The Upanishads

, the third sound.
This initiates measurement and merging.
Whoever awakens to this merges with the world yet has the measure of all things

This initiates upholding and unification.
Whoever awakens to this upholds the tradition of knowledge and unifies the diversities of life.
Everything that arises speaks to him of brahman.

Experience of the inner world corresponds to U, the second sound.