For home stereo systems—and especially home theaters—a subwoofer adds a dynamic quality that most speakers alone can’t match.
First, let’s describe what a sub is: a subwoofer is a speaker designed to reproduce the lowest bass frequencies your stereo system can provide. Regular speakers—even 3-way speakers—can display a wide frequency range but virtually all of them fail to achieve the deepest bass frequencies. A powered subwoofer combines a woofer with a built-in amplifier that selectively powers the signal to provide deeper, cleaner bass. Powered subwoofers require an electrical outlet to power their built-in amplifier. Passive subwoofers do not require their own dedicated power supply and use power from the receiver's amp.
As part of a traditional two-channel stereo system, the addition of a subwoofer can bring out lower frequency tones you’ve never heard before in your favorite songs. In a home theater system, the addition is even more dramatic, as explosions and other deep special effects take on a totally new reality.
The key to getting the most from your subwoofer is proper placement. The three most common subwoofer locations include corners, placement along a wall away from the corner, or beside a front speaker (which can create a better blend with the front speakers, at the cost of sacrificing overall volume). Experimentation with subwoofer placement is highly encouraged; slight adjustments can create big differences in bass reproduction.