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Passive and Active Subwoofers

Passive and Active Subwoofers
Put simply, subwoofers are designed to handle the ‘.1’ 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 – which typically feature a lot more deep bass than music – 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.
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EMP Tek Impression IMP-7.1-2 9/10 Recommended DEC '10

They come in a variety of shapes and sizes and can be split into two types – 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.
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Teufel System 8 THX Ultra 2 9/10 Recommended JUL '10
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 sub’s rear panel (or it can be controlled by your AV receiver if you prefer) and filters out frequencies it doesn’t need to reproduce because they’re handled by the speakers. So if the other speakers can reproduce frequencies down to 80Hz (coincidentally THX’s recommended crossover point) then the crossover dial should be set to 80Hz and it’ll take care of anything below that. It’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 ‘in-phase’ at the frequency at which they crossover – if they aren’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 ‘daisy chain’ another subwoofer and boost your system’s bass performance even further.

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