As explained by Siri Singh Sahib Bhai Sahib Harbhajan Singh Khalsa Yogiji
Naad means "the essence of all sounds". All languages contain sounds which relate to one or more of the five elements of air, fire, water, earth or ether. Gurbani is a perfect combination and permutation of sounds relating to all the five elements in complete balance. When Guru Arjan Dev, the fifth Guru Nanak, compiled the Siri Guru Granth Sahib, in 1604, he put in only those Banis which were in Naad. These compositions are called Gurbani.
There are eighty-four meridian points on the upper palate of a human’s mouth. One can feel that upper palate with the tongue and experience its different surfaces. There are two rows of meridian points on the upper palate and on the gum behind the upper teeth. The tongue stimulates those meridian points, and they in turn stimulate the hypothalamus which makes the pineal gland radiate. When the pineal gland radiates, it creates an impulsation in the pituitary gland. When the pituitary gland gives impulsation, the entire glandular system secretes and a human being obtains bliss. This is the science.
Gurbani is the combination of the tongue with the meridian points. When you read and recite Gurbani, it stimulates your hypothalamus. It is totally different than any scriptures because Gurbani is made in a scientific way. There are two banis: "kachee bani" is that which does not fit on the Naad. It is not a balanced sound current. "Pakee bani" is that which fits on the Naad and is balanced. One who knows the Naad knows the Aad, the Primal Creativity. One who knows the Aad is Parmayshwaar, the Supreme God.
The whole language of Gurbani has the power to make a person divine, just in its recitation, if done correctly. One need not be concerned with the meaning for a change in consciousness. Bani has to be understood by the heart, not by the head. There is no power in the head, it is in the heart. The head is for God and the heart is for you. That is why Guru Gobind Singh asked for the head, and not for the heart. Whosoever lives with the head to God and heart for self, that prayer is complete.
The entire Siri Guru Granth Sahib is the calling of the Beloved. It is in Naad. Torah, Bible and Vedas are not in Naad because none of them felt that much pain of separation between God and the beloved."