What is a sunken relief? where have many great examples of sunken reliefs been found?

Sunk or sunken relief is largely restricted to the art of Ancient Egypt where it is very common, becoming after the Amarna period of Ahkenaten the dominant type used, as opposed to low relief. It had been used earlier, but mainly for large reliefs on external walls, and for hieroglyphs and cartouches. a great example is  Relief is a sculptural technique where the sculpted elements remain attached to a solid background of the same material. The term relief is from the Latin verb relevo, to raise. To create a sculpture in relief is to give the impression that the sculpted material has been raised above the background plane.[

this musical excerpt is an example of piano volume. (true) !