefore has no influence of the timbre. Frequency, initial transient, stability, ease of blowing and timbre of a note are solely determined by the inner geometry of the entire instrument, including the player's mouth.
John Singer, in a fine article, talks about something called "chikuin" which he implies contributes significantly to the sound of certain shaks which possess this quality. A simple experiment will clear up a lot of the misconception.
Play Ro, Tsu, Re--those three notes. Did the vibrations also change pitch? Play Ro, Tsu, Re on a marimba. Did you play it all on the same bar? Of course not. A flute who's body will vibrate does so at a single pitch, so the sound arising from this vibration would be heard as a drone note as it's pitch would be unvarying. Those who insist on the idea that flute body vibrations contribute in any meaningful way to the sound of a shakuhachi would have to have flutes that, first of all, do vibrate and second, the length of the vibrating medium must change length in order to create different pitches. Thus, what they're claiming is that their flutes somehow magically change length while being played. Now that would be something to witness!
A flute who's body could (somehow) create audible sound would be deemed inferior as a strange drone note would be in the background of all play.
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