This is probably for a few reasons chiefly because it s easiest to wire there but you should know that the nature of the sound coming from a sub mean that it.
Front firing subwoofer on carpet.
Also subs have dedicated power on board lessening the strain on your a v.
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Today in the irc chatroom ericduckman wants to know if putting his down firing subwoofer on carpet is going impede the quality of sound.
For decoupling vibration pads can be placed beneath the shelf or cabinet so that minimum sound vibrations are transferred to the floor.
Subwoofers are less obtrusive than the sort of floor standing speakers with drivers big enough to pull off the same task.
The default position most subwoofers end up in is at the front of the room near the entertainment system.
The same rule of heaviness applies here.
My family room is fairly small 15 x 12 and carpeted.
If the subwoofer is front firing it can be placed on a shelf or in a cabinet.
Down firing subwoofers are popularly used with surround sound systems to increase the vibrations and rumbles heard and felt during film viewing.
Scott wilkinson and chris boylan have an answer.
A word of caution for placing subwoofers on tables cabinets or shelves.