If you are anything like me then when checking out the specs of a pair of monitors, the first spec you check is the frequency response and the number you look at first is the low frequency cutoff. Bass isn’t the only thing which is important but it is undeniably important, and unlike the time domain performance or the consistency of the off axis sound, bass is a crowd pleaser. Everyone grins when a huge dollop of sub hits, even people who should be too grown up for such things (yep, guilty…).
So in spite of the issues deep bass usually reveals in our rooms, we all want it. But how do you get that missing octave at the bottom into your studio? Back in 2017 we ran a survey on this and while nearly a third of respondents had a sub, nearly half of the respondents (47%) didn’t yet but were considering using one.
One of the questions we didn’t ask of the nearly two thirds of the respondents who didn’t have a sub was why not? Is it that they had full range monitoring and had no need of one or was it that they had an issue with using additional subs to extend their bass performance?
Full Range Monitors
Some people who don’t use subwoofers don’t need them. If you have a large pair of main monitors which extend down to 20Hz then there probably isn’t much to gain if you are working in stereo. If the room is suitable to accommodate sub bass then this surely is the preferred route isn’t it?
Many of us use subwoofers to supplement and augment the performance of smaller monitors. It is probably this “two satellites and a sub” format that most of us are thinking of when we talk about subwoofers and it is probably this format being done badly which has let to the opinion held by some that subwoofers are a bad thing. Certainly some horrors lie in wait for people who believe that a pair of computer speakers and an over-ported, too small “sub” will do anything other than a splashy tish with one note bass. But proper subs can be far better than this.
There are still some issues though. One of the first is the over enthusiastic application of a sub. My first pair of proper monitors had a sub and thankfully I had almost no control over it and I definitely had no way of turning it up inappropriately loud compared to the tops, a practice which will lead to bass light mixes and ultimately do more harm than not having a sub at all.
Timing And Subwoofer Placement
Another issue it the time performance of the sub. Many subs use complex, band-pass enclosures such as 4th or 6th order designs. These generate amazing quantities of bass from tiny enclosures but the group delay will always be significantly longer than the bass driver in the usually ported cabinet of the satellites in a typical two monitor with one sub arrangement.
Another consideration is crossover frequency. It’s an often repeated “fact” that bass is omnidirectional so it doesn’t matter where the sub is placed. This is going slightly further than can be justified. While it’s true that deep bass can’t be easily localised, it is still desirable for the path length to be at least roughly the same from the monitors and the sub. But when talking about subwoofers being used to extend the bass performance of less than full range stereo monitors then with a typical crossover frequency of 80Hz the wavelength is 14 feet. Even with steep 24dB/Oct filters in the crossover that means there will be audible amounts of 160Hz being put out by the sub which has a wavelength of 7 feet. The potential for phase issues between a carelessly placed sub and the monitors is definitely there.
Using A Subwoofer Purely As An LFE Channel
When using subs as an LFE channel in a 5.1 or 7.1 system the role of the sub is very different. If it is being used purely as an LFE channel then it shouldn’t share information with the other channels and these potential phase issues don’t apply. However there are a range of opinions on how the .1 channel should be used.
Using Two Subwoofers
Using two subs and locating each subwoofer physically close to each monitor in a stereo system offers similar performance to a larger, more powerful monitor with better bass extension and more headroom. The disadvantages of this are cost, many people will be put off by the additional expense of a second subwoofer, and when compared to using a pair of genuinely full range monitors, the issues which can be presented by band pass designs could make this less than ideal. However many manufacturers do produce front-firing, ported subwoofers which wouldn’t suffer from these differences to anything like the same extent.
Why Stop At Two Subwoofers?
If you see a subwoofer as some extra bass that can be bolted on to some monitors, which putting it crudely, is what they are, then the advantages of additional bass extension comes as soon as one is added to the system. Potential timing errors can be addressed by adding a second and placing them close to the monitors but along with the timing advantages comes additional headroom courtesy of the added power. Something I really enjoyed about the HEDD Tower mains when I went to a listening event at Rimshot Studios in February was the fact that a TMS36 subwoofer was necessary to extend the TM80 monitors beyond 80Hz, so you definitely needed one per speaker. They were installed physically close to the TM80 main monitor and shared the same infinite baffle design so the time domain performance was well matched between the main monitor and the sub but with the addition of a second pair of subwoofers the Extended Tower Mains allowed you to bolt on additional headroom if you needed it.