Buyers Guides > Audio

Subwoofers

It could be a riddle - 'What is it that you can't hear, but can make almost anything sound better?'. If you have a subwoofer, but can't work out the answer, you're probably doing it wrong; if you don't have a sub in your system, maybe it's time you thought about it.

Trouble is, these dedicated sub-bass speakers have a bad press, and a lot of it's to do with the way you hear them demonstrated: most hi-fi enthusiasts have only come across them in home cinema demonstrations. They rattle the windows and pin you back to the sofa every time a rocket takes off, a city block explodes, a car gets wrecked or that T-Rex foot crashes down.

It's all very exciting in a theme-park-ride kind of way, but what overblown bass like this has to do with music is beyond us. So why are we suggesting that a subwoofer can change the sound of your hi-fi system, and do so for the better? Simple, really: it's all a matter of how it's set up... Let's get a couple of things straight from the off. First, we're talking here about active subwoofers, not the less expensive passive types.

An active sub has its own mains-powered amplifier built-in, along with the crossover and control circuitry you need to set it up correctly; passive subs tend to be little more than a large drive unit in a big box and, unless they're of a design tuned to be used with specific main speakers, will produce nasty 'whoompah, whoompah' bass you'll get bored with in about two minutes.

Second, we're talking about tuning the bass to complement your system, not just shake the room. Any fool can set up an active subwoofer to annoy the neighbours. You just plug it in to the mains, hook it up to your amplifier, park it in a corner to maximise boundary reinforcement, crank every control on it to maximum, and hit the music. Don't worry even if you've just bought a huge sub - when the noise pollution people come, they'll soon find a van big enough to take it away.

Here we suggest a way to do it properly. Before you even buy your sub, consider the size of your room, and the system with which it's going to be used. There's no point in buying a massive subwoofer to use with tiny main stereo speakers, just as a compact unit will have less effect when partnered with big floorstanding speakers.

Similarly, you shouldn't buy a unit that only accepts line-level inputs if your amp doesn't have a spare set of volume-controlled preamp-level outputs (not fixed-level tape outputs) to drive it. Most subs offer a choice of inputs - either line-level on phono or XLR connectors, or at speaker level using a variety of terminal types.

The speaker level connections (sometimes called high-level connections) are the easiest to use, needing just a separate run of speaker cable from the amp to drive them, while line-level inputs could well need long (and thus expensive) interconnects.

Some manufacturers provide a long cable with their subs, perhaps even with stacking plugs so it can be used in parallel with the main speakers. Most subwoofers are designed to be used in mono or stereo, the former set-up taking a feed from both amp channels. If you use a pair of subs - in most hi-fi systems you won't need to - each one is fed from a single amp channel.

Positioning a subwoofer is done according to the same rules as normal speakers: against walls will give more bass, while corner placement will bring on the boom. Mono subs, however, don't actually have to be between your main speakers: the very low frequencies they'll deliver when set up properly are essentially non-directional, so an off-centre position isn't out of the question if it suits your room better.

If you're using a subwoofer with a home cinema amp with a dedicated sub output, you can it set to full level and adjust its effect with the amplifier. With two-channel hi-fi amps, however, the sub will need to be set up to match the speakers in use.

Controls vary from make to make, but in general terms you'll usually get one to set the roll-off-frequency - the upper frequency level at which the sub stops working - and another for gain (or subwoofer level). Manufacturers will often supply information on how to adjust the former to suit your main speakers, but if you're doing it by ear the trick is to set the sub so it takes over just before the main speakers' bass begins to roll off.

Having a test disc with a frequency sweep or tones of different frequencies will help you identify where the bass runs out on your stereo speakers, and the sub should then be set so it extends that limit downwards. Level? It's usually set too high, so should only be done once you feel comfortable with the frequency setting. Listen to some music, then turn the gain down to the point where you stop hearing the subwoofer as a distinct entity. That's got it! What should you hear? Erm...nothing.

The whole point of a well-sorted sub is that it just sounds like your main speakers have greater bass extension, better imaging and greater three-dimensionality. Disconnect your main speakers and it'll seem strange to you that the almost imperceptible murmurings from the sub can make such a difference, but they do.

Finally, tempting though it may seem to put a cloth over that sub and use it as a coffee-table or plant stand, don't. You won't harm it or whatever you put on it, but when the teaspoons start jingling in the saucers in time with the music, it can get very annoying!

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