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In Reply to: Re: 3rd subwoofer placement. posted by Duke on February 4, 2007 at 17:28:57:
Do you try to place subwoofers less than 1/4 wavelength apart? Or are you using the Geddes random placement scheme? The thing that always bothered me about Geddes proposed subwoofer placement (besides the localization problem you alluded to) is since it's random, you can't know response at any place in the room. It might be good in the listening area or it might not.Geddes idea is that dense interference smooths the sound field, averaging the peaks and dips from room modes as well as interference modes between sound sources. That much I agree with. The Geddes subwoofer placement proposal a good idea in concept, but isn't guaranteed of success. I have found several random placements that fit the Geddes proposal and had huge bass dips in the center of the room, right where you would probably be listening. Some Geddes placements work, others don't. It's a crap shoot with Geddes scheme because it's, well, random.
I think it's better to evaluate the room and use specific placement, don't you? In that case, you can't have a one size fits all approach. There are a few general rules that can be used as starting points, but to know for sure what placements work in a specific room takes measurements or at least nodal analysis of the room and treatments. Without proper analysis, the Geddes positions are probably OK, but so are Welti positions. I can't say for sure, and neither can Geddes.
Follow Ups:
Yes, my suggestion is based on Geddes.I see nothing wrong with evaluating the room and deliberately selecting subwoofer locations which are calculated to maximally stagger the path-length-related peaks and dips.
I don't see how multiple low frequency sources scattered around the room could possible produce greater amplitude path-length-related peaks and dips than a single low frequency source would. Maximum amplitude peaks and dips are produced when the reflected energy arrives coherent in time and phase, which is best accomplished by having it all originate from the same point. I can see how multiple low frequency sources with similar path lengths to the listening position could produce broader, shallower peaks and dips than a single source would - in which case I think that adjusting the subwoofer positions to increase the variance in their path lengths to the listening position would be beneficial.
In addition to the differing subwoofer setup principles proposed by Geddes and by Welti, David Griesinger has proposed a psychoacoustics-based subwoofer setup which assumes significant low frequency stereo content. In my room, I use a setup that attempts to meet the criteria of both Geddes and Griesinger - but it requires more than three subs.
Is Griesinger's setup one that is in a paper I just found about "externalization" and "envelopment"?The one here:
Hello Ben,David Griesinger has written many papers, and is an advocate of placing stereo subwoofers to the extreme right and left of the listening position. I've read several of his papers, including the one you linkd to.
Thanks,
Thanks Duke. I found his site right after I posted. Got some reading to do.That does sound like what he was suggesting. Have you messed with the 90 degree phase shift between the left and right that he suggests?
Hi Ben,I haven't tried the 90 degree phase differential that Griesinger suggests. So far I haven't found a plate amp that includes continuously variable phase along with the other features I want - namely fourth order lowpass filter that can be set to 40 Hz or lower, at least 200 watts output, line and speaker level inputs.
If you have any suggestions, I'm all ears!
Here you go:
Hello Ben,I thought I posted a reply to you yesterday but now I don't see it.
I wanted to thank you for going to so much trouble to help me! That was way, way above and beyond what I ever would have expected.
The Parts Express amp - I have a pair of those. And, I spoke with Rhythmik a couple of days ago also. Turns out their "4th order" crossover version isn't very adjustable. The crossover consists of a fixed 80 Hz second order filter cascaded with a frequency-variable second order filter. I want a fully adjustable 4th order filter.
Below is a link to another plate amp I found that looks promising. I'm communicating with the builder about a couple of custom features.
I'd like to get to know you a bit better now, but there's no way for me to e-mail you. So I invite you to shoot me an e-mail.
Thanks again.
Duke
Would this one do? Every amp I can find is missing at least one feature on your list. Probably what you've found too...
Looks like you have thought about this some. I bet your setup sounds great.
Thanks, V. I like to think my setup doesn't suck too bad... but then, don't we all??
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