Nada, along with shastra (scripture), bhakti (devotion), ahimsa (non-violence) and dhyana (meditation) is one of the five central tenets of Jivamukti yoga.
It is said that there are four levels of nada, listed here from the grossest to the subtlest:
1. Vaikhari: Audible, struck sound that can be heard by the human ear. It is a sound of speech and song, the sound that is made when two objects strike each other. This is the grossest level of sound.
2. Madhyama: A mental sound that is a little more subtle than vaikhari. This is when the subtle nada begins to emerge.
3. Pashyanti: A visual sound, one that can be seen. A dream of music or a visualization of the quality of a sound.
4. Para nada: A transcendent sound with infinite wavelength, associated with the sound Om (or Aum). It is the final stage before samadhi, heard in a state of super consciousness. This is the anahata, the unstruck or silent sound. Hearing this sound is a goal of practicing Nada yoga.