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With multiple sources and locations of the same frequency within a room, timing differences will cause some to combine and others to cancel. It is the same effect that room modes produce. If I understand the use of multiple subs correctly, if set up properly multiple subs can use these timing differences to counter the natural fixed room boundry issues. However, if set up wrong they can add to them.
The reason is pretty simple. The 2-channel loudspeakers are limited in placement by how they have to be placed for stereo reproduction. That location is pretty much never the optimal for bass reproduction. With a couple small subs, a DSP and some room measurement capability you can get much smoother in-room response and it won't matter that the unlimited budget 2-channel system is using some multi-thousand dollar drivers because their response in-room is determined by location and their dance with the room. FR isn't everything but it is damn near impossible to have a system with +/- 12dB FR swings down low and have it sound good. It won't matter how much you spend on it because the bass performance is determined by the device, its location within the room and the relationship with the listening location. Duke's method of using multiple smaller subs is an excellent strategy and if you can have DSPs on them, that allow you to adjust for delay and properly cross them over to the main speakers you will have much better bass reproduction than you will with just two stereo loudspeakers. Done right it is completely unidentifiable from a good 2-channel setup with the exception that the bass will be improved. The DSP will also allow you to use a couple bands of PEQ if you need them. Every room is different so I do believe in using a little EQ if it helps. You have to measure multiple locations for the reason Duke pointed out and using too much EQ is almost always worse than using none but there is no simple rule. You have to measure. In terms of what Earl says.... he is right. Under about 30-35Hz you start to universally pressurize a room (depending upon size/dimensions). At that point you are below the region where room modes matter and the response is pretty linear compared with 30-250Hz. Of course for 2-channel use response under about 25Hz is not needed for most music. But if you are spending $10K on a pair of loudspeakers there is absolutely no good reason not to use these techniques to improve the bass. It is simply insane to live with +/- 12dB ripples in the measured response in a system in that price range. Of course the final issue that isn't solvable with any loudspeaker or room is the fact that recordings are not mastered universally the same. Especially in the low frequencies you have significant differences in how they were mastered/recorded. You are ultimately limited by what is in the recording and since there isn't a universal standard for studio monitors or the room, there ultimately is no standard for the bass that goes on the recording. That is why it is sometimes more frustrating to have extended bandwidth than to have a limited bandwidth. Also... the less bandwidth you have the less you stimulate room issues so often people prefer to have a system with modest bass reproduction because it is the lessor of evils.
There is something I have not noticed being talked about enough and that is the possible problems with phase. I have experienced this even with stereo bass towers, but I would think it would be much worse with summed mono sources.With multiple sources and locations of the same frequency within a room, timing differences will cause some to combine and others to cancel. It is the same effect that room modes produce. If I understand the use of multiple subs correctly, if set up properly multiple subs can use these timing differences to counter the natural fixed room boundry issues. However, if set up wrong they can add to them.
My 2 channel speakers (Vandy 5A's) are sort of a hybrid to what people have been discussing here. They have self powered subwoofers built in so that the amps don't have to worry about driving big woofers. They also have 11 bands of eq (with Q and output control to fine tune) to help handle room issues. This approach means I don't have to worry about time and phase issues that placing multiple subs around the room can cause.George
Measurements have their place in the hobby, but I think no more than to provide general guidance. Sometimes getting in there any playing with your hobby will provide very enjoyable results to some ears, while it might drive a "meter reader" to drink.
The room you mention will have major peaks and dips from bass modes down to about 40 Hz. Small rooms are actually worse than big ones, as the peaks and dips end up being farther apart and therefore more audible and distinct. If the peaks and dips are very close together, the ear can average them out and the perceived sound is a lot smoother than what the eye would judge from looking at a measurement. Counter-intuitive as it may seem, smaller rooms benefit more from a scattered multisub system than larger ones do; this is because they are so much worse to begin with.
I completely understand everything that is being said- but cannot get over the thought that a mono sub system is oversimplified, even with multiple placements. I think it must improve many things and sacrifice others. I listen to allot of bluegrass. There are times when bass is stronger out of one or the other channel. (AKA: Stereo separation) I don't want a sub smearing it all over the place. If it's supposed to be in the left channel, shouldn't we keep it there? I don't want it all over the place. Same goes for the other recording I mentioned earlier on.
Measurements have their place in the hobby, but I think no more than to provide general guidance
Room treatments might have alleviated some of my initial issues (and still might be capable of improving things further - I'm not sure), but I'd rather have a couple of subs kicking around than a whole lot of panels on the walls.
I suspect you are hearing things higher than 150 Hz. It is generally agreed that that is the lower limit for localization (as long as there are no harmonics and nothing is rattling).
A good discussion and maybe I can add some thoughts if there are any particular questions.Earl Geddes
Good points here. In my latest multiple subwoofer setup adventure, phase and delay certainly were an issue until we applied individual settings to each subwoofer, then applied any necessary EQ. Three ended up having the same phase, while one was almost completely reversed. It depends on the room and placement, but if you don't get these right you get cancellations, which is like never having fixed the room modes to begin with. Of course, it's kind of moot concerning the lowest frequencies, but what about mid and upper bass? Is phase and delay that last step to take on the importance scale of order? I've always wondered why Geddes and Toole said its not really important in multiple subwoofer setups...so confusing.
I should have mentioned that room treatments are the only thing that can alleviate ringing, at least as far as I've learned. PEQ is great for removing peaks, but the room treatments are what reduce ringing. If necessary, apply both.
Sure - I'd love some guidance concerning this:
First off let me state that I do not think it is audio "blasphemy" to integrate subwoofers into a 2-channel system. However, I've been called a "blasphemer" on numerous occasions for doing it. Ironic...
Can I ask if the speakers pictured in your avatar are you current speakers, and if so are they TLs? TLs happen to have a fast changing gradient of phase through their lower passband. I've heard only a couple implementations of TLs and all have been "deep and tight" but also a bit one note and boxy in character.
PEQ can go a long way (there is an interesting discussion about that in Floyd Toole's book). I think phasing also, applied appropriately, can reduce ringing.
