In the Sanskrit tradition, this sound is called "Anahata Nada," the "Unstuck Sound." Literally, this means "the sound that is not made by two things striking together." The point of this particular distinction is that all ordinary audible sounds are made by at least two elements: bow and string; drum and stick; two vocal cords; two lips against the mouthpiece of the trumpet; the double reed of the oboe; waves against the shore; wind against the leaves. Things create all sounds within our range of hearing visible or invisible, striking each other or vibrating together, creating pulsing waves of air molecules, which our ears and brain interpret as sound.

So, sound that is not made of two things striking together is the sound of primal energy, the sound of the universe itself. We can compare this unstuck vibration to the humming of an electrical transformer, or the (to our ears) unheard humming of atoms and molecules.

And the ancients say that the audible sound which most resembles this unstuck sound is the syllable OM. Tradition has it that this ancient mantra is composed of four elements: the first three are vocal sounds: A, U, and M. The fourth sound, unheard, is the silence, which begins and ends the audible sound, the silence that surrounds us.