Because they carry primarily music and sound effects, the left and right speakers should have the same wide, smooth response as good stereo speakers.
The center speaker carries much of the dialogue as well as music and effects, so ideally should be identical to the left and right speakers (A, "Wide" center mode on Pro Logic units). A smaller speaker can be used, however, thanks to the "Normal" mode that feeds center channel low bass to the left and right speaker (B). If necessary, you can do without a center speaker–"Phantom" gives the illusion of one, but only for viewers seated right on center.
Surround speakers need not reproduce low bass, so can be small and inconspicuous. Nevertheless, their overall sound balance should be as close as possible to the front speakers. Proper placement is vital to establish an evenly distributed soundfield.
A true subwoofer designed to reproduce powerful low bass effects (explosions, the rumble of spaceships, etc.) dramatically heightens involvement with the action on the screen. Some models are powered by built-in amplifiers, while others are designed to be driven by your existing amplifier, or, for better results, by a separate bass amp.
Speakers placed less than two feet from the TV set must be magnetically shielded models to prevent picture distortion. Virtually all speakers designed specifically for center channel use are shielded, as are many other "A/V" models.
Center and surround models from virtually all speaker manufacturers make it easy to convert an existing stereo system to home theater.
Many speaker manufacturers offer high-quality in-wall systems that effectively "disappear" in the room. All five speakers can be in-wall units, or just the fronts or surrounds.
Satellite speaker systems, with a separate speaker unit handling the low bass that the ear cannot localize, are particularly appropriate when space is at a premium.
Align the center speaker evenly with (A), or slightly behind (B), the left and right speakers, but not ahead of them.
Placing the left and right speakers to form a 45 degree angle with your favorite viewing position will duplicate the soundtrack mixer's perspective.
The mid- and high-frequency drivers of the three front speakers should be as close as possible to the same height. This often requires placing the center speaker directly atop (A) or beneath (B) the TV set; be sure to use a magnetically shielded center speaker (see Home Theater Speakers, above about Magnetic shielding.)