A flute is any musical instrument in which the sound is produced by the musician blowing air which is split by a notch or an edge in the
mouthpiece, causing the column of air inside the (usually cylindrical) body of the instrument to vibrate. It's just about the oldest instrument of which examples have been found by archaeologists. Recently a flute constructed from a bear bone was found in a cave dig near
Cerkno in
Slovenia dating back to between 40,000 to 80,000 years ago, thought to have been made by
Neanderthals! Other than this, examples have been found which are about 22-35,000 years old, both in Asia and Europe. An 8,000 year old flute recently found in China was tuned so close to current Chinese musical scales that recognisable folk-tunes could be played on it.
What exactly happens inside the flute is quite a complicated bit of physics, and heavily dependent on the shape of the tube and whether the ends are closed. The frequency of the vibrating column of air, which is related to its effective length in a similar way to the relation between the length of a vibrating guitar string (say) and its frequency, gives the pitch of the note produced. The vibrations of the material of the flute itself are relatively unimportant for the pitch (though they affect the timbre.)
A flute with both ends open will have a still point (a node) at the center of the tube and the air will move most (an anti-node) at the ends. A flute with one closed end will have its node at the closed end, and its anti-node at the open end, giving a sound just less than an octave deeper in pitch.
Making holes in the tube is like creating an open end, and serves to effectively make the tube shorter - that is, the column of vibrating air will be shorter - causing an increase of pitch. And covering the holes back up with fingers or keys will lower the pitch.
Blowing with greater strength (or with a narrower embouchure) into the instrument can cause harmonics to be generated - which means that more nodes (still points) inside the vibrating air column will occur, and the frequency will increase, usually by a low integer multiple of the original (or 'base') frequency. This is the same principle as when playing harmonics on a guitar.
The width of the tube (or bore) also plays a role in determining the behaviour of the air-column - narrower bores are more easy to overblow (producing harmonics to get higher notes), but have less volume and sonority than wider bores. Tapering bores can subtly alter the effect of the position of the holes on the pitch, and so on. The size of the holes may also be a factor - if the size of the hole is below a certain ratio with the bore, it may just induce a harmonic rather than shortening the column. Things can get tremendously complicated within that column of air, and the calculations involved in an optimally engineered orchestral flute took inventor Jacob Boehm practically his whole life to complete.
The key to successful sound production on the instrument (apart from the diaphragmatic control of breath, which is required for any wind instrument) is the efficient direction of the player's breath onto the part of the flute which splits the air, causing the vibrations inside the flute. For a clean sound, the shape of the lips is used to exactly direct a tight 'beam' of air straight onto the edge at a proper angle (around 45 degrees). The particular way of pursing the lips, the shape of the mouth that this requires, is called the player's embouchure, and it's the mastery of the embouchure which distinguishes an expert player from a novice. An expert can control the flow of air so well that the sound can be tightened or loosened and given a cold clear or a warm breathy timbre at will, whereas the novice will struggle to produce anything other than a noisy, inefficient hoot.
Flutes may be divided into three main families, according to how the vibrating column of air is produced at the mouthpiece:
- The transverse flute or "cross-flute" is one in which the musician blows across a hole in the side of the instrument, usually closed at the end nearest the mouthpiece, in order to produce a note, and is held horizontally by the player. Examples are the fife, piccolo, and orchestral, or Boehm flute. The instrument may be tuned by altering the position of the stopper which closes off one end. In modern orchestral instruments, the different notes are produced by closing keys which in turn close the holes in the body of the instrument, but some fifes (and some Indian transverse flutes) have no keys, and the musician closes the holes with his fingers. The latter arrangement allows one to 'bend' the pitch produced by sliding the finger on and off the hole, and so some modern orchestral flutes have holes in their keys to allow this (which is usually more use in jazz or folk music than in the classical repertoire.)
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The end-blown flute, such as the Japanese shakuhachi, the Chinese jade flute, the Middle-Eastern ney, or the East European kaval. The flute is held roughly vertically, the flautist blowing into the end at the top. In the Japanese and Chinese examples, a notch is provided in the blown end to facilitate the splitting of the air. In the ney, there is no notch, just a sharp edge on the round mouthpiece, and this can make the production of a sound incredibly hard compared to the notched kind. To my knowledge, these kinds of flutes rarely have keys, and the different notes are produced by covering the holes in the body directly with the fingers.
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The duct flute (or "fipple flute") is (usually) an end-blown flute which makes things easier for the musician by providing a tube to blow into which directs the breath exactly onto the edge which splits the air and sets it vibrating. examples are the tin whistle (penny whistle or flageolet), the recorder and the taborpipe.
Information from:
http://memory.loc.gov/ammem/dcmhtml/dmpref3.html
http://realindy.com/anthronews.htm
and riverdale.k12.or.us/~audreyc/physics.htm which is gone, so had to be pulled out of the google cache.