Separation of mid/tweeter from woofer in rear speakers - diyAudio
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Old 24th September 2002, 12:21 AM   #1
dc is offline dc
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Join Date: Dec 2001
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Default Separation of mid/tweeter from woofer in rear speakers

I'm currently planning/building rear speakers for an HT setup. The woofers will be two Peerless XLS on each side, in an open baffle H-frame (as described at http://www.linkwitzlab.com/proto.htm). For the mids and highs, I plan to build a small satellite box to house one Manger on each side. The Manger satellite boxes are from an article from a German hifi magazine (don't know which one, I only have a photocopy).

I'm planning to use a 12db/octave active XO at ~150HZ. I've read that the Manger prefers a slightly higher XO point, but, I've also read that the woofers, as mounted in this H-frame, tend to roll-off around 150Hz, so I may need to do a little futzing to find the best compromise. Right now, I'm planning to leave the woofers (the box is 25" tall) on the floor, and elevate the Manger satellites to a position about 2-2.5' above the listener's ear when seated at the listening position (~45 or 50" above the floor). So, there would be at least 20" between the top of the woofers and the bottom of the Mangers.... Is this too much space? I'm concerned that the separation between the woofer and the Mangers will be too noticeable at 150Hz since the speakers will be 5' or less from the listening position....

Does anyone have any suggestions on my proposed speaker placement? Would I be better off elevating the woofers as well as the Mangers in order to keep them closer together?

Thanks in advance.

Brad
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Old 24th September 2002, 12:54 AM   #2
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Default Some quick thoughts

Have a look at the program (spreadsheet actually) used to model the line arrays and use 2 point sources(http://www.pvconsultants.com/audio/radiation/vpr.htm) to help with visualizing (a nice tool to help understand a lot of things about interactions between two/more drivers IMHO). 150 Hz has a wavelength of about 2m, so even when you are only 5' away from the two sources, this would not be significant on its own. Then one must not forget, that the crossover doesn't have an infinite slope, so one must take more into consideration than just the response (in magnitude and phase) at the crosover frequency itself but also above this (the extent of which is related to the order of the XO's order)

This problem is however a lot more complex and as mr Riley points out, the room interaction of the di-pole woofer arangement is different from that of a traditional direct radiator. With such an investemnt of money in drivers, a measuring setup would surely help a lot (I am straying clear from the subjectivist/objectivist thread) and Speaker Workshop and my cheap electret mic and pre-amp are turning out to be invaluable tools. Room interaction would overshadow the effects of varying the height of the woofers and have a much larger effect than some response ripple caused by combing. One could then make measurements at the listening position and model filter responses in magnitude and phase to optimize such a setup.

For more from the FRD, have a look at http://www.pvconsultants.com/audio/frdgroup.htm which is turning out to be an invaluable resource for me.

only my 2c

Martin
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Old 24th September 2002, 09:02 PM   #3
dc is offline dc
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Thanks for the information. I'd heard about Speaker Workshop before, but had never downloaded it. I did last night and was very impressed by its capabilities. The pvconsultants links also look excellent.
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Old 24th September 2002, 11:26 PM   #4
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In-room response curves will look rather ugly though (really ugly) and to get any useable results one would look at smoothing the info for optimisation purposes initially and then use the un-(or lightly) smoothed data later. ESP has a mic pre-amp (project 93 on http://sound.westhost.com) and mic capsules like the mentioned Panasonic modules can be sourced through Digikey (I just use a cheap capsule for the time being, shipping, handling and import duties would push the price for one capsule to as much as i spend on some speakers (frugalphilia and all). I run the thing through my trusty old Wallins Jig (the first one) which is the second best investment i have made so far.(http://www.gti.net/wallin/audio/jig2/jig2.html).

I don know your religious persusion regarding EQ, but IME the improvement is definitely worth the effort, especially regarding the bass (I just use a program on my PC, but with speakers and electronics on the opposite end of the price spectrum from what you are suggesting). 1/3rd octave EQ isn't a cure-all though and can only hide some imperfections.

I myself am only starting off in this for me exciting field of acoustics design and it seems to hold a lot of promise...

only my 2c

Martin
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Old 25th September 2002, 12:44 AM   #5
dc is offline dc
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You said the Wallins jig was the second best investment you've made.... What was the best investment you've made?

I plan to integrate EQ on the woofers, based on information from SL at www.linkwitzlab.com, so, I have no problem with using EQ, notch filters, etc.

You mentioned that you use a program on your computer to do EQ? Have you seen Don Maurer's digital filters? http://home.pacbell.net/donwm

brad
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