We also ordered 9 6-foot square sheets (measuring 24x36 inches) of Extremeliner (PN 21205). Extremeliner provides high acoustic absorption and excellent thermal insulation for a variety of automotive uses and is considered their ultimate low-frequency sound attenuator because its four-part lead septum composite acts as a sound barrier. The thickness is 0.392 inch. Dynamic Control recommends using Extremeliner with a layer of Dynamat for best results. So that's just what we did.