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Old 3rd October 2007, 08:41 PM   #11
GM is offline GM  United States
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Originally posted by JinMTVT

1- What about stereo use of the front sub array?
would it have to be a mono sub system to perform as it is supposed to ?? i was planning on stereo subs for my HT, but if it yields the correct benefits i'd go mono for that anyway ...

2- Do the side walls have to be paralell ?
i was planning on some deviation of paralellism to remove as much modes as possible for the mids
would that affect the performance of the planar wave?

3- overall, what do you think about the use of this system ? with and without cancelling rear louspeakers
( i'd personally go with an absobtive rear for my HT ...
probably something like 1-2' of loose wool )


1) As a general rule, music was summed mono below ~150-160 Hz until the cinema standard of XOing to a LFE sub system at 120 Hz became the new one AFAIK, so 'stereo' (two) subs only make sense WRT smoothing in-room response.

2) Yes, if you want it to sum correctly, though depending on the sound wall's width and XO point/slope it may be moot. To ensure adequate summing in an expanding (horn) shaped room though, using the listening position's room width to calc the number of subs required seems the thing to do.

3) 'What was once old is yet new again' , RCA installed vertical arrays on either side of the screen in cinemas circa 1926 to get around the excessive beaming of the large ultra wide BW horns of the day. Better, cheaper mono multi-way horn systems relegated these expensive LA systems to the history archives until C-V resurrected them in a horizontal sub duty spaced array format for the 'Earthquake', etc., movies and today a LFE horizontal spaced array is used in the few THX certified cinemas I've seen pics or descriptions of, so there's certainly some merit in using them in a HIFI and/or HT app. Indeed, folks have periodically championed them for the ~10 yrs I've been following audio on the net, including me, though without the far end active canceling sub system. Here's one from '97 (scroll down):

WRT the active canceling system, I prefer acoustic solutions to acoustic problems, so my one serious DIY attempt was to 'clone' a LEDE tapered room within a room studio design as shown in Don and Carolyn Davis's 'SOUND SYSTEM ENGINEERING'. It transformed a neighbor's home concrete block walled/poured concrete pad underground basement into a relatively spacious sounding small concert hall/cinema. That said, it was very labor intensive and I imagine the materials cost today would be at least as much as the inexpensive electronics/drivers required to do active, so overall it may be the better choice. Still, I can't help but think the passive way would sound closer to 'live' in the same way a passive Vs active speaker system usually does.

Semi-related reading:

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Old 4th October 2007, 04:38 PM   #12
JinMTVT is offline JinMTVT  Canada
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just a side question ..

how would multiple 18" or 15" perform as high as 200hz ??

thinking of it, i have heard a few times how single or dual 15 or 18" performs in the mid frequencies..
but never have i listen to a great number of those drivers performing the same task

i would tend to think that alot of 15" would play just very well up to near 200hz if the drivers would be choosen wisely for that ... could have impressive dynamics because of massive displacement ..
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Old 4th October 2007, 05:15 PM   #13
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The rule-of-thumb (ROT) I've seen most often is that the center-to-center (c-t-c) spacing should be < 1 WL of the XO's -24 dB point, so for two or four 18" (square pattern): ~13560"/18" = ~753.33 Hz = ~47.06 Hz/1st order, ~94.13 Hz/2nd, ~188.25 Hz/3rd, ~376.5 Hz/4th. Ditto the same ROT between the sub and mains. Note that some folks say as little as -12 dB is required, or ~188.25 Hz/1st, ~376.5 Hz/2nd, so as always YMMV.
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