The digraph th represents a dental consonant and corresponds to two different sounds (voiceless and voiced).
The sounds of th are:
th (/θ/): thing, think, three;
th (/ð/): father, that, the.
Th is a digraph, which means that two letters together represent just one sound. To pronounce it, you need to pass the tongue through the upper and lower teeth, that is why this digraph is called dental.
- th (/θ/) is voiceless so the vocal cords do not vibrate to produce the sound;
- th (/ð/) is voiced so the vocal cords vibrate to produce the sound.
External link to Sound: TH exercises (1158).