Srimad Bhagavatam Canto 11, Chapter 21, Text 36

SB 11.21.36

sabda-brahma su-durbodham
ananta-param gambhiram
 durvigahyam samudra-vat
The transcendental sound of the Vedas is very difficult to comprehend and manifests on different levels within the prana, senses and mind. This Vedic sound is unlimited, very deep and unfathomable, just like the ocean.
According to Vedic knowledge, the Vedic sound is divided into four phases, which can be understood only by the most intelligent brahmanas. This is because three of the divisions are internally situated within the living entity and only the fourth division is externally manifested, as speech. Even this fourth phase of Vedic sound, called vaikhari, is very difficult to understand for ordinary human beings. Srila Visvanatha Cakravarti Thakura explains these divisions as follows. The prana phase of Vedic sound, known as para, is situated in the adhara-cakra; the mental phase, known as pasyanti, is situated in the area of the navel, on the manipuraka-cakra; the intellectual phase, known as madhyama, is situated in the heart area, in the anahata-cakra. Finally, the manifest sensory phase of Vedic sound is called vaikhari.
Such Vedic sound is ananta-para because it comprehends all vital energies within the universe and beyond and is thus undivided by time or space. Actually, Vedic sound vibration is so subtle, unfathomable and deep that only the Lord Himself and His empowered followers such as Vyasa and Narada can understand its actual form and meaning. Ordinary human beings cannot comprehend all of the intricacies and subtleties of Vedic sound, but if one takes to Krsna consciousness one can immediately understand the conclusion of all Vedic knowledge, namely Lord Krsna Himself, the original source of Vedic knowledge. Foolish persons devote their vital air, senses and mind to sense gratification and thus do not understand the transcendental value of the holy name of God. Ultimately, the essence of all Vedic sound is the holy name of the Supreme Lord, which is not different from the Lord Himself. Since the Lord is unlimited, His holy name is equally unlimited. No one can understand the transcendental glories of the Lord without the Lord’s direct mercy. By offenselessly chanting the holy names Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare, one can enter into the transcendental mysteries of Vedic sound. Otherwise the knowledge of the Vedas will remain durvigahyam, or impossible to penetrate.
Srimad Bhagavatam Canto 11, Chapter 21, Text 35
Srimad Bhagavatam Canto 11, Chapter 21, Text 37