If you are not using full range speakers, then you will need a subwoofer, and even if you do have full range speakers, surround processors often have a subwoofer output jack, or alternatively, a "mono out", to connect such a speaker. You can also use the tape loop outputs to drive a subwoofer, but the volume control on the subwoofer will have to be changed when you alter the main volume setting. Subwoofers are designed to reproduce only the lowest frequencies in the music or movie sound track. There are 10 octaves in the range of human hearing. Each octave doubles the frequency. So, octave 1 is 20 Hz - 40 Hz, octave 2 is 40 Hz - 80 Hz, octave 3 is 80 Hz - 160 Hz, and so on, up to 20 kHz. Subwoofers reproduce the lowest 2 octaves, or 20 - 80Hz. Most of the speakers designed for surround sound will reproduce frequencies down to about 80 Hz, so this is where the subwoofer comes into play.
Several types of subwoofers are available. The drivers range in size up to 18" in diameter. Some have built in power amplifiers, and others are just the driver in the enclosure (you need a separate power amplifier to use them). For those readers who need an inconspicuous, yet powerful subwoofer, there are several designs with small enclosures, but have very long throw drivers and 1,000 watt amplifiers, like the one shown on the right, which has a 10" driver. The example on the left is a 12" push-pull subwoofer. Depending on which type of subwoofer you choose, and you decide on one of the power amplifiers that has several channels of amplification (3 - 6) all in one chassis, you will need to consider how many amplifier channels to purchase. If you are not going to use the center channel (in which case, you are using the "phantom" mode), and the subwoofer is self powered, then you only need a four channel power amplifier. If you use the center channel and a self powered subwoofer, then you need a five channel power amplifier, and if you use the center channel and a subwoofer that has no amplifier of its own, then a six channel power amplifier is in order. Remember to make your choices up front. The majority of commercial subwoofers are self powered.
One of the advantages of the self powered subwoofer is that it can be of a design called "servo feedback". This means that a sensor on or near the speaker cone feeds back information to the amplifier about how the cone is moving when a signal is passed through the voice coil. A special circuit compares this sensory feedback with the amplifier signal that is being sent to the speaker. If the two signals are different (and they are, because the speaker is producing harmonic distortion), the signal to the speaker is modified by adding electrical information that will cancel out the distorted movements of the speaker cone. The servo feedback subwoofer produces the most accurate sound quality (if properly designed), because most of the even and odd ordered harmonic distortion is cancelled out. The next most accurate (freedom from harmonic distortion) subwoofer design is the active push pull (both speakers electrically connected; see above), where the even ordered harmonic distortion components are cancelled out. Passive radiator subwoofers would probably be in third place, behind the active push pull subwoofer, and finally, a simple vented enclosure subwoofer.
Subjectively higher volume will be heard from non servo feedback subwoofers because some of the harmonic distortion is still present. However, this distortion is not as irritating at low frequencies as it is at higher frequencies, so, if you want a subwoofer with additional "punch", you might consider one of the other types listed above. As always, listen and compare, since a poorly designed servo feedback subwoofer could have more distortion than a well designed acoustic suspension subwoofer with no servo feedback or push pull.
When auditioning speakers, listen primarily for accuracy of the instruments (a piano should sound natural). The soundstage may be difficult to compare between different speakers since they are packed into the demonstration rooms at various distances apart. Rap your knuckles on the speaker cabinet. If it sounds like you are rapping against a piece of granite, this means the cabinet is well damped from vibrating on its own (the inside of the cabinet contains wool, fiberglass, sometimes lead or sand to dampen vibrations of the cabinet). If rapping your knuckles against the cabinet results in a drum-like sound, called resonance, the cabinet may not be well damped. Resonance can add artifacts to the sound (the vibrating cabinet acts like a speaker itself). Click here to listen to a wav file that was recorded when rapping on a speaker that has excellent damping (the speakers shown on the left).
Most speakers have a vinyl finish (usually black) with a woodgrained appearance. Some of these look like real wood, and it is difficult to tell that it is vinyl. If you want real wood veneers, many designs offer this as an option. Exotic woods add several hundred dollars to the final cost. The speakers on the left have a rosewood veneer that is exquisite. Other beautiful and unusual woods include jarrah and bubinga.
Move around the room during the speaker demonstration. Listen for how the sound changes as you move side to side, or from a sitting to a standing position. Ambience will be difficult to test in the dealer's demonstration room, since your own room at home will be much different. However, a certain degree of ambience can be heard with different surround systems, such as direct, bipole, and dipole. Careful listening pays off here. You should take a few of your favorite CDs with you to audition equipment. Include music with a wide variety of instruments and dynamics (loudness variations) such as an orchestral symphony, as well as sharp transient sounds (like steel string guitar), piano music, and solo singing voices (especially female). Several brands carry models specifically made for home theater.
At low volume, even the least expensive speakers can sound very good. Be sure to audition the speakers at the maximum volume you would be listening to them at home. At some loudness point, every speaker begins to produce significant amounts of harmonic distortion. A fundamental sinewave in the low midrange, such as 500 Hz, can have many harmonics that are audible, i.e., 1 kHz, 1.5 kHz, 2 kHz, 2.5 kHz, etc. This makes the sound mushy. The diagram on the right shows a sinewave in blue. When there is significant harmonic distortion, the waveform looks like the outline in red (like a squarewave). The red area is distortion. So, if the speakers you are considering sound mushy at the loudest volume you like to listen, try a different model or brand. Choir music is a good test for this.
Keep in mind that digital surround will be best with full range speakers even in the rear surround channels (which will be stereo), so, if you purchase small limited range speakers for the rear channel when using Pro Logic, they will not reproduce the low frequencies present in DD or DTS sound tracks. A subwoofer could be used to carry the low frequency sounds in the rear channels, and, in fact, could be used to carry the low frequency sound from all the channels. However, adding all the low frequency information together from several channels can result is "phase cancellation" which means that some of the sound in one channel would cancel sound in other channels where the voltage is moving in the opposite direction. Also, beginning at about 50 Hz - 60 Hz and above, we can localize the direction of the sound, so having all the <80 Hz sound go to one subwoofer would lessen the soundstage. Speak with your sales representative about how best to optimize your home theater with the system you are purchasing, and plan for the future at the same time, economically.