(Public domain image from Websters Dictionary 1911)
It is an open tube with circular holes, which produce different sounds when some are opened or closed with the fingers of the instrumentalist.
The sound of the flute is characteristic because of its sweetness and ability to blend with other instruments. These qualities occur because a flute's timbre and pitch are agile, under the instantaneous expressive control of the player.
A standard concert flute is pitched in C and has a range of roughly 3 octaves starting from middle C. The other common concert flute is the piccolo, pitched an octave above the concert flute. The alto and the bass flute are available, but difficult to play because the embouchere requires more skill. D-flat, B-flat and E instruments are rare, but available.
The modern concert flute is generally made of silver or another metal, and has a system of keys invented by Theobold Boehm (who gave his name to the system). Players select notes by a combination of fingering, embouchure, and breath control, in a way somewhat similar to horns.
Concert flutes have three parts, the head, body and extension. The head contains a tuning-cork (or plug) for precision tuning, adjusted by the head-end knob. Gross, temporary adjustments of pitch are made by moving the head in and out of the head-joint. The player makes fine, or rapid adjustments of pitch and timbre by adjusting their embouchure.
The chief modifications in modern flutes from Boehm's original design are "trill keys" intended to permit a player to rapidly alternate between two notes, and "extension keys" permitting the flute to reach lower notes (notably B-flat on expensive models). Modified partial fingerings using the trill keys also permit a skilled player to reach four octaves of range, though the "natural" range is three octaves. Boehm's fingering is also used in saxophones, and many flute players therefore "double" on this instrument for jazz and small ensembles.
A maladjusted flute is much more difficult to play, and beginning flute-players should invest in a professional adjustment if their instrument is not new. The most common problem as a flute ages is that its pads rot and leak. Also, rough handling can bend the pads and make them leak. The return springs can also weaken, causing slow or unsynchronized opening of the holes. Also, the pad-closure mechanisms can become misaligned or misadjusted. Occasionally the alignment pins can fall out.
The head end is the most difficult part to construct, because in form, it is a long thin hyperbola or parabola. The lip-rest and tone-hole have critical dimensions, edges and angles, whcih vary slightly in different models. Fortunately, once made, these never need adjustment.
The holes are stopped by pads constructed of fish skin over dacron, or in some very low-cost or ruggedized flutes, silicone rubber. Over time, the pads rot, and must be replaced. The highest-quality flutes use open-holed "french" pads so that the players fingers can open the pad more quickly than the return spring permits.
The pad return springs are phosphor bronze, and roughly the shape of a pin. The rest of the mechanisms are constructed by lost-wax castings and machining, with mounting posts silver-soldered to the tube. On the best flutes, the castings are forged to increase their strength.
Flutes fall broadly into four classes, depending on how the sound is produced, and whether the tube is open or closed.
The familiar concert flute, piccolo and fife are examples of transverse flutes, in which air is blown from the mouth across a small hole at the top of the instrument. In a transverse flute the embouchure (position of the lips and tongue) is the main determining factor in tone production (as well as having an effect on pitch).
End blown flutes, include the recorder, ocarina and the tin whistle. These typically have an arrangement whereby the stream of air from the mouth is directed against a blade; embouchure is less critical, though is still important in mastery of the finer points of playing. Nose flutes exist in some cultures.
Flutes may also be either open or closed-ended. The organ pipe, ocarina, pan-pipes, concert whistle, jug, police-whistle and bosun's whistle are common examples. Open-ended flutes such as the concert flute and recorder have more harmonics, and thus more flexibility for the player, and brighter, more pleasing timbres.
Production of sound
A flute produces sound when a stream of air directed across the top of a hole bounces in and out of the hole. Some engineers have called this a fluidic multivibrator, because it forms a mechanical analogy to an electronic circuit called a multivibrator.
The stream beats against the air in a resonator, usually a tube. The player changes the pitch of the flute by changing the effective length of the resonator. This is done either by closing holes, or more rarely, with a slide similar to a trombonist's slide.
Because the air-stream is lower mass than most of the resonators used in instruments, it can beat faster, but with less momentum. As result, flutes tend to be softer, but higher-pitched than other sound generators of the same size.
To be louder, a flute must use a larger resonator, and a wider air-stream. A flute can generally be made louder by making its resonator and tone-hole wider. This is why police whistles, a form of flute, are very wide for their pitch, and while organs can be far louder than concert flutes: an organ pipe's tone-hole is usually eight or more times wider.
The air-stream must be flat, and precisely aimed at the correct angle and velocity, or it will not vibrate. In end-blown flutes, a precisely machined slot extrudes the air. In organs, the air is supplied by a regulated blower.
In a transverse flute, especially the concert flute and piccolo, the player must form and direct the stream with his lips. This makes the transverse flute's pitch and timbre more instantly expressive than any other instrument. However, it also makes the transverse flute immensely more difficult to play than the recorder.
Generally, the quality called "tone color" or "timbre" varies because the flute produces harmonics in different intensities. A harmonic is a frequency that's an even multiple of the lowest, or "fundamental" tone of the flute. When a flute sounds harsh, or whiny, it is being played to provide more harmonics. Generally the air-stream is thinner (to vibrate in more modes), faster (providing more energy to vibrate), and aimed across the hole more shallowly (permitting a more shallow deflection of the airstream to resonate).
Almost all flutes can be played in fundamental, octave, tierce, quatre and cinque modes simply by blowing harder and making the air-stream move faster and more shallowly. Flute players select their instrument's resonant mode with embouchere and breath control, much as brass players do.
The timbre is also affected by the quality of the resonator. Generally, more rigid resonators (such as wood) have a "dead" sound, because they have a higher acoustic impedance, and do not resonate with the harmonics. Concert flutes are expected to produce a "brilliant" sound, with a wide range of harmonics. They are therefore generally constructed of thin tubes made of hard-drawn silver or gold alloys. These are more mechanically elastic than wood, and therefore vibrate in more modes. Theoretically, flutes constructed in thin tubes of elastic but heavy metals, such as alloys of gold, tungsten, platinum or osmium sound "richer" because they vibrate to a lower, therefore more audible range of harmonics. This effect also explains the excellent tone of bronze and brass flutes, which though inexpensive, are less massive, but more elastic.
Appearance and development
The precursors of the modern flute were keyless wooden transverse flutes, similar to modern fifes. Later these were modified to to be well-tempered, and include between 1 and 8 keys to aid in producing chromatic notes. The most common pitch for such flutes was and remains D, but other pitches sometimes occur. These simple system flutes continue to be used in folk music (particularly Irish traditional music) and in "historically informed" performances of baroque (and earlier) music.
See also pipe.