Even bass coverage in bassment

[xplod1236]

I just can't figure this out. I have four 12 inch subs that I'm using. The basement is 25 ft long by 20 ft wide. I can't seem to get even coverage in the marked (rectangular) area. I always seem to get a dead spot (no bass at all) just to the right of the stairs (shown by the circle).

One setup I had, shown in the second picture, the subs (green) on the right were out of phase with the subs on the left. That gave me tons of deep bass on the wall with the doors, but towards the middle of the basement, the bass would cancel. Then I had my full-range speakers (one in each corner) fill in the bass, and that sounded decent. This setup somehow took care of the dead spot in front of the stairs, but I didn't like the idea of having half the subs out of phase, and the cancellation towards the middle.

Does anyone have any ideas to get pretty even coverage? I have tried just about everything, from a sub in each corner, to all the subs in one corner, to the subs spread out along one wall and all firing one way. I got the best results with half the subs out of phase and the mids filling in. Give me something else to try, because I'm out of ideas.

[TWOJZ]

tried positioning the subs as far away from the stairs as possible, and facing up?

[jason_watkins]

http://www.harman.com/wp/pdf/multsubs.pdf

The short summary of lessons from that publication:
- use 2 or 4 subs
- put them symmetrically midpoints of walls for most even response

Granted, you have a non-standard room where the stairway, etc will skew the modes, but I think the general ideas behind the results still apply. I suspect all 4 of your subwoofers are coupling to the room rather well since they're either in a corner or not positioned at a low order null. The two along the long walls should cancel odd order axial modes for each other, but that's about all that's helping you.

One way you can attempt to map the modes in your room is to put a sub in the corner, play a tone, and try to find nulls with a spl meter. They should be the best locations for the subs for flat response. It's a bit error prone though.

[simon5]

Did you try all subs in the stairway corner ?

[xplod1236]

I guess I'll try one in the middle of each wall.

simon5, I cannot fit all the subs in the corner next to the stairs. I only have enough room there for 1 sub (due to the door).

[simon5]

You could stack them no?

[nunayafb]

a sub in each corner, to all the subs in one corner, to the subs spread out along one wall and all firing one way

It sounds like you are making too large of changes, what you might want to consider is starting with a logical position and tune from there.
Based on the room having two different dimensions (not square) it is going to have different resonant modes in one direction than in the other. So if you put two subs in the middle of each wall you will get symmetry and any resonant modes will add up, same with corner loading.

I would start with this:
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The "O's" representing starting woofer locations, remember you are dealing with phase relationships from multiple sources and with multiple reflections so start with moving each sub a couple of inches and one at a time.

BTW, I arrived at this arrangement by using, first an idea of asymmetry, then checking the result using a wave applet

Good luck, (be patient and you wont need luck)

-J

[y8s]

Originally posted by jason_watkins
http://www.harman.com/wp/pdf/multsubs.pdf

The short summary of lessons from that publication:
- use 2 or 4 subs
- put them symmetrically midpoints of walls for most even response

Granted, you have a non-standard room where the stairway, etc will skew the modes, but I think the general ideas behind the results still apply. I suspect all 4 of your subwoofers are coupling to the room rather well since they're either in a corner or not positioned at a low order null. The two along the long walls should cancel odd order axial modes for each other, but that's about all that's helping you.

One way you can attempt to map the modes in your room is to put a sub in the corner, play a tone, and try to find nulls with a spl meter. They should be the best locations for the subs for flat response. It's a bit error prone though.

hey that's one of the most useful audio posts i've seen in a long time that's totally accessible and easily implemented by the commoner.

thanks!

Matt

[jason_watkins]

Based on the room having two different dimensions (not square) it is going to have different resonant modes in one direction than in the other. So if you put two subs in the middle of each wall you will get symmetry and any resonant modes will add up, same with corner loading.

I don't believe this is correct.

There's are two symmetries you can use to put subwoofers in places that couple to the room less:

1. subwoofer along the midpoint line between two facing walls.
2. subwoofers each directly against two facing walls.

The first avoids even order axial modes because it locates the subwoofer at a null.

The second avoids odd order axial modes since the subwoofers cancel each other.

In an ideal rectangular room, the midpoints of the walls have both these properties at the same time, which is great. The room doesn't have to be square. If you check the publication/slide deck you'll see all of their analysis was in rectangular rooms.

In a more typical room with doors, windows, differing wall lengths, etc, these axial modes are still present... however, they may be significantly lower in impact, so that the room becomes dominated by tangental modes. Hard to do anything about that, but we still can at least avoid the axial modes.

Once I learn a bit more mathematica, I plan to replicate toole's analysis for multiple dipole woofers as well as more commonly found 'strange' rooms like L's.

[Eva]

How about adding different delays (or local phase shift in narrow frequency ranges with high-Q 1dB parametric EQs) to the signal going to each subwoofer?

Is there any ready-made software capable of modelling this?

There are low-cost tools to do the signal processing, like Behringer DCX2496, but the amount of possible adjustments is just too high to try to tune it by ear without a previous mathematic analysis.