Do You Need A Subwoofer?

If you need more low end from your monitors, adding a sub can be very tempting — but is it the right option for you?

If you’re reading this, chances are you’ve got a pair of studio monitors. But if, like many of us (myself included!), your ‘studio’ is also a guest room, bedroom, box room or home office, rather than a dedicated space, then your monitors are also likely to be fairly small, and that inherently means they won’t be particularly bassy.

Whether that’s a problem or not depends on a number of factors (the type of music you make, how tolerant your neighbours are...), but if you want more bass from your monitors, your options are either to buy a bigger set of monitors, or to augment your current pair with a subwoofer. The sub option has a number of advantages: it can be cheaper than buying a whole new set of speakers, and it allows for greater flexibility in terms of placement. There are limitations on where main monitors can be placed because of the need to establish a good stereo image, but because most subwoofers are essentially omnidirectional, they can be located almost anywhere, and so the way a sub excites the room’s acoustics can be chosen with greater freedom. When done skilfully, that can result in a flatter response at the listening position than might be possible with full‑range monitors alone.

There are pitfalls, though, and in this article I’ll explain some of them. Along the way, I’ll discuss the basics of how subwoofers work, what features to look out for, and how to get the best out of one if you decide a sub is right for you.

The Down Low

Back in SOS September 2021, I explored the many ways that loudspeakers can be designed. Since low frequencies are inherently unruly things, most of that article covered the fundamentals of bass reproduction, the principles of which are the same for subwoofers as they are for full‑range monitors, so for an in‑depth look, do give it a read: www.soundonsound.com/sound-advice/how-choose-studio-monitors

To recap a little: low frequencies require a lot of energy to play back, and given that sound waves are generated by both sides of the speaker cone, much of their energy is essentially wasted by firing sound away from you. Naturally, the sound that comes from the rear of the cone is in opposite polarity to that coming from the front, and will partially cancel if the two are allowed to mix. So, speaker cones are generally put into cabinets to occlude that rear‑firing sound.

In a sealed cabinet, that’s where things end: the rear‑firing energy is more or less eliminated, but much of the energy from the cone is wasted by being absorbed within the cabinet. The air inside the cabinet also inhibits the cone’s movement, reducing efficiency even further. For this reason, many speakers and subwoofers employ a ‘reflex port’. At its simplest, this is basically a hole in the cabinet that allows some of the rear‑firing sound to escape, reinforcing the sound from the front and increasing the speaker’s efficiency. By fitting a tube to said hole, you can ‘tune’ the port so that it increases the speaker’s efficiency around a particular frequency — usually somewhere around 60‑80 Hz for a typical two‑way monitor, and perhaps half that (ie. an octave below) or even lower for a subwoofer. The efficiency improvement you get from doing so is considerable, which is why the majority of subwoofers — from affordable models like the KRK S8.4 to high‑end designs like the Focal Sub 12 and Genelec W371 — are ported designs.

Ports can introduce problems of their own, though, especially in subwoofers. First, tuning them to a low enough frequency requires a very long tube, which means a very large cabinet. Second, the ports need to have a large enough cross‑section to not suffer from compression effects. Third, ports can generate their own resonance, which can cause something known colloquially as ‘one‑note bass’, where a particular note plays inordinately loud. This is particularly common among home theatre subs, where the exact pitch of an explosion or car crash doesn’t really matter, but can cause serious problems if you’re you’re trying to mix a song with a melodic bass line! There are other potential pitfalls involved in engineering a port, but suffice it to say it’s not a simple matter.

So, despite the efficiency savings that ports introduce, some manufacturers stick with the sealed‑box approach. That, however, requires a powerful, high‑spec driver capable of long excursions and which won’t suffer from overheating. It also requires a large, sturdy, non‑resonant cabinet, and both of these things are inherently expensive. Good examples include the ATC SCS70 Pro, Dynaudio’s 18S and the Neumann KH750. Tellingly, all those models use premium, in‑house‑built drivers.

Product Placement

So, subwoofers aren’t easy to design or build. Unfortunately, nor are they straightforward to install. They’re inherently large things for a start, so you need to find room for them! They’re heavy things, too, which is why you’ll most commonly see them sited on the floor. This actually has one excellent advantage, in that it increases the subwoofer’s efficiency: unlike a speaker on a stand, say, which will radiate its bass energy in all directions, a speaker (or subwoofer) placed on the floor will benefit from something called the ‘boundary effect’. Essentially, all the energy that it expends on downward‑firing sound is immediately reflected back up off the floor, reinforcing the sound that’s firing upwards. This is especially helpful at low frequencies, which require a lot of energy to generate, and means that a subwoofer won’t have to work too hard to produce a decent level — which is excellent news, because any speaker working too hard is liable to distort.

An easy way to identify the best position for your subwoofer is to put the sub at the listening position, and then crawl around your studio to identify the place where it gives the smoothest response.The down side of floor placement is that the position at which your sub performs best acoustically, in the room, might also be where your filing cabinet, spare bed, outboard rack or potted yucca plant is.

As to where that optimal position is, there are a few ways to find out. You can be scientific about it, using a measurement microphone at the listening position and a real‑time analyser to assess the subwoofer’s response at multiple locations in the room to identify the best spot... But that involves repeatedly moving a large and heavy subwoofer around, while tediously taking and re‑taking acoustic measurements.

A more practical but still perfectly valid method is to place the subwoofer at the listening position, play some test tones through it, and then crawl around on your studio floor to identify the spot where the response is smoothest — that is, where all the notes are heard at roughly the same level (as you move around the room, you’ll find that some notes become almost inaudible, while others sound significantly louder, which is the opposite of what we want). That spot is where you should place your subwoofer.

You can even combine the two approaches, by putting the sub at the listening position and taking measurements around the room (it’s a lot easier to move a microphone around than a subwoofer!). In either case, you’ll need a suitable test tone. You can either make one yourself using a software synth playing pure sine waves, or, if you’re lazy, you can simply download one from www.soundonsound.com/techniques/room-improvement-audio-file

Crossing Over

There’s more to setting up a subwoofer than just finding the right place for it, but the amount of tweaking that you can actually do afterwards will depend on your sub’s features. I’ll assume here that you’re considering a powered subwoofer, as that describes the vast majority of studio models. Being active, then, your potential sub will have its own level control. Setting the right level for your sub is rather like adding reverb to a vocal: it should be loud enough that you miss it when it’s gone, but not so loud that it draws attention to itself. The sound of a subwoofer doing its job well is essentially the sound of your monitors, but ‘filled out’. You can make this judgement by ear, or you can again get the measurement mic out and adjust the sub’s level until you get as smooth a response as possible.

Most subwoofers will feature a variable crossover control, which allows you to determine the frequency below which the subwoofer takes over from your main monitors.Next thing to consider is the crossover. Just as your monitors require a crossover to separate the low from the high frequencies so that your tweeter and woofer focus on what they do best, a sub needs to isolate the extreme low frequencies while sending everything else to your main monitors. Here, you’ll see some variation between subwoofers: some will have a switch to choose between a couple of frequencies, while others will have a continuously variable control. The exact frequency you set it to will depend on the subwoofer and your main monitors: ideally, it shouldn’t be higher than 120Hz or so (above that, you’ll start to hear the sub as a separate entity, rather than as an extra octave coming from your main monitors), and preferably lower if your mains can produce a healthy level below that. For typical two‑way studio monitors, 60‑80 Hz is usually about right; the sub’s output needs to ‘meet’ that or your monitors, in order to avoid a ‘gap’ in the overall frequency response.

If the sub has a polarity switch, it’s definitely worth experimenting with to see which setting gives the best results. What this switch does is reverse the sub’s output, so that ‘outie’ cone movements become ‘innie’ ones, and vice versa. What you want is for your monitors’ cone excursions to match the sub’s around the crossover frequency, but even if sound leaves them both at the same time, and in the same polarity, it might not reach your ears in the same condition! Your sub might be considerably further away from your monitors, which would incur a time delay, or you might have sealed‑box monitors with a very tight response and a ported woofer with a sluggish one, so it’s definitely worth experimenting with their relative polarities.

Some models also include a variable phase control, which can help you fine‑tune the combined response of your sub and monitors.A somewhat related control is ‘phase’, and some (but not all) subs will have a variable phase control. Think of this as a frequency‑specific delay and you’ll be in the ballpark; by adjusting a subwoofer’s phase response you can ‘tune’ how well your sub reinforces the output of your speakers, more precisely than by switching the polarity alone.

A useful way of checking the sub polarity/phase is to play a sine tone at somewhere near the crossover frequency (85Hz is often a good bet; it needs to be something that both the sub and satellites reproduce at the same time). You then adjust the phase or polarity switch and listen for which setting gives the loudest signal — indicating the sub and satellites are more in‑phase than not at the listening position.

It’s also worth mentioning that the crossover, level and phase controls (assuming they are all present) will interact with each other to some extent, so be prepared to go around the adjustment loop a few times to fine‑tune the alignment. The most effective order is to set the phase alignment first, then tune the crossover, and finally set the level.

If you’ve opted for a sub with DSP, then you’ll likely have even more options for tuning and tweaking. DSP‑enabled subs will usually have a delay option, to help you more accurately compensate for subwoofer placement: a good rule of thumb is one millisecond per foot, so if your sub is three feet further away than your monitors, a 3ms delay will bring it into alignment.

You might also have an EQ band or two to play with, to counter any acoustic anomalies in your studio. The crossover parameters may also be a little more involved than a simple frequency knob, perhaps allowing you to adjust the slope of the low‑ and high‑pass filters, as well as their frequencies. Some digital subs might even have an element of room correction built in (see ‘On Brand’ box).

Finally, many subwoofers will have an extra jack socket round the back to accommodate a footswitch, for bypassing the sub. Usually (but not always; check the manual!), this will let you instantly mute the sub while disengaging the crossover, so that you can hear your main monitors in their natural state — useful if you want to hear whether your sine‑wave sub‑bass part will be audible on more limited sound systems.

On The Up Side

As you can see, there’s rather more to installing a subwoofer than just plugging it in and turning the volume up. The huge wavelengths inherent to low frequencies also mean that, whatever you do, there are still likely to be areas of your room where the bass response will be uneven. Deceptively bass‑light or bass‑heavy monitoring will manifest as mixes that sound bass‑heavy or bass‑light when they’re played on other systems in other acoustic environments, and in the worst cases, will mean you’ll have to completely revise your mix balance.

What’s more, it’s not just the level of bass that’s important: distortion from a cheaper high‑excursion driver can easily run up into the kHz, even if the sub’s input is filtered at 80Hz or so, and this distortion can end up masking sound from your mains. So even if your main monitors are decent, adding a cheaper subwoofer can make the monitoring system as a whole worse. Good‑quality, low‑distortion subs are expensive, and even then, to get the most out of them will require an acoustically treated room, which further adds to the cost.

As long as any other monitoring issues have been dealt with as well as is practicable, and care is taken to set them up properly, subwoofers can be genuinely useful monitoring tools.

It’s understandable, then, that questions along the line of “What sub should I get?” are often met with a barrage of questions in response, such as “Is your room treated?”, “What monitors do you have?” and even “Why do you think you need one?” This caution makes perfect sense from a mixing and mastering point of view, but can have the effect of putting people off the idea entirely. As long as any other monitoring issues have been dealt with as well as is practicable, and care is taken to set them up properly, subwoofers can be genuinely useful monitoring tools. But those conditions do have to be met!

There is another reason for getting a sub, though, beyond the goal of pristine, full‑range monitoring. In our world of measurement mics and frequency responses, it often gets overlooked that Bass Is Fun. Ravers and clubgoers know this very well, and have for decades been enjoying sub‑bass on sound systems that would bring the average mastering engineer to tears. My point is that not everyone with a home studio aspires to be a mastering engineer. The electronic music producers I know aspire to have their music played on rigs with multiple 21‑inch scoop bins in reappropriated warehouses, not linear‑phase, DSP‑aligned systems in light, airy rooms containing nothing but 10 grand’s worth of hi‑fi speakers, a cactus and a Herman Miller chair.

Musicians and composers likewise have much to gain from having unscientific fun in the studio: whether you’re noodling away on a synth or guitar, playing bass lines, programming drums or DJ’ing, a little extra heft can be just the thing to help you get in the zone. So, from a creativity and enjoyability point of view, a sub — even a cheap one, in an unoptimal position and perhaps turned up a teeny bit too much — can be both enjoyable and creatively inspiring. Just don’t expect a cheaper ‘for fun’ system to give you anything like as much information as a high‑end, well‑configured monitor and sub combo — and remember to bypass the sub when you’re doing your rough mixes!