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Old 2nd November 2008, 03:47 AM   #1
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Default New to Forum - 1st Post - What to do with boxes, 2 twelve inch drivers, and 800w amp?

Hello All. This is my first post here, so I wanted to thank everyone first for all the info on this forum. I am very new to this so please forgive some of my lack of knowledge.

Here's my situation/setup:
1800 cubic foot room - 15x15x8 with one of the 15' walls open
Two sealed boxes each 4.76 cubic feet - They are built and I have to use one or both of these
pro amp (400w x 2 at 4 ohms)
2 twelve" subs (dual 8 ohm driver)

I was going to go with 2 separate boxes until I read about dual opposing subs in one box and the benefit of such a setup. Here are my questions:

1. What do you guys think is the better setup in my situation? Two singles or one dual opposing? I am afraid that each of my boxes might be a bit big for 1 twelve ", but i am also afraid that 1 of my boxes is too small for 2 12"'s.

2. If dual opposing is the way to go, how close should the drivers be to each other. Anything else I should be careful of when using a dual opposing setup?

Below are the specs for the subs I have in case someone wants to do me a huge favor and run some numbers. Due to my lack of knowledge, I do not know how to run numbers on the dual opposing box.


Subwoofer Driver Specs
Re: DC resistance of VC 3.60 Ohms
Le: Inductance of VC 0.90mH
Fs: Resonance frequency 19.6 Hz
Qms: Mechanical compliance loss 2.60
Qes: Electrical motor loss. .46
Qts: Total Q of driver .39
Mms: Moving mass 140g
Cms: Suspension Compliance 0.47 mm/N
Vas: Stiffness of driver scaled by cone size 169L
Sd: Area of the cone 506 cm^2
Vd: Total Diaphragm Displacement volume 2.65L
BL: Motor Strength 11.66
X-Max: One-way linear excursion 27mm
Pmax: Maximum power (music, not RMS) 600W
SPL: 86.3 dB 1W/1M 89.1 dB at 2.83 Vrms
Volume Occupied by Driver .15 cubic feet or 4.25L


Thanks for any help.
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Old 2nd November 2008, 04:51 AM   #2
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I can only answer one of your questions - 1 of those 12's in the box you specify will give a very slightly low Q response of 0.6, resulting in a more gradual rolloff towards the -3dB point at 38hz. In room, this is likely to be entirely unnoticeable, and indeed, some have said that slightly low Q alignments are actually desirable for giving even in room response.

Which dual woofer arrangement are you proposing? Both drivers on the same side of the cabinet, but one mounted with the motor facing outwards and wired out of phase, resulting, theoretically, in cancelling of mechanical non-linearity? That should model just like an ordinary pair of woofers in a box, but, while looking weird, has potential to sound better.

My vote is for using them as two single subs. That should make it easy to keep them in phase with the main speakers (by putting them close by to the mains). The room will dominate the response of the subwoofer, and using two separate boxes minimises the problem of room placement.
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Old 2nd November 2008, 02:27 PM   #3
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Thanks Seeker.

For the dual setup, I was looking at a push-push sealed box, side-mounted, where each driver was facing outward (i.e. back to back) and wired in parallel. From what I have read, this results in a cancellation of forces and less box vibration. Plus, there is a possible 6db gain from doubling cone area and doubling power in same box. My problem with this setup is that I do not know if there are any pitfalls to watch out for in the push-pushbox design. For example, my drivers have a mounting depth of 7.25" and the outsides of my box are 18.75" apart. This will leave 4.25" between the backs of each driver. Is that too too far apart to reap the benefits of the push-push design? If so, can I put some bracketing in between the drivers that brings the drivers closer together? Also, do they have to be exactly opposing to get maximum benefit?

I think you were referring to push pull, which I do n think I need b/c my drivers are decent and fairly linear.
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Old 2nd November 2008, 08:25 PM   #4
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Winisd tells me that sealed, those woofers won't do well as subs in those boxes without eq; port them, tuned to 20Hz and they'll go down to the low 20s nicely... go for 2 subs, more control over room resonances through placement.

If you want to model 2 drivers in the box, simply double Vas & halve Re (for parallel connection)
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Old 3rd November 2008, 03:35 PM   #5
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Thanks. I would like to stay with sealed boxes. I think I lose about 8db at 20 hz and, 13db at 15 hz, and 19db at 10hz. With two subs and some EQ, I thought that would be decent. But, if everyone mentions ported, even though I specifically stated I did not want to change my built boxes, then maybe I should re-think my plans.

Also, I guess there is no love out ther for the push push dual opposing sub box setup. Funny....b/c I thought there was something to gain with this design (less vibration, slight increase in output.
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Old 3rd November 2008, 04:43 PM   #6
badman is offline badman  United States
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Quote:
Originally posted by grateful72
Thanks. I would like to stay with sealed boxes. I think I lose about 8db at 20 hz and, 13db at 15 hz, and 19db at 10hz. With two subs and some EQ, I thought that would be decent. But, if everyone mentions ported, even though I specifically stated I did not want to change my built boxes, then maybe I should re-think my plans.

Also, I guess there is no love out ther for the push push dual opposing sub box setup. Funny....b/c I thought there was something to gain with this design (less vibration, slight increase in output.

I'd put the 2 boxes back-to-back and use one driver in each. You can have one of the drivers with the magnet out as an experiment, to see if you get some distortion cancellation, just reverse the wiring phase to that one.

Screw them together if you want, or otherwise couple the cabinets, lest you have them rattling against each other.
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