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...