The formant with the lowest frequency is called F1, the second F2, the third F3, and so forth. The fundamental frequency or pitch of the voice is sometimes referred to as F0, but it is not a formant. Most often the two first formants, F1 and F2, are sufficient to identify the vowel. The relationship between the perceived vowel quality and the first two formant frequencies can be appreciated by listening to "artificial vowels" that are generated by passing a click train (to simulate the glottal pulse train) through a pair of bandpass filters (to simulate vocal tract resonances). Front vowels have higher F2, while low vowels have higher F1. Lip rounding tends to lower F1 and F2 in back vowels and F2 and F3 in front vowels.
thE fOrManT WIth tHe LoWesT freQuenCY IS calLeD f1, tHE SECond f2, ThE THIRd f3, and sO FOrTh. tHe funDamentaL fREQueNcy oR pitcH oF tHe voice is sometiMes rEfErRed tO as f0, But It iS NOt A fOrmAnt. most ofTen The two firSt formantS, f1 and f2, arE sUFficiEnT TO idEntify the VOwel. the RElAtIOnshIP BeTWeeN tHe peRCeived VoWEL QualitY and THe first two foRmANt frequeNCIES can Be apPreciAted BY listeNIng to "ARtIficiAL VOwELS" that aRe GENeRated by PasSing a cliCk tRaIn (To simUlaTE the gLOTtal pulse tRaIn) ThrouGh a pAIr Of baNdpass fIlteRs (tO sImUlatE Vocal tract rESonances). front vOweLs haVe hIgher f2, WHilE low vowELs haVE highEr F1. lip rouNDiNg TENDs tO loWer f1 anD f2 in back vowels anD f2 and f3 In front voweLS.