In order to understand how flutes change pitch, you must first understand how flutes produce sound. A flute is a woodwind instrument, but unlike other woodwinds that make sound through a vibrating bamboo reed (such as clarinets, saxophones and oboes), a flute’s sound happens as the player blows air over the top of the mouthpiece hole. When the air is projected at just the right angle and pressure, it vibrates against the far edge of the hole. This creates a sound which then resonates throughout the hollow cavity of the flute, naturally amplifying it (like and echo in a cave) and giving it pitch.
As the mouthpiece vibration resonates throughout the instrument, the pitch of the flute is determined by the instrument’s size. The larger the flute is, the deeper the instrument’s pitch will be, since larger objects vibrate more slowly. This is why smaller flutes, like the piccolo, will have a higher pitch range than the larger soprano, alto and bass flutes.
Covering and uncovering the holes of the flute does not technically make the instrument larger or smaller, but it has the same effect on pitch because it changes how the instrument is resonating; when air is escaping from a hole at a certain point along the flute’s length, the flute will only vibrate up to that point.
A simple flute, such as a folk flute, has its holes covered and uncovered by a player’s fingers. With a classical flute, there are too many holes for this to be possible, so many holes are covered and uncovered by hinged and spring-loaded mechanisms attached to soft discs that cover the holes. The player uses keys that will operate several of these at a time.
Flute pitch is also changed by player’s control of air. When the air is blown at different strengths, it will cause the mouthpiece vibrations to occur faster or slow, making a higher or lower pitch set. These pitch sets are known as “partials,” and every flute player must learn to control them as a fundamental part of playing the instrument.