Nada yoga is union through internal sound. Nada in Sanskrit means sound. Nada yoga is meditating on the inner sound, also known as the sound of silence. Interestingly, in Spanish, nada means absolute absence of any being or thing, nonexistence.
The goal of Nada yoga is to achieve that stage where nothing is heard; the silence that is not heard with the ears, but that is felt and experienced.
The source of the sound can be external or internal. External sounds are gross, like the sounds caused by musical instruments, voices, friction, blows… The vibrations of sound penetrate all matter so a deaf person can experience sound. There are also less gross sounds in the human body that can be heard through physical instruments and through the practice of Nada yoga. Non-physical internal sounds are subtle and are not heard or felt by any of the five senses.
In Nada meditation, we focus first on an external sound, like the sound of the second hand of a wall clock. With practice, the mind calms down and the practitioner can start to become aware of the internal sounds. The sound of blood moving from one place to another through the veins, arteries, and capillaries is heard. Vibrations of the brain are felt… The goal is to eventually hear the sound of silence, the sound of nada.
Nada yoga is practiced in Tantra and Kundalini yoga.
In Hatha Yoga Pradikipa, Chapter 4, aphorisms 80 to 102, nada is explained.