Passive and Active Subwoofers
Put simply, subwoofers are designed to handle the â€˜.1â€™ of 5.1-channel sound, which is a low frequency effects channel found on the vast majority of Blu-ray and DVD movie soundtracks. Subs boost this bass content to heighten the intensity and excitement of movie playback.
Most home cinema speakers can only reproduce sounds down to a certain frequency, so for movies â€“ which typically feature a lot more deep bass than music â€“ a system requires a subwoofer in order to reach the low frequencies that the main speakers simply cannot. Typically subs handle frequencies from 200Hz down to 20Hz.
They come in a variety of shapes and sizes and can be split into two types â€“ active and passive. Most speaker packages come with an active sub, which means it uses its own power source and features a built-in amplifier. You simply have to feed a line-level signal to an active sub and its own circuitry does the rest.
Passive speakers are most commonly found with all-in-one home cinema systems, which are cheaper to make and are therefore ideal for budget systems. But active subs invariably deliver superior bass output for movies, and usually come with a range of equalisation controls that let you tune the in-room response and ensure that they fuse cohesively with the other speakers in the system.
features the S8000 Active Subwoofer, a 42KG beast rated to 500W and capable of 114 dB at 40Hz
The most important of these controls is crossover. This is usually adjusted using a dial on the subâ€™s rear panel (or it can be controlled by your AV receiver if you prefer) and filters out frequencies it doesnâ€™t need to reproduce because theyâ€™re handled by the speakers. So if the other speakers can reproduce frequencies down to 80Hz (coincidentally THXâ€™s recommended crossover point) then the crossover dial should be set to 80Hz and itâ€™ll take care of anything below that. Itâ€™s not an exact science though, and some overlapping of frequencies is inevitable.
Other controls you may find on the back of an active subwoofer include phase control, which is usually a switch that allows you to reverse the polarity of the subwoofer relative to the incoming signal. This helps keep both the subwoofer and other speakers â€˜in-phaseâ€™ at the frequency at which they crossover â€“ if they arenâ€™t, then it can cause problems with the reproduction of those frequencies and have a detrimental effect on sound quality.
Typical connections you can expect to find on an active subwoofer include a cinch input for feeding in the line-level (unamplified) signal into the sub, and sometimes audio outputs that allow you to â€˜daisy chainâ€™ another subwoofer and boost your systemâ€™s bass performance even further.