It is primarily f1 and f2 that are most important: the different numerical values (hz) for f1 and f2 determine how we perceive them, tongue and lips influence size and shape of vocal tract the most. Formants: properties of vocal tract itself (independent of whether a laryngeal source is present or not, caused by position of the articulators, not a property of the speech signal itself. Formant frequencies: length: average frequencies of vowel formants inversely proportional to the length of the pharyngeal-oral tract. The longer the tract, the lower the average formant frequencies, & vice versa: height of tongue: inverse relationship between height of the tongue and f2. The higher the vowel, the lower the f1 & vice versa: backness of tongue: inverse relationship between backness of tongue and f2. The basic idea is to model the vocal tract at a series of tubes: 2.