Nada means the flow of sound and Yoga means Union. Nada is called by different names - shabda, surat, nam, akashwani, dhun, nad-i-asmani, vani, logos, sraosha (in Zoroastrianism) etc. Nada Yoga is the process of the union of the individual mind with cosmic consciousness through the flow of sounds. Nada is also called Shabd or word and is produced either by striking two objects known as Ahat Nad or without striking two objects which is called Anahat Nad.
Nada Yoga as such is yogic practices which use sound to expand awareness. It is a powerful practice of tracing sound back through its psychic and subtle manifestation to the source. Nada is sound of chidakasha. It is a technique to turn the perception of hearing inwards to listen the pure sound of your inner spirit. This sacred sound practice also utilizes chanting Sanskrit mantra and listening to meditative external music like Indian classical music.
Pythagoras conceived that the whole universe and each individual is connected with the single string, the lower end to gross matter and at the upper end to pure consciousness. Modern science has postulated that everything in the universe is composed of wave forms, vibrating at different frequencies.
Different levels of nada:
The following fourfold classification has been given in the ancient texts:
Nada Yoga, an ancient Spiritual practice:
Sri Gorakhnatha (a great yogi) described various types of laya yoga (method of raising the kundalini). Amongst these nadanusandhana (discovering the source of nada) is the best. By this the brahma granthi is pierced and a wonderful bliss arises from the void. A tinkling sound can be heard emanating from the anahata chakra. Nada yoga is an important sadhana of many sects such as Radha Soami Satsang, Kabirpanthis etc. it is praised in Adi Granth (holy scripture in Sikhism).
In early stages of the practice, the aspirant hears gross, loud sounds. Gradually more subtle sounds are heard. The sounds are like those of the ocean, clouds…. Nada Bindu Upanishad. The nada from the flute is heard which arises from the underlying consciousness. The preferred time is midnight. The practise of Nada Yoga is done as follows:
Technique - I
Technique - II
Technique – III – Shanmukhi mudra (the attitude of the seven gates)
There four stages of attainment of the practice: beginning, continuation, ripening and culmination. In the beginning stage, brahmagranthi (associated with mooladhara chakra) is pierced and bliss appears from voidness. A sound like tinkling ornaments can be heard in the anahata chakra. Then the vishnugranthi (associated with anahata chakra) is pierced and bliss arises together with the sound of the bheri and vimarda (two kinds of musical instruments). In the last stage the prana enters the ajna chakra and pierces the rudra granthi and the sound of vina (flute) and a flute can be heard. The awareness should be fixed on the sound until it becomes soundless. This is the state super consciousness.
Reference: A systematic Course in the Ancient Tantric Techniques of Yoga and Kriya – by Swami Satyananda Saraswati