Balance: Balance is a speaker control that determines how much sound appears to come from the left versus the right channels. The control reduces power to one side (left or right) making the apparent volume of the other louder.

Crossover: This circuit splits the audio signal into frequency bands, routing the lower band to the bass speaker/subwoofer. Ideally, the crossover frequency, below which signals get routed to the bass speaker/subwoofer, should be 100 Hz, since humans can begin to localize (determine the position and direction of) sounds above that.

dB SPL (decibel sound pressure level): A measurement of how loud a sound is. Sound pressure is the value over time of the rapid variation, caused by acoustic waves, in air pressure at a fixed point. Sound pressure level, which is given in dB SPL, is a logarithmic ratio of the sound pressure of the measured sound to the sound pressure at the threshold of hearing. The range of human hearing is generally said to be around 120dB, with 0dB representing absolute silence and 120dB to 130db representing the threshold of pain (depending on the reference you consult), at which severe hearing damage can occur. The table* shows db SPL values for some common sound levels.

*Source: Safetyline Institute

Decoder: A device that can take an incoming digital audio stream, decompress it, and convert it into discrete channels that are, in turn, converted to analog signals the speaker system sends to the appropriate speakers.

DirectSound and DirectSound3D: Microsoft sound APIs (application programming interfaces) that are part of the DirectX family of gaming APIs. Access to sound devices in a Windows-compatible manner is one of the capabilities the APIs give developers. DirectSound3D extends the abilities of programmers, letting them do 3-D positional audio, giving listeners the impression that sounds are coming from particular points in space.

Dolby Digital: Dolby Digital, also called AC-3, is the standard 5.1 audio format for DVD movie discs. Dolby Digital can also be down-mixed to a two-channel format when a DVD movie is played on a two-channel speaker system. A recent addition to Dolby Digital called Dolby Digital EX is a 6.1-channel format that provides specific information for the center-rear speaker channel.

Drivers: The parts of a speaker that actually produce sound; also called transducers. Drivers come in several forms, with the most common being tweeters for high-frequency sounds, midrange drivers for the middle band of audio frequencies (from roughly 150 Hz to about 2,000 Hz), and cone drivers, sometimes referred to as woofers, to handle bass frequencies.

DTS: This is another 5.1 audio format that some DVD movies use as an alternative to Dolby Digital. DTS is favored by home theatre aficionados because it uses less compression than Dolby Digital, and some believe it sounds better. The differences are often subtle, though, and opinions vary as to which format delivers superior sound quality. A new arrival, DTS ES, is a 6.1 version of DTS that provides specific information for the center-rear channel.

DVD audio: A fairly new multichannel audio CD format that delivers 5.1, 24-bit audio sampled at 96 KHz, or a two-channel down-mix at 192 KHz. This format delivers better audio quality than current CD audio, which has a 44 KHz, 16-bit resolution.

Efficiency: Also referred to as sensitivity, this speaker measurement gauges how much sound a speaker can produce when being driven by a specific amount of power, usually one watt. Greater efficiency is desirable because an amplifier won't have to work as hard to drive the speaker, and as a rule, the less an amplifier has to strain to drive a set of speakers, the better the audio will sound.

Fader: A control that changes the level of sound between front and rear speakers by attenuating the power going to one set, making the other seem louder.

Frequency: This is the number of cycles per second  a periodic signal, such as a sine wave, makes. For example, a 1 KHz sine wave test tone completes 1,000 cycles each second. If you were to graph a sine wave, one complete cycle would look like a dollar sign ($) turned sideways.