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-   -   Planar bass wave to take care of room modes ?? (http://www.diyaudio.com/forums/subwoofers/109298-planar-bass-wave-take-care-room-modes.html)

JinMTVT 1st October 2007 04:00 AM

Planar bass wave to take care of room modes ??
 
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 :
http://img227.imageshack.us/img227/4...rrayee6.th.jpg
( i have not permission to use it..lets hope it was intended for public use :p )


DBA paper :

http://www.sennheiser.com/klein-humm...s/tmt2002.PDF/$File/tmt2002.PDF

JinMTVT 1st October 2007 04:04 AM

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 )

MaVo 1st October 2007 06:26 PM

the link doesnt work for me. but i am interested. my intiution tells me this cant work and my knowledge in acoustics isnt sufficient to know better.

can you post a working link, so i can read about it?

JinMTVT 2nd October 2007 02:37 AM

copy paste the link in ur browser ..


http://www.sennheiser.com/klein-humm...es/tmt2002.PDF

sorry for the non working direct link

GM 2nd October 2007 03:40 AM

Can't do it, it's been abbreviated. Let's see if mine works: http://www.sennheiser.com/klein-humm...le/tmt2002.PDF

MaVo 2nd October 2007 11:18 AM

thanks. GM's version worked. and its in german, great :)

MaVo 2nd October 2007 12:33 PM

Quote:

Originally posted by JinMTVT
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=JX4...J&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.

JinMTVT 3rd October 2007 02:52 AM

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?

MaVo 3rd October 2007 08:31 AM

Quote:

Originally posted by JinMTVT


i do not understand much about room shape
and how it alters the different frequencies

who are the experts here on room accoustics?


some links about room acoustics:

http://www.ethanwiner.com/acoustics.html
http://www.linkwitzlab.com/rooms.htm

MaVo 3rd October 2007 05:52 PM

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?


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