The alto flute is a type of Western concert flute, a musical instrument in the woodwind family. It is the next extension downward of the C flute after the flûte d'amour. It is characterized by its distinct, mellow tone in the lower portion of its range.
The tube of the alto flute is considerably thicker and longer than a C flute and requires more breath from the player. However, this gives it a greater dynamic presence in the bottom octave and a half of its range.
Said to be popularized by Theobald Boehm, it is pitched in the key of G (sounding a 4th lower than written). Its range is from G3 (the G below middle C) to G6 (4 ledger lines above the treble clef staff) plus an altissimo register stretching to Db8. The headjoint may be straight or curved.
Both have Boehm-system keywork (in fact, the modern alto flute was developed by Boehm), and the fact that they are transposing instruments means that a flautist doesn't need to learn a new set of fingerings for each instrument.
In addition to being used in various flute choirs and concert ensembles, alto flutes are also popular in many jazz groups. It is considered a doubling instrument for saxophonists (saxophone). It is a harmonic instrument which allows the changing of note without adjusting the keys.
In the classical literature, the alto flute is particularly associated with the scores of Igor Stravinsky and Maurice Ravel, both of whom used the colouring effect of the instrument in a variety of places. It is featured in Ravel's Daphnis et Chloé, Stravinsky's Rite of Spring, Franco Alfano's opera Cyrano de Bergerac, Sergei Prokofiev's Scythian Suite and Shostakovich used it in his operas The Gamblers (incomplete), Katerina Ismailova, Lady Macbeth of the Mtsensk District and only in his Symphony No. 7 (Leningrad), for example, and in several movements of Gustav Holst's The Planets.
British music that uses this instrument often refers to it as a bass flute, which can be confusing since there is a distinct instrument known by that name.
Depicted above is the 'curved head' version, frequently preferred by smaller players because it requires less of a stretch for the arms, and makes the instrument feel lighter by moving the centre of gravity nearer to the player. The straight version is the one more commonly preferred for comfort and for acoustical reasons. An Alto Flute may be supplied with either a straight or curved head.
The embouchure for alto flute is similar to that for C flute, but in proportion to the size of the instrument. Hence the embouchure-hole sits lower on the lower lip, and the lip-aperture is wider.