Here are the important characteristics of speakers:
Computer speakers are sold in sets. Two-piece sets include two small speakers intended to sit on your desk or attach to your monitor. Three-piece sets add a subwoofer, which resides under the desk and provides enhanced bass response. Four-piece sets include four small speakers, and are useful primarily to gamers who have a 3D-capable sound card installed. Five-piece sets add a subwoofer to that arrangement. Six-piece sets include a subwoofer, a center-channel speaker, and four satellites, and are intended for PC-based home-theater applications. Most headphones use only two speakers, one per ear, but some use two horizontally offset speakers per ear to provide true four-channel support.
Frequency response is the range of sound frequencies that the speaker can reproduce. The values provided for most speakers are meaningless because they do not specify how flat that response is. For example, professional studio-monitor speakers may provide 20 Hz to 20 kHz response at 1 dB. Expensive home-audio speakers may provide 20 Hz to 20 kHz response at 3 dB, and 40 Hz to 18 kHz response at 1 dB. Computer speakers may claim 20 Hz to 20 kHz response, but may rate that response at 10 dB or more, which makes the specification effectively meaningless. A reduction of about 3 dB halves volume, which means sounds lower than 100 Hz or higher than 10 kHz are nearly inaudible with many computer speakers. The only sure measure of adequate frequency response is that the speakers sound good to you, particularly for low bass and high treble sounds.
Manufacturers use two means to specify output power. Peak Power, which specifies the maximum wattage the amplifier can deliver instantaneously, is deceptive and should be disregarded. RMS Power (Root Mean Square), a more accurate measure, specifies the wattage that the amplifier can deliver continually. Listening to music at normal volume levels requires less than a watt. Home audio systems usually provide 100 watts per channel or more, which allows them to respond instantaneously to transient high-amplitude peaks in the music, particularly in bass notes, extending the dynamic range of the sound. The range of computer speakers is hampered by their small amplifiers, but computer speakers also use small drivers that cannot move much air anyway, so their lack of power is not really important. Typical dual-speaker sets provide 4 to 8 watts of RMS Power per channel, which is adequate for normal sound reproduction. Typical subwoofers provide 15 to 40 watts, which, combined with the typical 5-inch driver, is adequate to provide flat bass response down to 60 Hz or so (although subwoofers often misleadingly claim response to 20 Hz). Headphones are not amplified, but use the line-level output of the sound card.
Most computer speakers place the amplifier in one speaker, which has connections for Line-in (from the sound card), Speaker (to the other speaker), and DC Power (to a power brick). Many speakers also provide an output for a subwoofer. Some speakers also provide a second Line-in jack. This is quite useful if you want to connect both your PC and a separate line-level audio source, such as a CD player or another PC, to the amplified speakers, allowing you to listen to either source separately or both together. An increasing number of high-end speakers?particularly six-channel Dolby Digital 5.1 systems?provide direct digital inputs via a Digital DIN connector, an SP/DIF connector, or both.