All You Wanted to Know About Subwoofers

...But Couldn't Find On The Internet! ***

Published in SOS April 2007
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Technique : Theory + Technical

If you really want to craft your bass end, a subwoofer can be indispensable. Here's the low-down...

Hugh Robjohns

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When I was first getting interested in all things audio, home subwoofers were rather specialist devices that were only of interest to two groups of hi-fi enthusiasts: those who listened to large pipe organ recitals and those who enjoyed reggae! Things have moved on a great deal since those distant days, and today subwoofers are now considered — more for the 'domestic acceptance' factor than any potential sonic benefits — to be an almost standard part of modern hi-fi and studio monitoring systems

It is therefore becoming increasingly common to have smaller 'satellite' left and right speakers, plus one or more subwoofers — a format often referred to as 2.1. The two refers to the main pair of stereo (left and right) speakers, while the '.1' refers to the limited-bandwidth subwoofer. In the same way, a surround sound system is often denoted as 5.1, meaning five main channels plus a subwoofer to handle the low frequency effects channel (LFE).

However, in the case of 5.1 home theatre and surround sound systems, a dedicated subwoofer is required specifically to handle the low frequency effects (LFE) channel. The fact that this sub usually also doubles up, through the use of bass management (explained later), to handle the bass content of all five main channels as well, is just a convenience that allows the use of smaller satellite speakers. Again, I'll come back to this topic later, but I first want to consider 2.1 stereo monitoring arrangements.

The Subwoofer Advantage

When designed and used appropriately, subwoofers can be extremely effective and very convenient. Equally, though, it is very easy to destroy any chance of good monitoring quality with an inappropriate or badly set-up subwoofer — and it is worth stating that I have probably seen nine unsatisfactory installations for every good one!

Poor subwoofer installations usually suffer from too much, or poorly defined, bass. Often there is an obvious 'hole' in the frequency spectrum in the crossover region between the satellite speakers and the subwoofer. It is the 'integration' through this crossover region that really makes or breaks the system as a whole.

The worst kind of subwoofer system will only provide a boomy or monotonal 'woomf' of energy, regardless of the pitch or dynamics of the bass instrument, and the bass might thus appear to be slow or late relative to the main speakers. On the other hand, a well-designed and well-configured system will usually enable more accurate imaging, and have a clearer, more transparent mid-range (thanks to lower distortion and intermodulation levels) and higher overall output than could be achieved with the satellites alone.

From a practical point of view, a satellite and subwoofer combination is much easier to site and to move around. The individual speaker cabinets are more compact and lighter than full-range speakers, and that is often an important consideration — especially in small home studios and for location-recording rigs.

So the aim of this article is to try to explain the hows and whys of choosing and using a subwoofer, in the context of both stereo and surround sound applications.

Concept

The first thing to understand is the basic concept of a subwoofer system. What it is trying to do, and how does it work? Obviously, the fundamental idea is to reproduce low frequencies. In most cases, this is roughly the bottom two octaves, from 20Hz to 80Hz. However, here lies the first major issue we need to take onboard: having a special box that generates low frequencies doesn't guarantee good bass in the listening room — in fact, far from it!

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The diagram shows a bass-management system. Each of the five main channels goes through a high-pass filter to remove the low-frequency element of the signal, before being passed on to the appropriate amplifier and speaker.
The diagram shows a bass-management system. Each of the five main channels goes through a high-pass filter to remove the low-frequency element of the signal, before being passed on to the appropriate amplifier and speaker.
The diagram shows a bass-management system. Each of the five main channels goes through a high-pass filter to remove the low-frequency element of the signal, before being passed on to the appropriate amplifier and speaker.

The room's own acoustic properties are of paramount importance. If you put the world's best subwoofer in an acoustically poor room, you'll get very poor bass performance! I have frequently come across monitoring systems where the owner has added a subwoofer in the hope of curing a weak or lumpy bass response, only to discover the situation either doesn't improve or actually gets worse!

If the room has nasty standing-wave problems — and almost all home studios do — it's vital that these acoustical problems are sorted out first, before you spend money or time on a subwoofer.

Bass trapping to control and reduce room standing-waves is a subject that we've covered many times, and is also a popular topic for ongoing discussion and guidance on the Studio Design and Acoustics forum on the SOS web site. You can often improve a room's acoustics dramatically for minimal cost with some basic DIY. And with a treated room you may well find that your existing speakers actually deliver much more and better bass than you thought!

Another useful benefit of a subwoofer is the additional power handling accorded to the system as a whole. The acoustic energy in music is highest at low frequencies and tails off with increasing frequency. So employing a dedicated box to handle much of the power-hungry bass takes that burden from the satellites, with useful benefits in overall power handling and clarity.

One Or Two?

Most stereo systems have two main speakers, yet we have only one subwoofer. Why not two subwoofers as well? In some situations there can be advantages to having two (or more) subwoofers, but in general one is usually sufficient. The reason for this is connected to the fact that, for frequencies below about 700Hz, our sense of hearing measures the phase difference between a sound arriving at each ear, whereas above this frequency it uses mainly level differences. Out of doors, our ability to determine a sound's direction remains quite accurate down to remarkably low frequencies, but this ability collapses when listening indoors. Sources generating low-frequency sounds (below about 100Hz) tend to do so more or less omnidirectionally (the sound wave travels from the source in all directions) because the wavelength of sound is usually larger than the object itself. When a low-frequency sound is generated within an enclosed space, the spherical sound waves created will reflect off the boundary surfaces of the room to arrive back at the ears with a multiplicity of phase variances, due to path-length differences. This confusion of signals makes it impossible for the ear and brain to extract a reliable phase difference, so normal directional acuity fails.

So in theory, since you can't tell where the low frequencies are coming from in a room, one subwoofer will be entirely sufficient. The harmonics of the bass notes will be reproduced by the satellite speakers — which typically start to take over above about 90Hz — and these will provide plenty of directional information through phase and level differences, in the usual way. So, although the bass itself is folded down to mono, the impression of stereo imaging is actually preserved perfectly satisfactorily.

This theory is all well and good, but I often hear people comment that they can hear where a subwoofer is placed in the room. This isn't because of some special acoustic ability on their part, though —rather, it is because of the poor performance of some subwoofers! Designs constructed at a low cost, employing inferior drivers, and those designed to favour efficiency above all else, tend to generate a lot of 'out of band' noise — lots of harmonic distortion and audible port noises, or other artifacts. These occupy the mid-frequency range, which not only makes their position easily detectable, but also obscures and masks the critical mid-range frequencies from the satellite speakers. So adding a cheap subwoofer to quality satellites will actually tend to make the system less rather than more accurate.

A good subwoofer needs to have a very linear driver (which is expensive), an accurate and powerful amplifier (which is expensive), and a well designed and built cabinet (which is... expensive). But cutting corners on any of these aspects is a false economy. I've listened to and used a lot of different subwoofers, and the best are, for all the obvious reasons, produced by the same companies you associate with good monitor speakers. Blue Sky, ATC, Genelec and PMC all produce superb subwoofer systems that integrate extremely well with their intended partnering designs. They are all relatively easy to set up because of the inherent close matching and the appropriate electrical alignment facilities. In my own monitoring system I use the PMC TLE1 subwoofer (shown in the picture at the start of this article), both as part of a large 5.1 rig, and to extend the bottom end of the tiny DB1 or nearfield TB2 monitors. Whereas many subs are large cuboid boxes, the TLE1 has the form factor of a computer case, which I find both aesthetically and practically appealing.

When buying a sub, the key is to try it in your own listening environment, with your own satellite speakers — particularly if the subwoofer is from a different manufacturer. Some combinations will integrate far better than others, and only a home audition will reveal the success or failure of a particular combination.

Bass Management

Bass management is the process of removing the bass element of the signal fed to each satellite speaker, and routing it instead to one or more subwoofers. In essence this is no different to a normal crossover — it's just that the bass driver happens to be housed in a separate enclosure, and there needs to be some sort of mixing facility included to combine the low-frequency contributions from at least two channels.

In the case of a simple 2.1 stereo system, this bass management or crossover filtering is usually built into the subwoofer, and may be active or passive (most systems are active these days). There are various approaches to wiring, but most route line-level signals from the controller or preamp to the subwoofer first, which filters the signals and outputs them for the satellites. Some systems work the other way around, connecting the signal to the satellite first, and then down to the subwoofer. Systems intended for domestic use often work with speaker-level signals.

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If there is a phase control on your subwoofer, as on the PMC TLE1 pictured here, you will find that small phase adjustments can often make surprisingly large differences to the smoothness of the crossover region, especially if the subwoofer and satellites are from different manufacturers.
If there is a phase control on your subwoofer, as on the PMC TLE1 pictured here, you will find that small phase adjustments can often make surprisingly large differences to the smoothness of the crossover region, especially if the subwoofer and satellites are from different manufacturers.

For 5.1 surround systems, the bass management is normally performed in the surround sound controller or monitoring controller, rather than in the subwoofer itself. The diagram on the previous page shows such a system. Each of the five main channels goes through a high-pass filter to remove the low-frequency element of the signal, before being passed on to the appropriate amplifier and speaker.

All five channels are also summed and passed through a low-pass filter to remove the mid- and high-frequency content. This signal is then combined with the dedicated LFE signal (which is also low-pass-filtered and boosted in gain, according to the appropriate specifications), and routed to the subwoofer speaker. It is worth bearing in mind that, since each of the five channels in a 5.1 system is a full-bandwidth channel, the subwoofer has to be able to cope with the bass contribution of five full channels, plus whatever might be conveyed on the LFE channel — and that could be a lot of bass! So you shouldn't really expect a very small box to be able to cope if you like listening at serious levels.

Of course, different systems implement bass management in slightly different ways. Some employ active filtering everywhere, whereas some only low-pass-filter the signal feeding the sub, relying on the satellite speakers' natural roll-off for mechanical high-pass filtering. Some will allow the filter turnover frequencies and slopes to be adjusted. Professional units usually do this with meaningful technical parameters, while domestic controllers tend to have simpler 'large' or 'small' speaker descriptions.

The better systems often include some sort of limiting or overload protection for the subwoofer, and some also include facilities for delaying the sound to each speaker, in order to compensate for less than ideal physical positions. Most domestic systems only apply bass management to digital surround inputs (Dolby Digital and DTS sound tracks) but not to discrete multi-channel analogue inputs, and this can present problems if you want to use a cheap domestic surround controller for your surround monitoring. Another common trap is that some DVD players have their own bass-management facilities built in, which means that you need to make sure you don't end up duplicating the processing!

Subwoofer Cabinet Types
While it is relatively easy to generate high levels of bass over very small bandwidths (and that's what most cheap subwoofers tend to do), designing something that can generate a high output over a broad bandwidth, with very low distortion, and remain a sensible size, is pretty tricky. Creating low-frequency sound at studio replay levels requires the movement of a lot of air. This requires a powerful amplifier, a very large bass driver (or several smaller ones), and a lot of diaphragm displacement.
An easy way of achieving high efficiency is to place the driver in what's called a 'band-pass cabinet'. This is essentially a resonant, tuned box, with the driver hidden inside and the sound escaping through one or more ports. You see these quite commonly on cheap home theatre systems and in car 'boom boxes.' Although efficient (for which read loud!), this kind of design always tends to sound boomy, with a one-note kind of response. This is great for film explosions and crashes, but is not much use if you want to hear which notes the bassist is playing, so it is best avoided for serious monitoring duties.
The majority of subwoofers employ some form of 'reflex' design, which combines practical efficiency with useably wide bandwidth, and in convenient sized enclosures. The design principles are thoroughly understood, with the front of the driver radiating directly and its rear contributing via the enclosed cabinet volume through one or more ports. Not all reflex designs are born equal, but most studio-quality subs will be of this kind of design.
A rather less common alternative is the 'closed box' design. The cabinet is sealed, and only the front side of the driver contributes sound to the room. Efficiency is relatively low, and significant demands are placed on the amp and driver (the latter needs to be able to cope with unusually large excursions). However, this approach has considerable benefits in terms of its phase response, timing and distortion. Another close variation on this theme is the 'transmission line' approach, which aims to combine the best elements of both sealed and reflex cabinets. These two types tend to be the most expensive, but also the easiest to align and integrate, and with the most accurate sound.
Don't be fooled by the size of the subwoofer. Bigger doesn't necessarily mean better, or even greater low frequency extension, although it does usually equate with louder; that business about needing to move a lot of air, again.
Alignment

The physical and electrical alignment of a subwoofer is a much misunderstood process, but to get it wrong is to destroy the accuracy of the monitoring system as a whole. Firstly, it is vital that the subwoofer and satellite speakers are all in the same phase as each other — by which I mean their electrical polarity and time alignment. If this is not the case, the crossover region will have an obvious bulge or dip in level. Potentially, there are a lot of sources of phase shifts that can mess up the crossover region. The subwoofer and the satellite speakers have their own mechanical phase responses to take into account, plus the electrical phase characteristics of the crossover filters themselves. There is also the time delay caused by having speakers located at different distances from the listener, and some cabinet designs introduce further acoustic delays, which often vary dramatically with frequency (and in some cases can exceed 40ms — a whole video frame!).

A lot of the better specified subwoofers include a phase-adjustment facility (either switched or continuously variable), which can help to correct for the mechanical and electrical phase differences between the satellites and the subwoofer. Unfortunately, though, not all designs are equally effective and, more importantly, phase correction is not the same as delay compensation. If the subwoofer is located nearer to, or farther from, the listener than the satellites, some delay compensation will be required to achieve the correct time alignment. Though some bass-management or surround sound monitoring systems incorporate this function, not all do so.

When it comes to placing a subwoofer, there are several things to consider. Although a high-quality subwoofer should not output the higher frequencies that allow its position to be determined by listening, that doesn't mean it can be put down just anywhere. Firstly, the subwoofer's location in the room — especially its proximity to walls — will have a significant effect on its frequency and time-domain responses. In a typical room, there will be a few 'best' places amongst many more that are unacceptable. Secondly, unless delay compensation is available, the sub should ideally be located at the same distance from the listener as the satellite speakers.

It makes sense to place a single subwoofer in front of the listener, rather than behind, and directly facing the listening position too. It should be well away from corners, but you should also avoid placing it at the centre of the room width, in order to minimise excitation of standing waves. The closer the subwoofer is placed to the wall, the greater the bass boost will become. Some models are designed to be placed close to a wall, specifically to benefit from this, but some are not — which means you should always check the manufacturer's recommendations. Often, small changes of distance in relation to a wall can make a big difference in the balance of deep bass, so don't be afraid to experiment.

For the subwoofer to work properly and not be locatable, the crossover between satellite and subwoofer should be set below about 90Hz, and that means the satellite ideally needs to have a decent response down to 70Hz or so. Anything above that starts to intrude into the mid-range and the subwoofer will become locatable. The THX organisation recommends crossing over at 85Hz, and I've found that to be a good starting point in most cases.

Practical Placement

Ideally, a subwoofer and satellite system would be aligned using proper acoustic measuring equipment, but few of us have access to that, or the experience to interpret the results properly. Fortunately, though, you can usually get very good subjective results if you give it some time and patience, and take a logical approach.

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Placement of your subwoofer is important. Finding the right place is a matter of getting down on your hands and knees, moving around and listening to some well-recorded bass, with the sub in your normal listening position. When you've found where it sounds best, relocate the sub there.
Placement of your subwoofer is important. Finding the right place is a matter of getting down on your hands and knees, moving around and listening to some well-recorded bass, with the sub in your normal listening position. When you've found where it sounds best, relocate the sub there.

Start by placing the subwoofer in the listening position, with approximate filter and volume settings — 85Hz and a volume that seems roughly right. You'll then need to play through a collection of music tracks with well-recorded bass lines in different keys. Alternatively, create your own test track using a sound generator or keyboard, playing each note percussively (not continuously) and with uniform velocity settings. All you then have to do then is crawl around on the floor listening at each potentially practical subwoofer location for which position gives the most consistent and natural bass sound. You'll find some places exhibit some boomy and resonant notes, while others will have obviously weak or missing notes. Hopefully, you will discover one or two places where the sound is well balanced and all bass notes are pretty uniform. Having identified an optimal location, place the subwoofer there and reinstate your listening chair.

You can now optimise the subwoofer level and, if provided, the filter turnover frequency and phase/delay. These controls tend to be interactive, so you'll probably have to cycle around their setup before you arrive at the best combination. I usually start with the subwoofer turned right down and then play through a wide collection of music, concentrating on how the mid-range and upper bass notes sound.

Once familiar with what the satellites are doing on their own, I start to increase the level of the subwoofer until all the bass notes are even in level, regardless of pitch. It is very easy to overcook the subwoofer level, and although it may sound impressive, this quickly becomes tiring and leads to bass-light mixes, so take your time and listen critically.

If the deepest and highest bass notes seem right, but it all goes wrong in the crossover region, try adjusting the crossover frequency up or down slightly to find the smoothest transition. If there is a phase control, you will find that small phase adjustments can often make surprisingly large differences here too, especially if the subwoofer is from a different manufacturer from the satellites. Remember, all three controls will interact, so take your time, live with a good setting for a while, and don't be afraid to experiment.

I normally allow at least an hour to set up a subwoofer by ear, and I usually carry on with the fine-tuning for a couple of days afterwards, until I'm happy that I have achieved the best possible performance.

Finally, remember that low frequencies are very hard to contain. Adding a subwoofer will inevitably result in some of your new-found deep bass leaving the listening room. This may well annoy your neighbours, even if they previously tolerated the system without the subwoofer. Generating low bass may also excite room standing-waves that you were not previously aware of, and it may also result in various structures of your building rattling and resonating in ways you've not heard before! The last time I recalibrated the subwoofer in my own 5.1 system at home, my daughter came rushing down to complain that everything on her dressing table was rattling and falling over! 


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