This 15" Q Series enclosure have a unique acoustical fiber board permanently affixed to the inner walls of the enclosure which effectively eliminates the "hangover" and vibration normally associated with large metal enclosures. To achieve additional rigidity, unique ribs are formed on the back side of the enclosure. In addition, the inner walls are lined with 11/2" (38mm) thick fiberglass with a 11/2 lb./cu.ft. density to control unwanted resonances in the high end of the frequency spectrum.The "Q" Series enclosures are constructed of 18 gauge cold rolled steel. Compound 1/2" - 3/4" (13mm-19mm) knockouts are furnished on all four sides. The front mounting panel of the enclosure is constructed of sturdy 18 gauge steel and incorporates the baffle mounting plaster ring. This panel is isolated by means of a foam gasket to prevent metal to metal contact. Two sets of slotted mounting tabs are welded on parallel sides to accept Model QR30 mounting channels (optional) for mounting the enclosure in the ceiling. These mounting tabs may also be used for any specific mounting technique developed by the installer. It is important that the sound contractor take particular note of the weight of these enclosures to determine suitable mounting procedures. The entire metal enclosure is finished in black textured epoxy. Models with a volume of 1-3 cu. ft include 1/2" fiberboard. Models with a volume of 4-6 cu. ft include 1" fiberboard.