There is something called "Mandukya Upanishads" which explain the four stages of AUM_, which some chant during meditation. I found the explanations of these con

There is something called "Mandukya Upanishads" which explain the four stages of AUM_, which some chant during meditation. I found the explanations of these concepts by a monk named "Sarvapriyananda" very useful. Search his name and the word "mandukya" on youtube for the explanation of what these four stages are.