Physics of Woodwinds |  |
Woodwinds are instruments that are played by blowing through a reed. The reed converts the blown air into a series of puffs dictated at the bore and is therefore pressure-operated. The vibration opens or shuts a slit between the reed(s) and the mouthpiece, causing vibrations of a column air within the bore. These air columns can be excited by a single reed in a nearly cylindrical or a conical tube, a double reed in a conical tube, or an air jet in a nearly cylindrical tube (Fletcher 1635). The frequency of vibration is set by cyclic changes in the pressure of the vibrating air in the bore. The fundamental frequency of the note is determined by the mode frequency of the resonator, which changes as holes in the instrument are closed or opened. Woodwinds tend to use the lowest three or four natural modes, so gaps in pitch are filled through the drilling of holes. Smaller holes tend to reduce the radiation of higher frequency components. The ratio of the volume of air contained in a closed hole divided by the volume of air in the length of the bore between adjacent holes must be the same in all parts of the bore. In a cylindrical tube, the tube is a quarter of a wavelength at the fundamental resonance. In a conical tube, however, the tube is half of a wavelength at the fundamental resonance, so the tube must therefore be longer to accommodate this. In the flute family, the tube length is half of a wavelength as well. The shape is designed to flatten the lowest resonance frequency to improve response (Fletcher 1637).
Key Points to Remember
- "Air blown into the instrument through the reed sets up vibrations in the column of air within the bore, and this vibrating air column produces the sound of the instrument" (Hutchins 35).
- Reed converts air from player's blowing into a series of puffs dictated at the bore.
- Pressure-operated.
- Vibration opens or shuts a slit between the reed(s) and the mouthpiece.
- Frequency of vibration set by cyclic changes in pressure of vibrating air in the bore.
- Length and shape of bore affects the frequency of the vibration.
- Woodwinds mostly use the lowest three or four natural modes.
- Gaps in pitch between the natural frequencies of the bore are filled by drilling a row of holes in the side of the bore.
- Covering the holes with the fingers alters the pitch.
- "The lowest note in the musical range of a woodwind is the lowest of the natural frequencies of the complete bore" (Hutchins 40).
- Ratio of volume of air contained in closed hole divided by volume of air in length of the bore between adjacent holes must be the same in all parts of the bore.
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