The full compass of the
Boehm flute is chromatic, from middle C to C, two octaves above the
treble clef C, a range of three octaves, which is common to all
concert flutes, and is not peculiar to the Boehm model. Of course this
compass is partly produced by altering the pressure of blowing.
Columns of air inclosed in pipes vibrate like strings in sections,
but, unlike strings, the vibrations progress in the direction of
length, not across the direction of length. In the flute, all notes
below D, in the treble clef, are produced by the normal pressure of
wind; by an increasing pressure of overblowing the harmonics, D in the
treble clef, and A and B above it, are successively attained. The
fingerholes and keys, by shortening the tube, fill up the required
intervals of the scale. There are higher harmonics still, but
flautists generally prefer to do without them when they can get the
note required by a lower harmonic. In Boehm's flute, his ingenious
mechanism allows the production of the eleven chromatic semitones
intermediate between the fundamental note of the flute and its first
harmonic, by holes so disposed that, in opening them successively,
they shorten the column of air in exact proportion.
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