2002-12-10 - 11:58 a.m.
From Introduction to flute acoustics:
The design of a flute involves compromises and many notes required slight pitch adjustment by the player. Players lower the pitch mainly by a combination of rolling the flute's embouchure hole towards them, or drawing the chin back. These actions do two things: (i) they increase the fraction of the embouchure hole that is covered by the lower lip, thereby decreasing the size of the hole opening to the atmosphere, and (ii) they decrease the solid angle available into which the sound wave can radiate. Both of these effects increase the effective length of the flute and so make the resonant frequencies lower and the note flatter. Rolling the embouchure away and/or extending the lower jaw have the reverse effects, and so raise the pitch. Technically, these actions work because they change the radiation impedance at the embouchure: when a note is "lipped down", the hole and angle are smaller so there is more impedance to radiation from the bore to the external field.
