It is a sub I built and tuned using winISD, has a scan speak 12 in it.
My room, while treated, definitely suffers from some issues as I can hear nulls etc during REW sweeps etc. I'd very much appreciate any advice you may have regarding placement etc!
Well there is no secret to positioning a subwoofer... you will inevitably have a big peak-and-dip pattern no matter where you place the sub, and no matter where you sit. You can move your chair or move the sub and that will change the peak-and-dip pattern, and you might make some improvement, but it will never go away. You can equalize for your listening position and make a significant improvement at that location, but at the same time you will be making the bass region WORSE at other locations within the room. This may not matter, but if one of your system's duties is "impress the clients", you don't want your clients to hear lumpy bass.
The problem is not that there are too many room-induced peaks and dips - the problem is that there are TOO FEW! Up in the midrange region the reflection-induced peaks and dips are spaced so close together that they essentially form a continuum (and they look like "grass" on an unsmoothed response curve), but unless we have an unusually large room, that situation doesn't extend down into the bass region. The ear/brain system tends to average out peaks and dips that are within about 1/3 octave of one another, but when they are further apart, they are more likely to stick out like sore thumbs. In general the smaller the room, the fewer and farther apart the bass region's peaks and dips are going to be.
If we look at a set of equal loudness curves, we can see that they bunch up south of 100 Hz:
One of the things these curves tell us is that a roughly 5 dB change in SPL at 40 Hz SOUNDS LIKE as big a change in loudness as a 10 dB change at 1 kHz! So our ears actually have a heightened sensitivity to dips and peaks in the bass region! This is one of the reasons why it normally takes a long time to dial in the correct level for a subwoofer - a small change in SPL makes a relatively large chance in PERCEIVED bass loudness.
At low frequencies, speakers + room = a
system. In the size rooms we have in our homes, the ear literally
cannot hear the bass apart from the room's effects on it (I can explain why if anyone would like). And those effects are typically huge.
One more bit of theory before we get to the solution: Perceptually, smooth bass is FAST bass. Speakers + room = a "linear phase" system at low frequencies, which means that the time-domain response tracks the frequency response (and vice-versa). So an in-room bass peak is ALSO where the bass energy takes longer to decay to inaudibility. I'm going to propose something that most directly addresses the in-room frequency response, but at the same time it's improving the time-domain response, so the net result is perceptually FAST bass.
Okay enough theory. The solution is, take multiple subs and spread 'em around the room asymmetrically. Each will generate a unique peak-and-dip pattern at the listening position. But the SUM of these multiple dissimilar peak-and-dip patterns is far smoother than any one alone. We end up with not only a lot less total variation, but also a much denser net peak-and-dip pattern, so the ear/brain's smoothing mechanism can kick in.
The ear/brain system doesn't hear the slight variation in arrival times; at low frequencies the ear's time-domain-resolution is quite poor, but (as the equal-loudness curves show) its frequency-response-domain (or loudness-domain) resolution is heightened at low frequencies.
This can be done working with the subwoofer you already have. Maybe build three small subs to go with the one you have, same impedance as the one you have, probably not going as deep and/or not being as efficient. That's okay. Connect the four subs in series-parallel and drive them all with the same amp (to avoid the expense of needing three more channels of amplification).
Caugh....swarm..caugh caugh...
Yeah, I make a commercial four-sub system called the Swarm. It got a 2015 "Product of the Year" ("POTY"?) award from The Absolute Sound magazine. But there are many ways you can apply the general principle of smoothing the in-room bass by having a lot of bass sources distributed around the room.
In general the more bass sources you can spread around the room, the smoother the in-room bass will be. And recalling that small rooms have worse bass response than big room, we come to the highly counter-intuitive conclusion that the smaller the room, the more subs you should have if accurate bass is the goal! Of course they can be small subs.
Regarding distributed multisubs vs EQ vs room treatment, it's NOT either/or. You can do all three! And because distributed multisubs significantly reduce the spatial variation (variation in frequency response from one location in the room to another), any EQ that is needed is probably going to make a global (room-wide) improvement rather than merely a local one.
There are a few things you want to watch out for if you're going to "roll your own" distributed multisub system, just let me know if you'd like me to describe them.
Credit to Earl Geddes for the asymmetrically distributed multisub concept, which I use in the Swarm with his permission.
