Where woodwind joints fit in an other, cork makes them airtight usually, or formerly a wound and impregnated thread. I try heatshrink sleeve at my bassoon's bocal presently.
The tighter fitting immobilizes the bocal as I wanted, but tighter cork would do it too. The result is stiff and can minimize the dead volume, which should ease the altissimo, but up to C# (just below the Sacre) where the comfort zone of the musician and the reed end presently, I notice no difference; maybe things change at higher notes.
These are the bassoon bocal without its cork, pieces of heatshrink sleeve, the bocal with the sleeves, and a zoom.
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The Chinese made the material affordable. It comes in many diameters and colours, length is commonly up to 1m. The diameter shrinks strongly and irreversibly at heat, from a hairdryer, a soldering iron, or with care from a lighter. No skills needed.
Two plies happened to fit at my bocal, first with grease, later without. At first try, the shrunk sleeve moved around the bocal, so I held it with instant glue, which could be applied because the bocal is conical.
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What can improve?
Each ply is too thick and stiff for an adjustment, I had luck. Also, the conical bocal holds in an inverted cone at the bassoon. Maybe double-sided adhesive tape below the sleeve can adjust the diameter and slope if it resists the heat. Or a thread wound below the sleeve, which would give some elasticity.
Heatshrink tape exists too, which enables big diameters, but I expect leaks where the tape ends. Below a sleeve maybe. Would it be thinner?
Inner layers could stop before the outer sleeve to provide a smooth, airtight and sturdy taper.
Marc Schaefer, aka Enthalpy