Subwoofers put the 'point one' into surround sound systems, refreshing low frequency parts that other speakers cannot reach. Integrating a subwoofer into your home cinema system is a tricky task, but you can do it, with a little help...
In home cinema speaker systems subwoofers are responsible for delivering bass from a dedicated low frequency effects (LFE) channel as well as supplementing low frequencies derived from other speakers that are simply too small to reproduce them.
Integrating a subwoofer into your system to produce a seamless sound is a challenge for any home cinema enthusiast. Experimentation is essential but using this four-step guide to subwoofer success will ensure sonic highs from the low frequency performance of your system.
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| B&W PV1
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1. Subwoofer Placement
Although low frequency sounds are considered virtually non-directional, it's important to find a place in your listening room that allows your sub to perform to the best of its ability. No two rooms are the same and it takes a little experimenting to find your room's 'bass nodes', where your sub will sound best.
The simplest way of finding the best position is to begin by placing your sub in your usual listening position. Then, play a familiar soundtrack with a consistent bass line and move around your room listening for where the bass sounds best - you might have to get down on your knees since most subs tend to sit close to the floor. Experiment by moving closer or further away from walls, in corners etc until you find a place where the bass sounds smooth and extended without exaggeration or boom. And that's the ideal site for your sub.
Be aware that placing subs close to hard walls or floors will enhance low frequencies through reflections off the stiff surfaces, while allowing your sub some free space does the opposite. Corners are often suggested as ideal places as they act like horns to emphasise the sound, which is fine if you have a weak sub but can lead to distortion and loss of detail otherwise. No room dimensions are ideal and compromises will always have to be made according to both room acoustics and aesthetics.
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| Revel B12
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2. Bass Management
In order to integrate your sub with the rest of the speakers in your system you'll need to access the bass management settings of your home cinema receiver. These allow you to determine speaker sizes and crossover settings that tell the receiver how to handle low frequency information, which is inherent to the success of your system's sound performance.
Speaker Size
Selecting speaker size offers a straightforward choice between Small and Large speakers, with anything up to the size of a bookshelf speaker considered small while large speakers are generally floorstanders that can produce a full flat range (20Hz-20kHz) of undistorted frequencies.
Setting your speakers to Small automatically redirects low frequencies to the sub at a predefined crossover point allowing small speakers to reproduce sounds in their frequency spectrum without straining while low frequencies are handled by the sub. With Large speakers you'll need to manually select a crossover point telling the receiver when to redirect low frequencies to the sub. If your system features a mix of speakers, say one using a pair of large front speakers accompanied by smaller surrounds and a centre, you'll need to set different speaker sizes accordingly.
Crossover
The crossover is the point at which low frequency sounds are redirected to the subwoofer while the frequencies above it continue to play through the other speakers.
Setting the crossover too low will produce a gap between frequencies produced by the sub and the speakers leaving sounds distorted or lost altogether. Setting the crossover too high however, forces the sub to create high frequency sounds which it isn't designed to reproduce, leading to a muddied sound overall. You'll need to refer to the user's manual to find the recommended settings for your particular sub but it's still worth experimenting with a range of crossovers as speaker bandwidth doesn't necessarily correspond to the dynamics of your room.
Try aiming for a crossover point that's around 10Hz above the lowest frequency that your smallest speakers can reproduce without distortion. This will vary depending on your speakers but you can use these figures as a guideline:
Satellite speakers 100 - 120Hz
Bookshelf speakers 60 - 100Hz
Floorstanding speakers 40 - 80Hz
A crossover point of 80Hz is seen as the recommended standard for most systems and is a good starting point. It's the same crossover used in all THX certified systems but needn't be taken as gospel and, once again, experimentation is the key.
A simple way to determine crossover is to completely open your sub's crossover filter so it is operating with a full bandwidth with no roll off. Then turn the volume level right down and play a track with plenty of bass, slowly adjusting the volume until it's at the same level as the rest of the track. Now, adjust the crossover filter until the sound increases in depth, but not volume. And that's your crossover point.
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| REL Storm 5
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3. Phase Control
Phase control is a switch on your subwoofer that electrically reverses its polarity allowing sound waves to synch with those coming from your main speakers. It's especially useful if your receiver's sub output is electrically out of phase with speaker outputs.
Let your ears judge if the sound is out of phase - usually the difference is obvious, so if you can't hear it, chances are it's fine. If you're not sure then try reversing the polarity of connections to your main speakers. Play a bass-heavy soundtrack and adjust the phase until you can barely hear the bass and then reconnect your speakers properly. Your sub should now output optimum bass in phase.
4. Level control
Finally, you'll need to adjust the level of your sub so that it's operating at the same volume as your other speakers. You can attempt this by ear but if you want to be accurate, use your receiver's test tones together with a sound pressure level meter (SPL - around £30-£40).
To use an SPL, make sure the room is quiet and take a seat in your ideal listening position, then set the SPL to 80dB, select C weighting and slow response. Hold the SPL in front of you facing upwards and turn the volume of the test tone up until the SPL reads 75dB, then adjust each channel until you get the same reading. It's tempting to have it too loud (the most common mistake) but this will ultimately upset the whole balance of your system.
A perfectly integrated sub should solidly extend low frequencies and increase a sense of space without drawing attention to itself. Once you've got that, you're well on your way to home cinema sound.
