the THX standard for playback always include high-capability subwoofers. Some professional applications require subwoofers designed for very high sound levels, using multiple 12-, 15-, 18- or 21-inch drivers (30 cm, 40 cm, 45 cm, 53 cm respectively). Drivers as small as 10-inch (25 cm) are occasionally used, generally in horn-loaded enclosures. The number of subwoofer enclosures used in a concert depends on a number of factors, including the size of the venue, whether it is indoors or outdoors, the amount of low-frequency content in the band's sound, the desired volume of the concert, and the design and construction of the enclosures (e.g., direct-radiating versus horn-loaded). A tiny coffeehouse may only need a single 10-inch subwoofer cabinet to augment the bass provided by the full-range speakers. A small bar may use one or two direct-radiating 15-inch (40 cm) subwoofer cabinets. A large dance club may have a row of four or five twin 18-inch (45 cm) subwoofer cabinets, or more. In the largest stadium venues, there may be a very large number of subwoofer enclosures. For example, the 2009–2010 U2 360° Tour used 24 Clair Brothers BT-218 subwoofers (a double 18-inch (45 cm) box) around the perimeter of the central circular stage, and 72 proprietary Clair Brothers cardioid S4 subwoofers placed underneath the ring-shaped "B" stage which encircles the central main stage. The main speakers may be 'flown' from the ceiling of a venue on chain hoists, and 'flying points' (i.e., attachment points) are built into many professional loudspeaker enclosures. Subwoofers can be flown or stacked on the ground near the stage. One of the reasons subwoofers may be installed on the ground is that on-the-ground installation can increase the bass performance, particularly if the subwoofer is placed in the corner of a room (conversely, if a subwoofer cabinet is perceived as too loud, alternatives to on-the-ground or in-corner installation may be considered). There can be more than 50 double-18-inch (45 cm) cabinets in a typical rock concert system. Just as consumer subwoofer enclosures can be made of medium-density fibreboard (MDF), oriented strand board (OSB), plywood, plastic or other dense material, professional subwoofer enclosures can be built from the same materials. MDF is commonly used to construct subwoofers for permanent installations as its density is relatively high and weatherproofing is not a concern. Other permanent installation subwoofers have used very thick plywood: the Altec 8182 (1981) used 7-ply 28 mm birch-faced