in Yamaha FS1R

How the Vocal Tract Filters Sound

The two largest spaces in the vocal tract, the throat and mouth, therefore, produce the two lowest resonant frequencies, or formants. These formants are designated as F1 (the throat/pharynx) and F2 (the mouth). In singing or speaking, it is these two lowest formants that are controlled by shaping the resonant areas with lip and tongue movements to produce vowels.

via The National Center for Voice and Speech – Tutorials