,,This word (OM) indicates the coexistence of the articulated and unarticulated sound of the heart, and of the melody that is hearable and not hearable – of the reverberate sound and the non-reverberate sound, Anahata Nada. Sound can be described by its triple nature – audible sound, inaudible sound and everlasting sound. The audible sound is the one that can be heard by the human ear. The inaudible sound is that which belongs to the higher, or lower, octaves, so that it cannot be sated by normal hearing. But there is also the third category of sounds, which is that of everlasting sound. The sound is made up of vibrations, and all vibrations have a beginning and an end. But if there was a sound that was not produced – Anahata Nada – then alone it could not have an end because it does not have a beginning. To talk about a vibration – without sound is indeed a paradox. In the sacred word OM, there is such a paradox. He is at the same time heard and unheard, reverberating and unreverberating. It is at the same time perishing and everlasting." - Rohit Mehta - The Call of the Upanishads
The sound or pranana AUM is metionate in Upanishade, and is treated specifically in Taitrirya, Chandogya and Mandukya Upanishd. It is used as an object of deep meditation, being considered to have a special spiritual efficiency, this fact is attributed not only to the whole word but also because of the three sounds of which it is composed: a (a-kāra), u (u-kāra), m (ma-kāra).
A – kāra means form, and refers to the earth element, to what grows on the earth, such as vegetation, trees or any other forms or material objects. U-kāra means devoid of form and refers to the element water or air or to the fire that burns. Ma-kāra means without any form (which exists) and can be associated with the dark energy of the Universe. When we combine the three sounds together, the sacar syllable is formed, AUM , it brings together all the elements, mentioned above.
In Hindu philosophy it is believed that when creation began to manifest itself, divine consciousness first took the primary and original form manifested by the vibration of the AUM sound. Before creation there is only the vacuum or Shunyākāsha. Shunyākāsha means in literal translation "without heaven" which is more than nothing, because everything then exists in a state of potentiality. The vibration of the AUM sound symbolizes the manifestation of God in the form of "Brahman Saguna". "OM" is the reflection of absolute reality also known as "Adi Anadi",that is, the one without beginning or end, and embracing all that exists, representing the ultimate vibration. When pronounced in the letters A-U-M, it represents the divine energy Of Shakty, united in its three aspects: Bhrahma Shakti (creation), Vishnu Shakti (preservation or maintenance) and Shiva Shakti (destruction and/or liberation).
Pranava represents the primordial sound that existed before creation and the sound that will exist after its resorption, after pralaya. This is NatharUpa – that is, the form of sound that belongs to the Supreme Enlightenment.
There are hundreds of references about the AUM pranava in the sacred writings of Upanisade. One of them refers to the mantra pranava (or mantra bija) as follows:
pranavo dhanu sharo hyaatmaa brahm tallakshya muchyate.
apramatten vedadhavyam sharavattanmayo bhavet..
In translation this would mean – The Sacra AUM syllable names the bow, the arrow represents the soul and Brahman is the target; he will be pierced by the one whose attention does not deviate. Then (the archer) will become one with Brahman, as the arrow becomes one with the target when he hits it.