(1) Vowels:
These are periodic sounds with a regular pattern of vibration. When their spectrum is analysed, it is possible to see peaks of energy at different frequency, rather like the notes in a musical chord. These peaks of energy (which we call formants) are different for every vowel, and acoustic phonetics has analysed the frequencies of many different vowels so that we know a lot about how formants are related to vowel quality. Although the relationship is certainly not exact, it has been found that the formant with the lowest frequency (Formant 1) corresponds roughly to the traditional open/close dimension of vowels: a low Formant 1 corresponds to a close vowel like i or u . Formant 2, which is higher than Formant 1, corresponds roughly to the front/back dimension of vowels: a vowel with a high Formant 2 is likely to be a front vowel like e or a , while a vowel with a low Formant 2 is more likely to be a back vowel like o or u . It is not possible to give exact frequency values for the different formants, because these vary from speaker to speaker. You should look carefully at textbooks which describe the acoustics of vowels, and notice how depressingly often they assume that an adult male voice is "normal" and give little or no detail of women’s and children’s voices.

Look at the following figures, which show formants in vowels produced by different speakers.