We should call “empty tops” or “Double top” to the process or making the guitar tops with Nomex. This process consists in a kind of sandwich made with two outer pieces of wood separate by the Nomex. Nomex is a lightweight honeycomb fiber . Cells are 0,2 mm thick and the size is 3,5 mm between vertices.
This material is extremely light, made of Kevlar fiber . It was developed as a thermal insulating for the clothing and shoes manufacturing for astronauts. Actually it is used in the panel building and in aviation industry.