What subwoofers do
Subwoofers have proliferated since the advent of home cinema, but many people don't realise that they can also be an asset for stereo hi-fi systems. The .1 in a 5.1 channel surround system indicates the subwoofer, and as a result even the least expensive home cinema in a box systems have something that is at least described as a subwoofer. The purpose of a sub is to provide the low bass that all but the largest regular loudspeakers cannot reproduce, they can do this because they have onboard amplification and equalisation that marries the drive unit to the power amp and squeezes surprisingly low frequencies out of what are often not terribly large boxes.
How they do it
A sub is an active or amplified speaker and as such does not require a separate amplifier to drive it, you can add a sub to a regular stereo system to enhance its bass performance or you can use it with a surround system where specific low frequency effects (LFE) can be sent directly to it from a home cinema amp.
In a stereo system the sub can be connected to a spare line output on a preamp for instance or with some designs you can use the same speaker output that drives your left and right channels. This latter approach is called 'high level' and the sub has to have a suitable input to work with it - the advantage with this system is that both speakers and sub are getting exactly the same signal. A potential drawback is that the sub will still be amplifying and equalising the signal which causes a very small delay.
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| Wharfedale Diamond SW150
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Integration
One of the reasons why subs have never been very popular with audiophiles is that it is difficult to integrate them with the main speakers and create a seamless result. Some manufacturers have addressed this by offering fine control over the crossover point at which the sub becomes active. The idea behind this is that you can adjust the sub so that it does not crossover with the main speaker at all but merely takes over where it runs out of low frequency steam. The tricky bit of course is finding that point, since the specified bandwidth of a speaker will not necessarily relate to the depth of bass it produces in your room. Walls and other solid objects tend to reinforce bass and given the variance in speaker placement no manufacturer can tell you precisely how low its speaker will go in your room.
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| REL Storm III
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The set-up
Subwoofer set-up necessarily involves a fair amount of experimentation. It makes sense to begin with placement and the most popular place to start is a room corner - this is because it reinforces the bass and gives the sub a helping hand. To find the precise position it's best to play a piece of music that features consistent low bass and use this to set up the sub by ear.
Start by setting the phase to the position that gives the highest output then gradually move the sub into the room until its output is again at its loudest point, usually this will be within a few inches of the wall(s). When you have placed the sub correctly you can begin to integrate it with the speakers. To do this turn the filter so that it is fully open, effectively operating the sub at full bandwidth with no roll-off. Turn the volume down to minimum and play the bass track, then adjust the volume so that it matches your speakers. Next turn the filter right down and then gradually increase it until the sub is increasing the depth of bass but not necessarily its volume.
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| ATC C4
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A correctly integrated sub in a music system should not stand out or draw attention to itself, it should extend the low frequencies and increase the sense of space in the sound. Tempting though it might be, if you have it set too loud it will upset the balance of the system. A good rule of thumb is that a correctly set-up sub shouldn't really be noticeable until you switch it off.
Home cinema
In home cinema systems the difficulty of integrating a sub is removed by the processor which will have a crossover point that sends everything above it to the surround channels and everything below it to the sub. So you can turn the sub's filter and volume level to maximum and use the processor to integrate it with the system as a whole. The positioning suggestions above do however apply in order to achieve best results.
If you have the space and budget there's a lot to be said for using two subs, combined they are greater than the sum of their parts and bring a sense of ease and power that only the best single subs can compete with.
Three to try
Some sub worth checking out
Budget
Wharfedale Diamond 9 SW150
£250
www.wharfedale.co.uk
A remarkable sub for the money, Wharfedale's latest creation delivers both weight and authority that defy its price point. If this is all you can afford you won't be suffering.
Midrange
REL Storm III
£800
www.rel.net
Small but reassuringly heavy, this 150W design has a full range of set-up controls with high, low and balanced high and low-level inputs. It will be equally at home in home cinema and music systems and provides genuine grunt.
Serious
ATC C4
£2,350
www.atc.gb.net
If you want to know what the term fast means when applied to bass look no further than this 650 watt brute, it comprehensively redefines the notion of low frequency resolution and musicality.
Have any sub set-up tips of your own? Share them with other AVR members in our forum.
