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Old 13th September 2005, 07:33 AM   #1
Pallas is offline Pallas  Pakistan
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Default Placement vs. Power

I'm in a bit of a quandary about my subwoofer options. I plan on using two 15" low-inductance drive-units with a crossover of 80Hz. (I'm leaving brand/model out intentionally, so as to focus the discussion on the placement considerations rather than on the merits of the drive-units.)

My first thought is to follow the recommendations of the Harman white paper, and put a at the midpoint of the front and back walls. My current living room has a fireplace in the middle of the front wall with a blocked off chimney, and I was eyeing that space for a subwoofer. However, I'll only be able to carve out enough space for a Qtc=~0.65 sealed box, with an anechoic F3 of about 37Hz. I'd use the same enclosure style for the back sub as well. In general I've never heard a good sounding ported bass subsystem that was smaller than a Bessel sealed aliignment (Qtc=0.577) so I'd run the subs sealed. In this case, the front sub would be ~12' from the primary listening position, and the back sub about 4'.

However, if I use the back corner exclusively I can build up to a well-braced EBS ported box for both drivers and have a natural (no EQ, though I'll be running EQ anyway eventually) F3 of around 16Hz, with an appropriate infrasonic filter. The sub would be ~7' from the primary listening position.

Sixteen Hz sounds really nice, but is it worth the extra lumpiness and risk of localization compared to the Harman placement? I'm not sure, which is why I'm throwing the question out there.
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Old 13th September 2005, 02:13 PM   #2
simon5 is offline simon5  Canada
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It depends on what do you want. In the corner you'll also get a louder subwoofer so it will need less power.

For the best soundquality possible, probably the corner or the middle of the room are both not the best place. If you have a small subwoofer, place it in the center of the room and crawl around and you'll find where your new subwoofer should be...
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Old 13th September 2005, 03:38 PM   #3
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The Harman paper says that their conclusions are only valid for rectangular rooms. If your room is rectangular with few openings, their suggestions should work. Otherwise, place a sub at the listening position and check for the smoothest response in acceptable postions.

As for the lumpy EQ'd respose portion of your question, IMHO it IS worth it. I use a pair of JBL 2245H in an EQd ported enclosure that gives me an F3 of around 20 Hz. A variable Q high pass filter is a good idea, as it will allow you to compensate for room gain. The boost required for flat anechoic response gives me way to much bottom end for music. It is fun for HT though

Another option is stay sealed and use a Linkwitz transform to get the desired F3. This will give much lower group delay than an EBS or equalized ported alignment and should sound like a sealed box of the target Q. The catch is amplifier power available and excursion limits. As expected, larger boxes require less boost. This is on my "to try" list, but I haven't built the filters yet. I haven't figured out how to easily vary the boost for room gain compensation though.
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Old 14th September 2005, 03:46 AM   #4
Pallas is offline Pallas  Pakistan
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My room is perfectly rectangular...well, except for the 5'2" (give or take) open entranceway into the next room.

Luckily, the walls of this built-in-the-1940s place are made of real materials (masonry and concrete) rather than the cardboard-and-spit construction that characterize most American construction of the last quarter-century at least, so at least the sub will have real boundaries against which to work.

Right now, I'm leaning slightly towards the middle-of-the-front-and-back-wall placement, with an upgrade path to the suggested optimum four subs halfway along the walls open to me. I'm not crazy about LT's, and truth be told I'm not sure a high-30Hz F3 is that much of a problem given room gain and a low-Q rolloff below Fc that results in a modeled anechoic F10 of a right around 20Hz. Of course, as stated it won't be as efficient as a corner sub.

The problem with the advice that both of you have given me is that it assumes a stationary listening position. I want the bass to be as even as possible over an AREA rather than a spot, so that everyone who may be in the room to listen to music or watch a movie gets decent bass. That - and the thought of a giant sub right next to a bookcase with some pictures on it, which could make localization an issue even with an 80Hz, 4th order acoustic crossover - is what's making me hesitate on the intuitively obvious solution of a low-Q vented sub (i.e. EBS) in the pretty-much nearfield corner.
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Old 14th September 2005, 06:20 AM   #5
ruerose is offline ruerose  Canada
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If you want a wider area of more-or-less even bass, make sure you have sufficent absorbtion in the corners to soak up some of the low end. This will even out the peaks and valleys somewhat at the expense of room gain.


The best reference I've found for understanding room acoustics is this book, I managed to find a 3rd edition at a used book store for a paltry $20, best money I ever spent:

The Master Handbook of Acoustics 4th Edition

Here is a page that might help also:

http://www.ethanwiner.com/acoustics.html
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Old 14th September 2005, 06:35 AM   #6
ruerose is offline ruerose  Canada
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To answer your question though, if the sub was to play anything above 40, I would try and find another alternative if you don't think the fireplace would play deep and loud enough. At that alignment my personal preference is to not go for it.

But, if there is no program material that the subs will play above 40hz or so, then I personally would use the back corner. At the SPL I think you are aiming at, there will not be localization problems. Although I am a little concerned about time-alignment/phase being a more of an issue if the sub is behind you and in a corner.


Is this a HT or 2.1 setup?

EDIT: Time alignment and phase are only concerns at a static listening position, if coverage is more important that perfection, don't worry about it.
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Old 14th September 2005, 11:47 AM   #7
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With your room it sounds like Harman's conclusions should apply.

Is crossing to the sub at a lower frequency an option? A lower crossover would help prevent localization, as do steep crossover slopes. When I only had one sub, with a variable XO I found that with a second order low pass electrical XO, set anything over 60 Hz or so the sub could be located. Bumping to fourth order electrical at 90 Hz I cannot locate the sub (except when the amp clips)
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Old 15th September 2005, 06:18 AM   #8
Pallas is offline Pallas  Pakistan
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Quote:
Originally posted by BobEllis
With your room it sounds like Harman's conclusions should apply.

Is crossing to the sub at a lower frequency an option? A lower crossover would help prevent localization, as do steep crossover slopes. When I only had one sub, with a variable XO I found that with a second order low pass electrical XO, set anything over 60 Hz or so the sub could be located. Bumping to fourth order electrical at 90 Hz I cannot locate the sub (except when the amp clips)
I'm pretty sure the Harman solution will get first crack, though now I'm thinking "build both sets of boxes, listen, and then finish the pair that sounds best.)

Unfortunately, I can't change the sub's crossover right now. The receiver I'm using (Panny XR55) only goes down to 80Hz, 3rd order electrical. However, as I want to keep bass energy out of my mains - they use 8" Tannoy 2046 dual concentrics, and sound their best when not asked to move much - a lower crossover isn't in the cards anyway. (Later on, when I upgrade the mains to Tannoy 2046 + JBL 2235H, I will run them with a 2nd order filter at 60Hz (4th order acoustic) and put a 4th order electric filter on my sub at about 60Hz. Yes, that will be a dome tweet on a waveguide, 8" midrange, 15" midbass, and 2-4 woofers that each displace 1.8L of air or 3.6 if you measure Vd like Adire does. I like my Mahler and Sibelius to have scale!)


The system is technically an HT (7.1 discrete channels) but I enjoy music far more than I enjoy movies. However, I've found that I really enjoy listening to symphonies and jazz combos in DPL2 music mode, so that makes the extra speakers worth it. Electronica doesn't do so well with DPL2 for some reason, though.
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