I want to add a subwoofer to my monitoring system, but how do I work out where to place it in the room?
Technical Editor Hugh Robjohns replies: Adding a subwoofer to a monitoring system is not a trivial matter, and great care is required if the potential advantages are to be realised. Firstly, every room will suffer from low-frequency standing waves unless properly treated with adequate bass trapping. Standing waves cause a very lumpy bass response in the room, where some bass notes are boomy and much too loud and others may not be audible at all, and the balance changes dramatically as you move around the room! Bass trapping essentially soaks up some of the LF energy at the room's corners, preventing it from being reflected back into the room to interfere with the direct sound from the speakers.
If the monitoring system has a restricted bass response (perhaps because they are small nearfield monitors) then these troublesome standing waves may not be excited and thus they may not cause too much of a problem. Introducing a subwoofer to the system will generate lots of very low-frequency energy which may well excite a range of low-frequency standing waves, with the result that the monitoring quality and accuracy is reduced, even though you have extended the theoretical bandwidth of the system (and spent lots of money!). So, before purchasing a subwoofer, invest in proper bass trapping at the very least.
If the room's acoustics have been sorted out, then the next challenge is to place the subwoofer in the optimal position. The most pragmatic way to achieve this is, having connected the subwoofer and set approximate levels, to place it temporarily where you normally sit when mixing. You can then crawl around the floor near the walls listening to the quality of the bass. You are trying to find the place where the bass notes are the most even and balanced — where none are excessively loud or quiet. When you have found the best place, reposition the subwoofer there and align it as the manufacturer advises in terms of its level, crossover frequency and, if provided, phase or delay.
Ideally, a subwoofer will only generate low-frequency sound, and as humans are rather poor at locating the source of low frequencies, it should be possible to place the subwoofer almost anywhere in the room without side-effects. Sadly, the reality is that most budget subwoofers generate large amounts of harmonic distortion, so even if you restrict the operating bandwidth to 80Hz, the subwoofer may well generate audible harmonics at 160, 240 and 320 Hz, all of which can be easily located. This will tend to cause distracting spatial images, and will reduce the transparency of the mid-range part of the spectrum. High-quality (but expensive) subwoofers — such as those from ATC or PMC, for example — tend not to suffer from this problem as much, but even so it is generally advisable to place the subwoofer somewhere between the main monitors so that any harmonics generated are located roughly where the intended source is panned.
Do not place the sub exactly at the mid point, though (assuming the main speakers are placed symmetrically in the room), as this will coincide with a fundamental standing wave and produce poor results, as your crawling around on the floor will hopefully have revealed!
Depending on the design of the subwoofer, you may find altering the distance from the rear wall as effective as moving the sub a few inches to one side or the other in optimising the eveness of the sound.