The way I was taught in meditation involved focusing on your “third eye” and listening to the frequency of your state of being.
I don’t know what has been reported on this but this frequency is often heard when it’s really quiet and you are totally relaxed(i.e when you are out camping) It sounds like a consistent but mild static. when you are more use to hearing this frequency. It won’t matter how loud it is around you.
Now this guru mentioned this is only the first state of existence. When you meditate on this sound and third eye more deeply. You can hear a progressive change in the tone. Such as birds chirping, drums, the gong, or even a choir-like symphony.
I am really interested in maintaining theta gama sync and I wonder if this technique is utilized at all if ever.