long-throw 15-inch (40 cm) MTX speaker drivers each powered by its own 1000-watt amplifier. The manufacturer claims that 53 13-ply 18 mm sheets of plywood were used in its construction, though one of the fabricators wrote that double-thickness 26-ply sheets were used for convenience. A diesel generator is housed within the enclosure to supply electricity when external power is unavailable. Of the constant tone output capability, designer Tom Danley wrote that the "target 94 dB at 250 meters is not the essentially fictional 'burst' or 'peak SPL' nonsense in pro sound, or like the 'death burp' signal used in car sound contests." At the annual National Systems Contractors Association (NSCA) convention in March 2007, the Matterhorn was barred from making any loud demonstrations of its power because of concerns about damaging the building of the Orange County Convention Center. Instead, using only a single 20 amp electrical circuit for safety, visitors were allowed to step inside the horn of the subwoofer for an "acoustic massage" as the fractionally powered Matterhorn reproduced low level 10–15 Hz waves. Royal Device custom installation Another subwoofer claimed to be the world's biggest is a custom installation in Italy made by Royal Device primarily of bricks, concrete and sound-deadening material consisting of two subwoofers embedded in the foundation of a listening room. The horn-loaded subwoofers each have a floor mouth that is , and a horn length that is , in a cavity under the floor of the listening room. Each subwoofer is driven by eight 18-inch subwoofer drivers with voice coils. The designers assert that the floor mouths of the horns are additionally loaded acoustically by a vertical wooden horn expansion and the room's ceiling to create a 10 Hz "full power" wave at the listening position. Concept Design 60-inch A single diameter subwoofer driver was designed by Richard Clark and David Navone with the help of Dr. Eugene Patronis of the Georgia Institute of Technology. The driver was intended to break sound pressure level records when mounted in a road vehicle, calculated to be able to achieve more than 180 dBSPL. It was built in 1997, driven by DC motors connected to a rotary crankshaft somewhat like in a piston engine. The cone diameter was and was held in place with a surround. With a peak-to-peak stroke, it created a one-way air displacement of . It was capable of generating 5–20 Hz sine