A wind instrument is a musical instrument that contains some type of resonator (usually a tube), in which a column of air is set into vibration by the player blowing into (or over) a mouthpiece set at the end of the resonator. The pitch of the vibration is determined by the length of the tube and by manual modifications of the effective length of the vibrating column of air. In the case of some wind instruments, sound is produced by blowing through a reed; others require buzzing into a metal mouthpiece.
Methods for obtaining different notes
Changing the length of the vibrating air column, by changing the effective length of the tube through opening or closing holes in the side of the tube. This can be done by covering the holes with fingers or pressing a key which then closes the hole. This method is used in nearly all woodwind instruments.
◘ Changing the length of the vibrating air column, by changing the length of the tube, through engaging valves (see rotary valve, piston valve) which route the air through additional tubing, thereby increasing overall tube length, thereby lowering the fundamental pitch. This method is used on nearly all brass instruments.
◘ Changing the length of the vibrating air column, by lengthening and/or shortening the tube using a sliding mechanism. This method is used on the trombone and the slide whistle.
◘ Making the column of air vibrate at different harmonics, without changing the length of the column of air (see harmonic series).
All wind instruments use a combination of the first or second or third and the fourth method to extend their register.
Types of wind instruments
Wind instruments are typically grouped into two different families:
◘ Brass instruments
◘ Woodwind instruments
Although brass instruments were originally made of brass and woodwind instruments have traditionally been made of wood, the material used to make the body of the instrument is not always a reliable guide to its family type. A more accurate way to determine whether an instrument is brass or woodwind is to examine how the player produces sound. In brass instruments, the player's lips vibrate, causing the air within the instrument to vibrate. In woodwind instruments the player either:
◘ causes a reed to vibrate, which agitates the column of air (as in a clarinet, oboe or duduk)
◘ blows against an edge or fipple (as in a recorder), or
◘ blows across the edge of an open hole (as in a flute).
For example, the saxophone is typically made of brass, but is classified as a woodwind instrument because it produces sound with a vibrating reed.
On the other hand, the wooden cornett (not to be confused with the cornet, which is made of brass) and the serpent are both made of wood (or plastic tubing, in the case of modern serpents), but belong to the family of brass instruments because the vibrating is done by the player's lips.
In the Hornbostel-Sachs scheme of musical instrument classification, wind instruments are classed as aerophones.
Physics of Sound Production
Sound production in all wind instruments depends on the entry of air into a flow-control valve attached to a resonant chamber (resonator). The resonator is typically a long cylindrical or conical tube, open at the far end. A pulse of high pressure from the valve will travel down the tube at the speed of sound. It will be reflected from the open end as a return pulse of low pressure. Under suitable conditions, the valve will reflect the pulse back, with increased energy, until a standing wave forms in the tube.
Reed instruments such as the clarinet or oboe have a flexible reed or reeds at the mouthpiece, forming a pressure-controlled valve. An increase in pressure inside the chamber will decrease the pressure differential across the reed; the reed will open more, increasing the flow of air. The increased flow of air will increase the internal pressure further, so a pulse of high pressure arriving at the mouthpiece will reflect as a higher-pressure pulse back down the tube. Standing waves inside the tube will be odd multiples of a quarter-wavelength, with a pressure anti-node at the mouthpiece, and a pressure node at the open end. The reed vibrates at a rate determined by the resonator.
For Lip Reed (brass) instruments, the player controls the tension in their lips so that they vibrate under the influence of the air flow through them. They adjust the vibration so that the lips are most closed, and the air flow is lowest, when a low-pressure pulse arrives at the mouthpiece, to reflect a low-pressure pulse back down the tube. Standing waves inside the tube will be odd multiples of a quarter-wavelength, with a pressure anti-node at the mouthpiece, and a pressure node at the open end.
For Air Reed (flute and fipple-flute) instruments, the flow of air over the mouth of the instrument forms a flow-controlled valve. Some of the air-stream flows into the instrument's mouth, leading to an increase in internal pressure, while some of the air-stream flows across the top of the mouth--through a Bernoulli effect this reduces the pressure at the mouth, drawing air out of the mouth and leading to a decrease in internal pressure. When the pressure inside the chamber decreases, more of the air-stream will enter the mouth, and less will flow across the top of the mouth. A pulse of high pressure arriving at the mouth will direct more air across the top of the mouth; this will decrease the internal pressure, and send a low-pressure pulse back down the tube. A pulse of low pressure arriving at the mouth will draw more air into the mouth; this will increase the internal pressure, and send a high-pressure pulse back down the tube. Standing waves inside the tube will be multiples of a half-wavelength, with pressure nodes at both ends. The air-stream across the mouth vibrates at a rate determined by the resonator.
To a rough approximation, a tube of about 40 cm. will exhibit resonances near the following points:
◘ For a reed or lip-reed instrument: 220 Hz (A3), 660 Hz (E5), 1100 Hz (C#6).
◘ For an air-reed instrument: 440 Hz (A4), 880 Hz (A5), 1320 Hz (E6).
In practice, however, obtaining a range of musically useful tones from a wind instrument depends to a great extent on careful instrument design, and playing technique.
Parts
The bell of a wind instrument is the round, flared opening opposite the mouthpiece. It is found on horns, trumpets and many other kinds of instruments. On brass instruments, the acoustical coupling from the bore to the outside air occurs at the bell for all notes, and the shape of the bell optimizes this coupling. On woodwinds, most notes vent at the uppermost open tone holes; only the lowest notes of each register vent fully or partly at the bell, and the bell's function in this case is to improve the consistency in tone between these notes and the others.
