nādānusandhāna, नादानुसन्धान 

Nadanusandhana (नादा, nāda means "sound", अनुसन्धान, anusandhāna – "search" or "research") is the contemplative practice of focusing on the inner sound.

Nadanusandhana is associated with the absorption of the mind by the sound of anahata-nada. A disciple (Natha-yogi aughar) receives knowledge from the Guru through sound (anahata-nada), and through absorption in this sound he attains Brahman. The practice is also called nada-upasana – worship through sound, cultivation of inner sound, or nada-yoga.

Concentration on the inner sound allows to detach consciousness from external objects of feelings and redirect it inward – to Atman, which is empty and omnipresent in nature and identical to the universal soul – Paramatma. Thus, nada is a tool for realization (samarasattva) and nonduality.

“Sounds from Anahata unite with Chaitanya (Pure consciousness), thought is absorbed (Paravairagya) and ceases, devoid of its objects. This dissolution is laya, leading to the highest state of Vishnu (Vishnu Parampada)."

Swatmarama, Hatha-Yoga Pradipika 4.99