Voicing

The vocal folds may be held against each other at just the right tension so that the air flowing past them from the lungs will cause them to vibrate against each other. We call this process voicing. Sounds which are made with vocal fold vibration are said to be voiced. Sounds made without vocal fold vibration are said to be voiceless. There are several pairs of sounds in English which differ only in voicing -- that is, the two sounds have identical places and manners of articulation, but one has vocal fold vibration and the other doesn't. The [θ] of thigh and the [ð] of thy are one such pair. The others are:

voiceless	voiced
[p]	[b]
[t]	[d]
[k]	[ɡ]
[f]	[v]
[θ]	[ð]
[s]	[z]
[ʃ]	[ʒ]
[tʃ]	[dʒ]

The other sounds of English do not come in voiced/voiceless pairs. [h] is voicess, and has no voiced counterpart. The other English consonants are all voiced: [ɹ], [l], [w], [j], [m], [n], and [ŋ]. This does not mean that it is physically impossible to say a sound that is exactly like, for example, an [n] except without vocal fold vibration. It is simply that English has chosen not to use such sounds in its set of distinctive sounds. (It is possible even in English for one of these sounds to become voiceless under the influence of its neighbours, but this will never change the meaning of the word.)