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Concept
Bore (wind instruments)
Summary
In music, the bore of a wind instrument (including woodwind and brass) is its interior chamber. This defines a flow path through which air travels, which is set into vibration to produce sounds. The shape of the bore has a strong influence on the instrument's timbre.
The cone and the cylinder are the two idealized shapes used to describe the bores of wind instruments. Other shapes are not generally used, as they tend to produce dissonant, anharmonic overtones and an unmusical sound. Instruments may consist of a primarily conical or cylindrical tube, but begin in a mouthpiece, and end in a rapidly-expanding "flare" or "bell". This flare reduces the acoustic impedance mismatch between the instrument and the air, allowing the instrument to transmit sound to the air more effectively.
These shapes affect the prominence of harmonics associated with the timbre of the instrument. A bore that flares from the mouthpiece reduces resistance to the breath, while a bore that narrows from the mouth increases it, compared to a cylinder.
The diameter of a cylindrical bore remains constant along its length. The acoustic behavior depends on whether the instrument is stopped (closed at one end and open at the other), or open (at both ends). For an open pipe, the wavelength produced by the first normal mode (the fundamental note) is approximately twice the length of the pipe. The wavelength produced by the second normal mode is half that, that is, the length of the pipe, so its pitch is an octave higher; thus an open cylindrical bore instrument overblows at the octave. This corresponds to the second harmonic, and generally the harmonic spectrum of an open cylindrical bore instrument is strong in both even and odd harmonics. For a stopped pipe, the wavelength produced by the first normal mode is approximately four times the length of the pipe. The wavelength produced by the second normal mode is one third that, i.e. the 4/3 length of the pipe, so its pitch is a twelfth higher; a stopped cylindrical bore instrument overblows at the twelfth.
Official source
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