Sub-Woofers: Choosing & Using

Subwoofers have often been tarnished with the reputation of being for the less discerning who simply want quantity of bass over quality, but the benefits to a good hifi system should not be over looked. Their effect on our whole perception of sound can be quite remarkable, as improving low frequencies can have perceived benefits at the top end too.

I have a few simple ‘rules’ that I find useful in picking and using subwoofers, this is of course not definitive, as there are always exceptions, but I find this a very good starting point and helps make a seemingly complex topic somewhat easier to understand.

I will be dealing with the topic of subwoofers purely for hifi applications here, use in a home cinema setup can put slightly different requirements on the sub woofer (most importantly the dedicated bass channel). Do beware that many so called sub woofers are actually nothing of the sort, they cannot reproduce really deep bass and just add boom to the sound often in the 50-100Hz region, these should not be used anywhere near a decent hifi system!

Active Vs. Passive

To get good sound straight out of the box I would recommend only considering an active sub woofer. This actually means two things, firstly that it’s self powered (I.e. there’s an amplifier built in) and secondly that the crossover that selects which frequencies the subwoofer plays is placed before the amplification stages, meaning the amplifier has greater control over the directly connected bass driver. This also potentially gives the designer the ability to make allowances for the drive unit and box that he knows the electronics will be working with.

The Right Connections

To get a seamless blend with your stereo speakers it’s imperative to connect the sub woofer to the speaker outputs of your main amplifier. This is often called the high level feed. This means that the sub woofer is seeing the same signal that your speakers are, and the tonality remains as consistent as possible. An active sub woofer is a very high impedance load and this means that your existing amplifier isn’t having to work any harder to drive it.

Cabling

As covered above, on a high level connection we’re presenting a high impedance to the amplifier, which means very little current will be drawn, so whereas you might assume a thick cable would be required for such low frequencies this is not the case. However I would still recommend using a good quality cable, equal to that used elsewhere in the system. In many cases the ones supplied in the box by some manufacturers really aren’t appropriate for use in a decent hi-fi system.

Design

To reproduce believable low frequencies we don’t want to use a tiny cone or a small cabinet. Whilst it’s truly remarkable what measurements can be achieved from a small box nowadays, they are invariably lacking musically, sounding stressed, compressed and subjectively slow. It’s just basic physics, small drive units have to have a large excursion (back and forth movement) and all things being equal this will result in higher distortion.

The same rules that apply to your main speakers also apply to the subwoofer, an inefficient driver will turn lots of the signal to heat rather than sound, and this just kills dynamics and the sense of life. Ditto the material the cone is made from is very important - whilst low frequency use should in theory make mean that the drivers resonance’s and break up modes are far away from the frequencies being reproduced I still find the material has a clear distinct sound. I have yet to hear anything that betters paper cones for tonality and perceived speed.

When it comes to the box design the best sound has always come from a design that faces the drive unit towards the listener, however there are certain models that get away with aiming them at the floor or wall that also work very well, and there are actually some benefits -we want to prevent any unwanted higher frequencies getting to our ears, and sometimes due to driver resonance’s or a crossover slope that is not steep enough the losses of bouncing the sound off other surfaces help attenuate these unwanted frequencies. However as sub woofer designs have improved, and DSP has been adopted which allows very specific response tailoring (one of the few places where I feel it’s use does not cause more problems than it solves), this problem has diminished.

Stereo Vs. Mono

Due to the distance between our ears we are not able to ascertain the location of low frequencies, once we get below about 80Hz it should become impossible to locate the source of the bass, so the brain automatically ties it in with the upper frequencies produced by your stereo speakers.

However this is a little simplistic, as a subwoofer may be producing higher frequencies, especially if its crossover isn’t steep enough to prevent higher frequencies getting out (as mentioned above). If you’re running the sub woofer with a speakers whose own bass response is very limited then we can also start to get into the directional region of sound quite easily. There are other factors too that can give the game away, port chuffing and cabinet resonance’s, and even local room rattles could all spoil the illusion.

Two sub woofers connected in stereo do give the best performance, the benefits include driving the room more effectively (this tends to result in a flatter bass response) and reducing distortion, so the sound becomes more effortless and dynamic. However doubling the cost and the extra domestic intrusion this presents means it is rarely done in most systems.

Positioning

This is a huge can of worms, and I’m not going to be able to cover every eventuality here, but there are some basic guidelines that could help. In an ideal world we want the speaker to be approximately the same distance from us as the main speakers - imagine a curve that intersects your stereo speakers , we would ideally place a mono subwoofer on its arc directly in front of us. If using stereo subwoofers then they should be positioned as close to the main speakers as possible, again on the arc. However, there’s no need to get too anal about this listening distance, as time alignment is not that critical at very low frequencies due to the long wavelengths involved (and the phase control found on many subwoofers does allow us to correct for some positioning compromises).

Sometimes it’s impossible to place subwoofers where we would want to, and it is often possible to place them to the side or even behind the listener with care, without it impacting the sound quality dramatically.

Some sub woofer manufacturers recommend placing their designs into corners, this can work very well in many rooms so experimentation here is worthwhile, and with the advent of self measuring DSP controlled designs the worst of the room resonance’s can be attenuated to give an overall flatter response.

A trick that can also work well is to position the sub woofer in the listening position, play a variety of music with a varied bass response (walking bass lines on an acoustic bass can be particularly helpful) and move around the room. Wherever the bass sounds most even is likely to be the best place to position the sub woofer.

Setting up

I will cover setting a sub up by ear here. In theory it can be done by taking lots of measurements if you have the right (good quality) measuring equipment, but in my experience the results are never terribly predictable when dealing with low frequencies, and ultimately you still need to do the fine tuning with music anyway, so I would suggest doing the whole process by ear.

Some sub woofers have outputs from a filter that allows you to restrict the low frequencies going to your main speakers. In a serious hi-fi system these should be avoided, whilst they do reduce the stress on the mains speakers (allowing them to play louder and reducing distortion) the additional filter components are sonically far worse than any benefits they bring.

When you get your sub woofer home naturally you will want to get it sounding great immediately, but I would suggest trying this will only lead to frustration. The new subwoofer will need time to run in, and as the drive unit materials stretch so their characteristics change, making substantial differences to the bass performance. So just do a quick setup that is bearable and leave it to run for several hours. Only then would I recommend starting to work on fine tuning the setup.

Most modern subwoofers have a microphone and will set them selves up to correct for the worst of the rooms resonance’s, they will never get a totally flat response (it’s not possible in the real world) but they will attenuate problem frequencies to a degree. Once you’ve followed the manufacturers instructions to complete this step we then normally have three things to get right, the crossover frequency, the volume and the phase (on some sub woofers there is also a variable crossover slope rate, but this is sadly quite rare), we’ll cover these in a moment.

One important thing to remember is that a sub woofer can also give us freedom to reposition our main speakers too - chances are they were originally positioned so that the bass was reinforced by the rear (or side) walls, but when the sub woofer can reproduce the bass it does allow us to bring the speakers away from the room boundaries (if you have space) which can bring substantial benefits to the width and depth of the sound staging.

Many people make the mistake of setting a sub woofer up so that it impresses, but the reality is that if we can hear it’s there then it’s probably not setup properly. This quickly leads to fatigue and simply makes it unlistenable for more than a few minutes. What we need is finesse when setting up.

Firstly the golden rule is that the best way to know what the sub is doing is to unplug or mute it - that’s when you hear what it has been doing, do this regularly throughout the setup phase to give yourself a reference. Do not rush, it can take small tweaks over the course of several days to get it spot on.

There is probably no right way to start setting up a subwoofer, but generally I would tend to work to the following procedure. Remember that all listening should be done from the listening position, this is much easier nowadays as most sub woofers have a remote control that will cover the majority of features, if not, a willing assistant is ideally required.

Frequency

Only once should you pay any attention to the numbers on the crossover frequency control - the very first time you run it, for an average floorstander start at 50Hz for an average stand mounter start at 60Hz. Set the phase to 0. From then on it’s all done by ear (as these numbers are pretty meaningless in the real world). Slowly increase the volume of the sub woofer so that it sounds similar to the main speakers and then try to leave it alone. Now lower the crossover frequency so that there’s obviously a gap between the output of the main speakers and the sub woofer. Play a variety of tracks, particularly those with bass lines that span a wide range of notes. If the frequency is set too low then notes between the sub and the main speakers will sound light or missing, if they overlap some notes will be to pronounced. Once you’re happy that the blend is in the right ball park then we can move onto the other controls.

Phase

The phase is simply about making sure the drive units all push the air at you at the same time. If the sub woofer is 180 degrees out of phase with the main speakers then it will try to cancel out the frequencies that both reproduce (basically, one will be blowing whilst the other is sucking), this will cause the bass to become lacking at the crossover point. So when the phase is right it will be the loudest on the notes that both speakers and subwoofer are reproducing at the listening position. Some subwoofers give you more than 0 and 180 settings, some are infinite, the same applies it just takes a bit longer to find the right point (always try 180 degrees different from whatever starting point, i.e. 90 degrees against 270, for example). Some people may choose to set the phase before fine tuning the frequency, and there’s nothing wrong with that, in fact it can sometimes be easier when the overlap is more pronounced.

There are two issues with phase, sometimes, in some rooms and setups the phase control does not seem to make any audible difference, this could be a sign that the crossover frequency is set too low, but sometimes it’s just not clear cut. If that’s the case don’t sweat it! Secondly, sometimes a slightly cleaner bass is achieved by intentionally having the sub woofer out of phase either completely or to some extent. Some speakers, particularly smaller designs have an intentional boost at their very lowest frequencies to make them sound larger, sometimes the effect of attenuating this by cancellation gives the flattest, cleanest crossover (you can also try free space positioning the speakers or partially stuffing the reflex port of if they have one and so on). As ever, if it sounds right, it is right.

Volume

Ok, I mentioned this needed to be set approximately in the ball park in the frequency setting paragraph above, now make small changes and preferably over a wide range of recordings. Do not be surprised if some recordings have lighter or heavier bass than the norm - it’s perfectly possible that you actually have wider bandwidth at your disposal than some engineers had when recording or mixing, so there can be some oddities.

What takes the greatest patience to get right is the volume and crossover frequency combined - it is all too easy to confuse the effect of having too high a crossover frequency with too much volume level (and vice versa) and get yourself in a mess. This is why small changes using multiple recordings over a long period is the way to go.

Results

Once you’ve got your sub woofer properly dialled in then, somewhat obviously, you will experience much more extended low frequencies, with realistic power to the fundamentals of bass notes. What you might not expect is a sound that is often more expansive, effortless and open. You are likely to be able to listen at lower volume levels, as the correct frequency balance has been achieved. On good live recordings, where the information is present, you will hear the size of the venue, the feeling of scale that comes from low frequency air movements. On music that demands it, the difference in menace and tension, particularly with larger classic works can be quite profound. Not every system needs a sub woofer, and no sub woofer is always better than a bad or badly setup one, but get it right and it can add a dimension to the sound that only the largest speaker systems can normally achieve.