Shaku
Design
Tapering PVC Pipe

Updated 4/24/08

Aside from PVC pipe not being bamboo, what's the primary objection to using PVC for making shaks? It's the taper thing. So let's solve that. Think about a garden hose and how the water flow is reduced by squeezing the hose. If a circle is turned into an ellipse, the area decreases. Same perimeter, reduced area. As the ellipse continues to flatten its area decreases to nothing. Tapering the bore is just a method (there are others) of restricting air movement by reducing the area of the bore.

That's what we're talking about in the tapered shakuhachi bore--the cross-sectional area of the bore decreases to the choke point and then flares to the foot of the flute. So how can we easily alter the cross-sectional area of the bore of a PVC pipe? Yes, you guessed it! We'll heat the pipe and squeeze it where we want the bore area decreased--gradually shifting the shape from a circle to a flatter and flatter ellipse.

What bore shape do we want? For this exercise, let's keep things simple.

1) Start the taper between the second and third tenths of the flute's length. A quarter of the length is a good general starting point.

2) Continue the taper (squeezing) until about 86% of the length, then flare back to the foot.

3) How much to squeeze? The choke point in some shaks is half the area of the mouth, but that's a pretty tight constriction. That's where the ellipse will be flattest.

There is another consideration: In woodwinds the cross-sectional shape of the bore does make a difference. In a pipe organ, a circular and square pipe of the same area don't act (or sound) exactly the same. A square pipe has more surface per area than does a circle and the same is true with an ellipse. Anyway, start by squeezing the diameter at the choke point to 1/2 that of the mouth as shown in figure 1. Using these measurements you'll be well within the ballpark for a tapered bore. The rest is just tweaking for the particular sound your ears enjoy.

A simple clamping jig can be built from two-by-fours. You can make any number of squeezing anvils, each pair with a different geometry. Once the clamp is ready, make a PVC shak in your usual manner, then heat with a heat gun and clamp. In less than a minute, the PVC will set rigid and you will have tapered the bore to the geometry of the anvils. The anvils will be thinner than you might have imagined. For Schedule 40 PVC pipe the bore has a diameter of about 20.7mm. Squeezing that in half means the thickest part of each anvil will be 5-6mm. For a 1.8 shak the anvils will be approximately 410mm long, so they're long skinny suckers. We're talking 16" long and a quarter inch thick at the thickest point.

If you're handy with tools you don't have any excuse for not having PVC shaks with bores tapered to your own specifications. Another interesting part of this ellipse business is your flutes can begin to vibrate as an elliptical bore shape is the biggest single factor is the production of flute body vibrations.

This method of creating tapered bores is well suited to the investigation of bores in general. Because PVC pipe can be reheated and pressed back to a straight pipe this method lends itself to forming and reforming bores. It's a quick, easy, low-tech method through which one can learn a tremendous amount about the mysterious shakuhachi bore--aside from knocking out some inexpensive high quality shaks.

See The Synthesis for a final flute design.

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