I would like to talk a little about Nādānusandhāna, which is one of the important and special sadhanas of Natha yogis.
Nādānusandhāna is translated as “study, investigation of nada; aiming at nada” This is immersion in the sound Nāda, connection with it, dissolution in it. This practice belongs to Laya Yoga, the yoga of dissolution. It is also called Nāda-upāsana (nādopāsana), which means honoring Nāda or worshiping Nāda. Nāda is not just a sound in the ordinary sense of the word, it is Shakti herself, the Goddess. Nāda is the primordial vibration, the unstruck sound “anāhata”, the spanda that creates the Universe. This is Shabda-brahman, the Absolute in the form of sound.
The Goraksha Vachana Sangraha says:
One who achieves hearing the sound “anāhata-shabda” reaches [gradually] the highest state of Shiva: first he goes deep into the sound and comprehends [what is beyond it], then he plunges the mind into this realized, and then the mind achieves dissolution there. (148) Everything that is heard as Nāda is Shakti herself. That which is behind reality, devoid of image, is the Supreme Lord himself. (149)
How to start practicing Nādānusandhāna?
⚡️The most natural method I can advise is to find a quiet place where against the background of external silence, you begin to hear your inner vibration, the sounds of the body. Observe them, trying to catch ever subtler sounds, immerse your mind, your attention there. You can also meditate on the "sound of silence" if you can hear it.
⚡️The practice with the mantra OM (ॐ, auṃ), consisting of chanting A-U-M aloud with each exhale, with the last sound M being longer than the others, into which you should immerse your mind. This sound M in Sanskrit is called an anusvara (nasal sound), and it contains Nada and Bindu (the point where everything dissolves). After 10-15 minutes of OM practice, you can sit in silence and listen to the internal sound, the vibration. Through practicing this mantra, which by the way represents the manifested Universe, your perception is gradually changing and you become capable of perceiving subtle sounds.
⚡️The practice of Bhramari pranayama - on the exhale, you produce a humming similar to the buzzing of a bee and try to immerse yourself in it, dissolve into it. The exhale should be as long as possible. It is recommended to close the ears with the thumbs or perform Shanmukhi Mudra (when, in addition to the ears, the eyes, nose, and mouth are symbolically closed to redirect perception inward). The principle of action is similar to the practice of OM: after some time of focusing attention on the gross sound (humming), consciousness becomes more receptive to subtle sound.
⚡️Ujjayi pranayama practice can also be considered as leading to Nādānusandhāna. Ujjayi pranayama consists of producing a hissing sound (using slight constriction of the throat) on both inhale and exhale. Here too, you immerse your mind in this hissing sound.
⚡️The practice of Soham. Soham is the mantra of our breath, it is our inhales and exhales. By consciously repeating Soham during breathing (“So” on inhale and “Ham” on exhale), you can gradually come to the point where inhales and exhales merge into each other, unite into something singular, and this will be Nada, the transcendental vibration of the Absolute.
In conclusion, I want to note that most practices are aimed at enabling you to transition from gross sound to subtle sound; gross sound is used as a support from which one can push off. This transition is not easy for many, and it occurs when the state of prana (energy) changes, which is why these methods involve either pranayama, or a mantra, or both aspects. Mantra also influences prana, so Mantra-yoga can also open the door to the realization of Nāda. Mantra is a sound form of Deity, and the base of all mantra’s sounds as well as the essence of Deity is Nāda-Brahman.