The Indian Bamboo Flute · Laws, Acoustics and Principles
Laws, principles and acoustical properties of the Indian bamboo flute — drawn from classical musicological tradition.
Classical scholarship identifies a definitive list. "The principal factors that determine the pitch, intensity and quality of the flute-note are" the following seven — each acting upon the sound independently, while all others remain constant.
Each law holds good provided all other factors remain constant. "Here and elsewhere the pitch of the flute means the pitch of the note, middle octave Sa of the flute."
"The length of the vibrating column of air in the flute is equal to the length of the tube from the mukha randhra down to the first svara randhra or stop-hole from which the air can make its escape. This length is equal to one-fourth of the wave length of the note heard."
"If the length of the vibrating column of air is 6 inches, the frequency of the note produced will be equal to 1100 ÷ 24 (i.e. 6 inches multiplied by four) = 550 vibrations per second. This is of course subject to certain end corrections."
How the speaking length changes: The classical texts are clear that it is not only the distance between the mouth-hole and the nearest open finger-hole that determines pitch — "but also the open or closed condition of the subsequent two finger-holes."
Overblowing: "By over-blowing, the octave of the whole series of notes is obtained." This is the mechanism by which a single flute achieves two and a half octaves — not by changing finger positions, but by adjusting breath pressure to excite higher harmonics.
Partial opening: "The frequency of a full note is heard when the svara randhra is fully open. When it is open three-quarters, the pitch is reduced by one shruthi; when the finger-hole is half open, there is a reduction by two shruthis; when quarter-open, a reduction by three shruthis."
This is the physical basis of the bansuri's capacity for infinite gradation — the microtonal language of Indian classical music.
"A flute-note is sounded by blowing a stream of air across the opposite edge of the mouth-hole. The air is not blown into the orifice; only a part of it enters the tube. The flat stream of air blown through the small aperture between the lips results in edge-tones."
"When all the finger-holes are closed, the lowest note is produced and the speaking length of the air column is from the mouth-hole to the open end. With the opening of each finger-hole from the open end, the speaking or the effective length of the tube is lessened and this results in a corresponding increase of pitch."
The flute-tone's purity: "The flute-tone, unlike the human voice and stringed instruments, is pure and simple and is accompanied by few and feeble upper-partials. The acoustic spectrum of the flute reveals this character of the instrument."
The air-chamber: "The air-chamber between the node and the embouchure and where the end is artificially closed with a stopper, the stopper's size, position and form, play a part in giving the peculiar timbre to the flute."
Underblowing: "Underblowing will result in the production of notes an octave lower down." — the mechanism by which the Mandra (lower) octave is accessed.
Harmonics on the European flute: For comparison, "in the European flute, it is possible for a trained flutist to play notes up to the 5th harmonic, by mere adjustment in blowing and without the change of fingers" — producing: mandra shadja, madhya shadja, madhya panchama, tara shadja, and tara sthayi antara gandhara.
"The flute is an ideal instrument for the analytical study of shruthis (quarter-tones) and gamakas (graces)."
Semitones and quarter-tones are produced on the Indian flute "by the partial opening or closing of the finger-holes." The Indian flute's direct finger-to-hole contact means "the problem of playing ragas which take consecutive shruthis does not arise."
The classical texts are precise about the acoustical consequence of shruthi deviation: "If during flute playing, there are a few moments when the shruthi (tonic note) is slightly departed from, it is not the fault of the player. It is a defect inherent in all wind instruments, including the human voice."
"Intricate gamakas are produced through cross-fingerings. The kampita madhyama of Yadukulakambhoji raga and the kampita rendering of the kaisiki nishada are instances in point."
"The fingers reveal to the flutist the delicate nature and the frequency relationships of such subtle notes as the Saveri dhaivata and Kuranji nishada."
"The human voice and the flute are alike in many respects. They possess the same compass, viz., 2½ octaves, and are both monophonous — i.e., capable of producing only one note at a time."
"In singing, the air from the lungs sets the vocal cords in vibration and produces music. In flute-playing, the air from the lungs first passes through the aperture between the vocal cords without setting them in vibration; this air after passing through the mouth, enters the flute through the mouth-hole and gives notes of different pitch."
Key similarities: Same compass (2½ octaves) · Both monophonous · Both capable of the subtlest gamakas · Both subject to shruthi drift inherent in all wind instruments
Key differences: "A very high degree of speed is possible on the flute, which is not ordinarily possible in the voice." Unlike stringed instruments with strings of gut and metal, "the flute, like the voice, has a constant tone-colour over the whole available compass."
The discovery of harmonics: "It was the flute that gave the idea of larger intervals to early man. When a note like shadja (C) was played with some force, the octave note was immediately heard." — the flute thus gave early man his first encounter with the harmonic series.
The flute has played many roles as an acoustical instrument — not just a concert instrument:
"In the days when the tambura and the present acoustical devices were unknown and notes of absolute pitch could not be readily sounded, the flute served as an instrument of absolute pitch for tuning stringed instruments like the vina and the yazh."
"The flute served as an acoustical meter to study the nature of gamakas and the lore underlying various musical phenomena. It was of use in working out to their logical conclusions the cycles of fifths and fourths."