Wind instrument

Wind instruments are musical tools that use a column of air to create sound. In the Hornbostel-Sachs system of musical classification, these instruments are known as aerophones.

Most wind instruments contain a resonator, which is usually a long tube that is shaped like a cylinder or a cone. To make music, a player blows air into or over a mouthpiece located at one end of the resonator. This action sets a column of air inside the tube into vibration. The pitch, or how high or low the note sounds, depends on the length of the vibrating air column. Musicians can change this pitch by manually modifying the effective length of that column.

There are several specific mechanisms used to change the pitch of a wind instrument. One method involves using valves, such as piston or rotary valves, to route air through additional tubing. This increases the overall length of the tube and lowers the fundamental pitch. This technique is used on nearly all brass instruments. Another method uses a sliding mechanism to lengthen or shorten the tube, as seen on the trombone or slide whistle.

Many woodwind instruments change the pitch by opening or closing holes on the side of the tube. Players do this by covering the holes with their fingers or by pressing a key. Finally, some instruments, like the pan flute, use different air columns of varying lengths to play different notes. Some of these instruments can even play several notes at the same time.

Wind instruments are typically divided into two families: brass and woodwind. It is important to note that these categories are based on how the sound is produced rather than what the instrument is made of.

For example, the saxophone is made of brass, but it is a woodwind because it uses a vibrating reed. Conversely, the didgeridoo and the serpent are made of wood, but they are considered brass instruments. This is because the player's lips initiate the vibration. In brass instruments, the player's lips act as a vibrating source. In woodwind instruments, the player might use a reed, blow over a hole against an edge, or blow through a mouthpiece called a fipple.

Physics explains how these vibrations become the sounds we hear. When air enters the resonator, a pulse of high pressure travels down the tube at the speed of sound. This pulse reflects off the open end as a pulse of low pressure. Under the right conditions, the valve reflects the pulse back with increased energy until a standing wave forms.

In reed instruments like the oboe, a flexible reed acts as a pressure-controlled valve. In air-reed instruments, such as the flute, a thin sheet of air, or a planar jet, flows across a sharp edge called a labium. The interaction between this jet and the labium creates a reaction force. This force drives the acoustic oscillation of the pipe, creating the sound.

We can use math to predict the notes produced by these standing waves. In an open-open tube, standing waves are multiples of a half-wavelength. For a tube approximately 40 cm long, a reed instrument might exhibit resonances at 220 Hz, 660 Hz, or 1100 Hz.

An air-reed instrument of the same length might produce notes at 440 Hz, 880 Hz, or 1320 Hz. However, the exact tuning depends on the speed of sound in the air. Because the speed of sound changes with air density, factors like temperature and humidity can affect how an instrument is tuned. While temperature changes the air density significantly, the thermal expansion of the instrument material itself has a much smaller effect.

Many wind instruments feature a flared opening at the end opposite the mouthpiece called a bell.

On brass instruments, the bell is where the acoustical coupling from the bore to the outside air occurs for all notes. The specific shape of the bell helps optimize this coupling and transforms the instrument's resonances. On woodwind instruments, the bell serves a different primary purpose. Most notes on a woodwind instrument vent through the uppermost tone holes. Only the lowest notes in a register vent through the bell. In these cases, the bell helps make the tone of those low notes consistent with the rest of the instrument's range.

Finally, playing wind instruments can have physical effects on the musician. Some instruments require very high breath pressure to play. A 2011 study observed that playing certain brass and woodwind instruments can cause temporary and dramatic fluctuations in intraocular pressure, which is the pressure inside the eye. This has been linked to health risks like glaucoma. The magnitude of this pressure increase often correlates with the intraoral resistance of the instrument. Interestingly, the pressure required for ethnic instruments, such as Native American flutes, is generally lower than that required for Western classical wind instruments.