Starting a new build, please advise. - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 4 Old 05-13-10, 04:44 PM Thread Starter
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Mario
 
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Starting a new build, please advise.

Just ordered 2 Fi IB 18" drivers for my hometheater-in-a-bedroom. The room is pretty small (11'x13') so I was conflicted on which sub to build. I wanted something that can play low and loud and that usually means a big box. Then I realized I already had a ginormous enclosure right in front of me: the walk-in closet!
I was initially thinking 4 drivers, but that would mean only 3.5*Vas, so I had to go with 2. It's still below the optimal 10*Vas, but when modeling I saw very little difference. Anyway, there are a couple of questions I wanted to ask before I start cutting wood and connecting wires.

1. What's the optimal placement for the manifold on the vertical axis? Since it's replacing the door to the closet I don't have much flexibility in terms of subwoofer location. But I can put it closer to the floor or move it higher. Playing with REW and my current sub it seems that moving it closer to the vertical center of the room would provide a smoother response, but I'd like to hear an expert opinion on the subject.

2. When modeling with WinISD I noticed that even a simple first order highpass at 10 Hz does wonders for the excursion and allows me to bump the power to 1200W. So I was thinking of putting a simple capacitor in the signal path (matched to the input impedance, obviously) to create a DIY subsonic filter. Has anyone tried anything like that with their IB setup?
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post #2 of 4 Old 05-14-10, 05:11 PM
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Re: Starting a new build, please advise.

1) There are many theories about this. Floor bounce, as Roy Allison called it, is a way to reinforce the output from a low frequency driver but it also can reinforce room modes. My usual approach when building an IB in a closet is to mount the drivers at equidistant points along the baffle. But bottom line, it does not matter all that significantly for subwoofers.

2) Building a cap large enough to create a first order high pass at 10Hz (almost 2000uF @ 8ohms) is going to get quite expensive. You'd most likely be better off utilizing an electronic filter.
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post #3 of 4 Old 05-15-10, 02:44 PM Thread Starter
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Mario
 
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Re: Starting a new build, please advise.

Thanks for your response. I'm actually building a manifold since I want my baffle to stay in one place . And I'm more interested in smooth FR, so I guess I'll place it as far from the floor as possible, which in my case is about 4-5 feet.

As for the capacitor I was actually thinking putting one before the amp or equalizer, matching it to the input impedance of the device. But after you mentioned making it a passive filter, I checked the prices and it doesn't look too bad. What's the voltage rating I would need, about 60V ?
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post #4 of 4 Old 05-15-10, 05:38 PM
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Re: Starting a new build, please advise.

Use crossover caps rated for at least 100 volts (higher is better) like these. It gets expensive to build up to the cap value required to create the subsonic you want using a passive crossover. Don't use electrolytic caps, BTW.

Building a line-level subsonic filter would be much less expensive approach.
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