Professional audio subwoofers must be capable of very high output levels. This is reflected in the design attention given in recent years to the subwoofer applications for sound reinforcement, public address, and concert systems. Consumer applications (as in home use) are considerably less demanding due to much smaller listening space and lower playback levels. Subwoofers are now almost universal in professional sound applications such as live concert sound, churches, nightclubs, and theme parks. Specifically, movie theatres certified to the THX standard for playback always include high capability subwoofers. Some professional applications require subwoofers designed for very high sound levels, using multiple 15", 18" or 21" drivers. Drivers as small as 10" are occasionally used, generally in horn loaded enclosures.
People accustomed to bass in home audio systems and car audio may think the subwoofers in a concert system don't have much low frequency output, since the human ear recognizes loudness (ie, sound pressure) logarithmically. Concert subs can be 10 times the power rating which equates to 10 more deciBels, but will only be twice as loud. Sound intensity obeys the inverse-square law in relation to distance from the source, and at outdoor events the crowd is many metres away from. The human ear is less sensitive to sound at lower frequencies, so very high levels are needed to hear 20 or 25Hz sounds.
The main speakers may be 'flown' from the ceiling of a venue on chain hoists, and 'flying points' (ie, attachement points) are built into many enclosures. Since subwoofers are larger, heavier, and least localizable components, they can be stacked on the ground near the stage. These enclosures may contain several 18-inch drivers, and the enclosures are typically built from Baltic Birch, sourced from Finland, Estonia or Russia. This material is void free, has many plies, and is exceptionally durable. There can be more than 50 double-18-inch cabinets in a typical concert system.
Subwoofers may or may not be fed with signal from the main program mix, instead having kick drum, bass and keyboards routed to them. This avoids the low frequency 'drone' of feedback that can occur if the vocal microphone signals are reproduced by to the subwoofer system. Some concert sound companies use subwoofer systems made by the same manufacturer of the flying speakers, but many others select subwoofers from other manufacturers, better suited to their truck-pack, cost, or other criteria.