Planar bass wave to take care of room modes ??

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I have read recently on the double bass array subject,
using planar wave bass launch to take care of 2/3 room modes and a delayed reversed bass unit to take care of any longitudinal mode

What do you think about the idea of using a planar wave bass lauch with a big array of drivers?

i was already planning something like 16 18" drivers
in an infinite baffle configuration, using the garage
as the rear enclosure

What if i'd dispose them all on the wall to create a completly flat 2d wave as suggested in the DBA idea??

i was suggested to this method from FoLLgoTT
over at AVSFORUM


this is the formula for placement of the drivers

pX = (2 * n + 1) * widthWall / (2 * numDriversX)
pY = (2 * n + 1) * heightWall / (2 * numDriversY)
n = 0, 1, 2, 3,...

and an image from him :
An externally hosted image should be here but it was not working when we last tested it.

( i have not permission to use it..lets hope it was intended for public use :p )


DBA paper :

http://www.sennheiser.com/klein-hummel/globals.nsf/resources/tmt2002.PDF/$File/tmt2002.PDF
 
So to continue on that topic ...


I have questions about the use of this method


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 )
 
JinMTVT said:
So to continue on that topic ...


I have questions about the use of this method


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 )


Since i just read the pdf, ill try to awnser your questions. Well, its more guessing than answering ;)

1) I think the Bass Array concept allows no stereo, since you want to have a flat wavefront across the room. If you place the subs as intended, you cannot have stereo, since you cover the whole frontwall with the subs for one signal. And if you place them in stereo configuration, you wont get a planar wavefront.

2) There is nothing said about parallel walls in the paper, but i read elsewhere (cant remember where) that room modes dont care about the exact shape of your walls, they still occur. Applying this on our context, i would guess the bass array solution would still be functional with rather odd roomshapes.

- wouldnt it be easier/better to build constant and high directivity speakers like cd waveguides instead of complicated wall structures? there are some interesting discussions here about this.

- btw... i think the mids dont have modes like you said (or rather, the modes are so close one cant distinguish them). in the mid frequencies you get problems with reflections which lead to comb filtering and distorted stereo images. well, what i wanted to say is, i dont think wall shape has that great influence on mid performance, since it can scatter echoes but it cannot avoid them. thats why i would go for controlled directivity speakers.

3) i think you need the rear speakers, since the response is (at least on one axis) still ugly without them.

edit: i would like to know if this works better than passive room treatment for bass problems. Patent 5,975,238 (http://www.google.com/patents?id=JX4XAAAAEBAJ&dq=5,975,238) displays an interesting bass absorber which could be easily build. would be definitely cheaper than at least 8 bass speakers.

i am quite interested, since i soon move out of an attic which has godlike bass response because of really leaky walls into a room with concrete walls which should suffer from serious modal problems.
 
that is what i thought about the stereo also ..
wouldn't work quite good with all the drivers not radiating the same content

i will have more reading to do ..
i do not understand much about room shape
and how it alters the different frequencies

since i will be doing all nearfield loudpseakers,
i am not too sure that a non paralell walls room will have much benefits ...

who are the experts here on room accoustics?
 
Concerning the usefullness of this bass array approach...

It has to be mono to work. The frequency where room modes are not problematic anymore (perhaps around 200hz or higher - i dont know exactly where the modal range starts to fade away) is definitely higher than the frequency i would put the sub/mains crossover (below 100hz). This leaves quite a large area without proper treatment. Or one has to sacrifice parts of the stereo experience with sub/mains crossover above the modal range. So unless one wants mono midbass, that solution is no solution.

Broadband bass absorbers like the ones in the patent i posted earlier cover the whole problematic frequency range.

Did i forget something which puts this into another perspective?
 
JinMTVT said:

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 )


Greets!

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): http://www.sonicdesign.se/subplace.html

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:

http://www.harman.com/wp/pdf/multsubs.pdf
http://www.harman.com/wp/pdf/LoudspeakersandRoomsPt3.pdf

GM
 
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 ..
 
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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