Voiceless fricatives such as s , S are aperiodic, and do not have formants in the way that vowels do. They do, however, have concentrations of energy at different frequencies. If you make s and S alternately, you will hear that s sounds more high-pitched than S . This is particularly noticeable in English, since English speakers tend to round their lips for S , making this fricative sound even lower-pitched.
Voiced fricatives combine aperiodic with periodic sound. This is because they have the aperiodic hissing noise typical of voiceless fricatives such as f , s or S , combined with vibration of the vocal folds. The vibration of the vocal folds happens in a very regular way, which means that the buzzing sound made in the larynx is periodic.