The TS-4 Grille silencer is only 4" deep and is ideal for installation behind Return Air Grilles to reduce the transmission of sound into the return air duct/chase/plenum. Silencers are constructed with a 22GA galvanized steel outer casing and 22GA perforated galvanized steel baffles filled with inorganic glass fiber. Silencers are undersized by 1/4" in both the width and height. Sizes are listed "A" x "B" with the largest single module size being 48" x 48", however, multipule modules can be assembled to form a larger overall bank size.