Bore (wind instruments)

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. These shapes affect the harmonics associated with the timbre of the instrument. The conical bore has a timbre composed of odd and even harmonics, while the closed cylindrical bore or closed tube is composed primarily of odd harmonics. The harmonic characteristics of instruments such as the clarinet (closed cylinder) are more variable than a given waveform and bore alone is not the only determining factor. The timbre of a clarinet, for instance, mainly depends on the construction of the mouthpiece and the properties of the reed. Furthermore, minute changes in air pressure and pressure applied to the reed (vibrato, slurs) modulate the tone.

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. This corresponds to the third harmonic; generally the harmonic spectrum of a stopped cylindrical bore instrument, particularly in its bottom register, is strong in the odd harmonics only.

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