Poly cylindrical bore, The bore is wider at the barrel side and becomes slowly smaler.
The bore is the heart and soul of the clarinet. It determines the sound, response, focus, resistance and intonation of the instrument.
The bore of clarinets is often described as cylindrical in contrary to oboes, bassoons or recorders. This was historically true but doesn?t apply for modern clarinets anymore. Only the lower clarinets like alto-, basset horn, bass clarinet and lower still have a cylindrical bore.
Modern Boehm clarinets have a bore size between 14,75-15mm at the barrel, German style instruments between 14,65-15,00mm and British instruments might even be as wide as 15,3mm.
From the top-joint to the open G-hole will the bore be reduced to anything between 14,3mm to 15mm depending on the brand and model. Only from there, until the low G-hole, will they be cylindrical.
Further down they flair out in different degrees, again depending on the brand and model.
When you compare different models, the bore dimension will be one of the most important parameters.
The smaller a bore is, the more resistance the instrument will have and the sound is darker and the airspeed will be increased which helps to focus the sound. If you like the sound, but find, that the instrument has too much resistance,
1. first make sure that it is really perfectly sealing and
2. try it with a more open mouthpiece.
The wider the bore is, the less resistance the instrument will have, the sound is rich on overtones and sounds brighter but the airspeed will be reduced which will result in less projection. If you like this sound but the instrument plays to freely and uncontrolled, try a mouthpiece with a smaller opening.
The bore of older instrument will have increased over the years do to wear, caused by swabbing. This will make the sound very open and the pitch will be sharp. Lohff & Pfeiffer developed a technic to restore the bore, to bring the old sound and pitch back again.
See also Bore restauration