The vibrating source of the human voice are the vocal cords in the throat. Their vibration, at a relatively low frequency of 125 hertz, creates oscillations in the air that fills the throat and mouth. By varying the size and shape of your mouth and position of the tongue you can change the frequency of the sound as well as the relative amplitude of the harmonics. Try it yourself! Sing a constant pitch, while you vocalize the vowel sounds—”a,” “e,” “i,” “o,” and “u.” Note how you change the shape of your mouth and position of the tongue when you go from one vowel sound to another.