F1 & Formants: The Vocal Tract Is a Filter
The vocal folds are the source (producing all frequencies); the vocal tract is the "filter" — amplifying certain frequencies into formants.
As the sound from the vocal folds passes through the mouth and pharynx it gets "colored": formants are the frequency regions amplified by the vocal tract (F1, F2, F3…), appearing as "humps" on the spectral envelope. The key rule: lower larynx → longer vocal tract → lower F1. A low F1 means a "deep", grounded sound — this is the core of the app's resonance depth score (30% weight).
Experiment 9 · Move the Larynx, F1 Follows
The left side shows a vocal tract cross-section: the larynx (red) position decides the tract length. Drag "larynx position" and watch the tract lengthen/shorten while the F1 hump on the spectral envelope (yellow dashed line) moves left and right.
Formants = frequencies amplified by the vocal tract "loudspeaker". Lower F1 = lower larynx = deeper sound. Related: resonance depth chart