my first diy sub. noob looking for advice - AVS Forum
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post #1 of 10 Old 10-14-2012, 09:43 PM - Thread Starter
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Hi,

I have been re-doing my theatre. I have an old Onkyo THX certified HTIAB. I have replaced the speakers with:
Definitive:
ProCenter1000
2xProMonitor1000 fronts
2xProMonitor 800 rears.

I also put the 12" 250 watt Onkyo sub in the corner.

I also covered the walls and ceiling with 1" soundboard and an additional 1/4" drywall, so the room is 'dead'.

I am still using my existing receiver. I do not intend to replace it.

Man, what a difference. I watching Avatar, and I have to keep turning the sound down because of the bass.

Or am I just kidding myself, and I'm missing out on true bass?

My downstairs is pretty open, 21x24x8, then there is the open stairway to the 2nd floor.

The area under the stairs I have available is 29" w x 45" h x 73" d.

It would exit under the stairs next to a wall.

I would like smooth response that would be a good match for my existing speakers, I'm not interested in very much SPL, since I live in a townhome. I also would like to keep the budget to under $800.

It would be cool to have a couple of ported 18"s, maybe? Can 18"s fill the gap between my speakers and themselves?

Thanks,

Carl
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post #2 of 10 Old 10-14-2012, 11:02 PM
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A single 18 would probably be enough to keep up with those mains. A Mach 5 IXL18, an EP2000 and a MiniDSP plus some wood etc would be about $700 or so. You could even make it an LLT and tune it really low as you have space for a big enclosure.
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post #3 of 10 Old 10-15-2012, 11:50 AM
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Quote:
Originally Posted by Intrepidman09 View Post

...I also covered the walls and ceiling with 1" soundboard and an additional 1/4" drywall, so the room is 'dead'....I have to keep turning the sound down because of the bass. Or am I just kidding myself, and I'm missing out on true bass?...
It sounds like you've added sound isloation to the room, meaning that the room is not "dead" but rather "quiet." If it were truly "dead" in the sense of very low reverbration time, you'd never turn "...the sound down because of the bass."

Far more likely that by soundproofing the room, you increased RT, especially at low frequencies, and so have a strong set of room modes. It's the old "reflection + transmission + absorption = 1" principle. By reducing transmission (the goal) without increasing absorption, you've increased reflected energy, and not uniformly at all frequencies. I have a room about 2/3 your size, but it's leaky, with several features that absorb or leak bass. My single 15" will make you jump if you're not expecting it, but it's hardly overpowering.

The solution is to find out what your RT looks like vs. frequency, then add acoustic absorbers of the appropriate type to flatten out the RT vs. frequency curve. Bass needs special attention as the vast majority of acoustic treatments aren't terribly effective in the range where room modes dominate. Then there's the room dimensions...
Arranged in
ascending order
23.5 | Adjacent mode spacing
26.9 | 3.4
47.1 | 20.2
53.8 | 6.7
70.6 | 16.8
70.6 | 0.0
80.7 | 10.1
94.2 | 13.5
107.6 | 13.5
117.7 | 10.1
134.5 | 16.8
141.3 | 6.7
141.3 | 0.0

This shows the room modes below 150Hz from the Harman "roomcalculator.xls" which you cna easily download. Ideally, room modes are evenly spaced. Yours are not: L and W are close, and H is 3x the other two, so modes coincide. Note that this assumes rectangular room with 6 flat, parallel walls. Any departure from that adds complexity, thus the recommendation to do some RT measurements at the low end to see what's actually present in the room. Without measurement, you may be able to confirm aurally, by changing locations to verify which mode you're hearing. My bet is that the 70-80Hz trifecta is killing you.

If this makes sense, here's a link to the master reference where you can find out how to measure, and what you cna do to address the issues, if room treatment is, in fact, the solution.
http://andrealbino.wikispaces.com/file/view/Master+Handbook+of+Acoustics+-+5th+Edition+-+F.+Alton+Everest,+Ken+C.+Pohlmann.pdf

Have fun,
Frank
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post #4 of 10 Old 10-15-2012, 12:54 PM - Thread Starter
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Wow

Thanks for all the info! I'll bet having hard flooring (tile/pergo) is making it worse. Would replacing the pergo in the living room (the ht room, its about 3/5 of the space) with carpet make things better or worse?

Thanks,

Carl
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post #5 of 10 Old 10-15-2012, 02:20 PM
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^^ At LF, carpet makes no real difference.
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post #6 of 10 Old 10-15-2012, 08:31 PM - Thread Starter
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Hi,

You recommended a minidsp. Would that flatten my response curve without going through all the work with adsorbers and whatnot? Is Avatar beastly in the bass department?

Thanks,

Carl
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post #7 of 10 Old 10-15-2012, 08:41 PM
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Quote:
Originally Posted by Intrepidman09 View Post

Hi,
You recommended a minidsp. Would that flatten my response curve without going through all the monkey motions with adsorbers and whatnot?
Yes. It has a lot of EQ capability as well as allowing you to set HPF and LPF filters for the sub.
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post #8 of 10 Old 10-15-2012, 09:10 PM - Thread Starter
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That's good news. I think that should be my first purchase. It goes between the receiver and subwoofer RCA, right? What model and plug-in do you recommend? How do I test my room? Is there a Mic, SPL meter, and software you recommend?
Thanks,


Carl
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post #9 of 10 Old 10-15-2012, 11:21 PM
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Quote:
Originally Posted by Intrepidman09 View Post

It goes between the receiver and subwoofer RCA, right?
Between the AVR LFE output and the input to the amp powering the sub.
Quote:
Originally Posted by Intrepidman09 View Post

What model and plug-in do you recommend?
I'd suggest the 2x4 either balanced or SE depending upon what amp you choose and levels required. For one or two subs, either plug in would do.
Though I have been using DSP xovers for a decade (I have 3 DCX), my first MD is still in the mail, a second hand 2x8 and Advanced plug in.
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Originally Posted by Intrepidman09 View Post

How do I test my room? Is there a Mic, SPL meter, and software you recommend?
Calibrated ECM8000 from Cross Spectrum, a Behringer Xenyx 502 to give gain and phantom supply, some leads, a mic stand and REW for software.
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post #10 of 10 Old 10-16-2012, 09:29 AM
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I'm going to agree and disagree with the intervening responses.

Agree
- carpet makes no difference at LF.
- EMC8000 with calibration, or EMM6 (same mic) from Parts Express, which comes with cal file. Both need 48v phantom power.
- Mic stands are not to be discounted. Stable mic mounting is critical to good measurement; I use a pro photo tripod.
- REW just keeps getting better and better...

Disagree
- Xenyx 502 is phantom powered, from USB port. You need the 802 which provides 48v phantom power and will work with either mic.
- You can't "fix" room modes with EQ. Collateral damage is unavoidable.

I'm not going to argue the virtues of devices like minidsp in some applications, just not for room mode issues. Spend your money as you see fit, but temper your expectations severely. You will be ablke to achieve flat response at one location in the room using EQ, but you won't achieve global benefits "...without going through all the work...." Unfortunately, folks tend to respond with whatever they're comfortable doing themselves, regardless the effectivity. Pose the question differently (without mentioning an 18" LLT) and you may get useful responses.

Regardless, do some measurements, get some data before you implement a solution, so you have a basis for assessing how well the solution fixed your problem.

Have fun,
Frank
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