Ancient teachings and modern science agree on this score: you, I, all living beings and all things in existence are, at their most essential level, made up of vibrating, pulsing energy.
For millennia, mystics have recounted their experience of this energy, which is said to manifest in our hearing awareness as a humming vibration around everything else.
In the Sanskrit tradition, this sound is called Anahata Nada, the “Unstruck 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… All sounds within our range of hearing are created by things 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 cosmos itself. Don Campbell, an authority on the subject and from whom we have borrowed liberally, likens this unstruck vibration to the humming of an electrical transformer, or the (to our ears) unheard hummings of atoms and molecules.
The ancients say that the audible sound that most resembles this unstruck sound is the syllable AUM (or 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 which surrounds it.
There are several traditional and allegorical interpretations of this ancient sound.