Nadanusandhan means research of sound. There are two types of Naads- Aahata Naad and Anahata Naad.
Aahata Naad like- are popular like Tabla, Sarangi, Harmonium, Dholak, Manjira, Veena etc., which are played by blowing. These are useful for music.
Anahata Naad is that which the seeker has to hear from within in his spiritual practice. For these, one should first try to concentrate on the gross and gradually leaving the gross and meditating on the subtle. This concentration is what gives the state of samadhi. When rhythm of the mind, the work of nadanusandhan of is completed. First of all, different types of sounds are heard. As the practice becomes firm, subtler and more subtle sounds begin to be heard. First Sea, Cloud, Waterfall etc. In the middle the soft sound of Bheri etc. and in the end the sound of Kinkini, Flute, Veena, Bhaura etc., many types of subtle sounds are heard. While doing the practice of going from subtle to gross and from gross to subtle, if it becomes subtle from subtle, then the state of meditation will be firm. On attaining the state of samadhi, the state of Kundalini awakening will come automatically and success in sadhana will be achieved.
In the fourth chapter of Hatha Yoga Pradipika, Swami Swatmaram ji has described the practice of Nadanusandhan. The following stages of Nadanusandhan are as follows.
1. Arambhavastha: When the Brahmagranthi in the heart is pierced through by Pranayama, a sort of happiness is experienced in the heart and the anahata sound like various tinkling sounds of ornaments are heard. In this state, a yogi’s body becomes divine, glowing, healthy and emits a divine smell. The heart becomes void.
2. Ghatavastha : The airs are united into one and begin moving in the middle i.e., the susumna, The posture of a yogi becomes firm and he becomes wise like a God. By this means, the Vishnu granthi in the throat in pierced which is indicated by highest pleasure experienced then the Bheri sound (like the beating of a kettle drum) is evolved in the throat.
3. Parichayavavtha: The sound of the drum is known to arise in the Sunya between the eyebrows. Then the Nada goes to the Mahasunya, the home of all the Siddhis, Conqueting the pleasures of the mind, ecestacy (Sahajananda) is spontancously produced which is devoid of evils, pains, old age, discase, hunger and slecp.
4. Nispatti avastha: When the Rudra grantht is pierced and the air enters the seat of the Lord i.e., the space between the eyebrows the perfect sound like that of a flute ia produced.
The contemplative man, having closed his ears with his fingers, should hear the sound, till the mind becomes steady in it. By practising it, all other external sounds are stopped and the yogi becomes happy by overcoming all distraction.