Anahata is the heart (fourth) chakra, the center of compassion, love and devotion. It is also the unstruck sound, the sound of the celestial realm.
Total silence is perfection. Silence is necessary for sound to exist, it is the aural canvas for melody’s brush in our three dimensional world. The sounds we know are produced by at least two elements – the waves on a shore, wind in the trees, the blades of a bagpipe reed vibrating together, two lips, drum and drumstick, fingering a guitar string and so on.
One of the koans of Zen asks, "What is the sound of one hand clapping?". This sound is known in the Sanskrit tradition as "Anahata Nada," the "Unmade Sound". This means a sound not made in the way we know of – it is the "sound" of the universe, the primal sound of energy itself.
Ancient tradition says that the audible sound which most resembles this unmade sound is the sound of "AUM" (OM). Aum is not, in itself, the un-struck sound, but leads one to it. The mantra is composed of four elements. Three are vocal sounds: A, U, and M, while the fourth element is the silence which begins and ends the audible sound, the silence which supports it. The objective of intoning AUM is not only the mantra itself, but the experience of perceiving the unmade sound that supports it. This is the same as seeking the space which "supports" the universe and its galaxies : the "emptiness" or nothingness of space is necessary for the existence of everything seen and unseen.
distilled from text by Mr. Roger Gould-King