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Old 16th December 2005, 12:50 PM   #1
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Default 3way as subwoofer?

I'm going to build some OB line arrays that will be used in a dedicated HT. The line arrays use 16 NSB per tower but the problem is that they will not go low enough to cross nicely to a subwoofer.

I therefore need to make the line arrays into a 3 way system. I was thinking of using 4 12" woofers in the base of each tower. Woofer spec says they play 20-5000hz. I will install them in a V configuration with 2 woofers high one each side of the V. They will also play dipole as the rest of the line array.

Since this is a 7.1 system I will therefore have 28 12" woofers in a 15' wide and 23' long room to play bass from about 140hz downwards. Could this setup provide very low bass say in the 15-20 region at decent SPL?

If my room (15' wide 23' long) will not work for really low dipole bass then I might incorporate a sealed sub for the really low end stuff, but I hope I dont have to.

I read that optimal subwoofer placement is usually not the same place where the surrounds go. But I assume that since its dipole bass, room modes issues are mitigated.

Could this work?
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Old 17th December 2005, 03:12 PM   #2
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go dipole down to 40hz and sealed below is my suggestion

28 12" woofers sealed will give you some swept volume
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Old 17th December 2005, 03:44 PM   #3
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Don't give "20-5000Hz" printed specs too much credibility.
1) Numeric specs of this sort are notoriously inaccurate.
2) Any 12" driver claimed to go to 5kHz is immediately suspect. That's not a rational spec for a driver of that diameter. This casts further doubt on the 20Hz rating.
3) Yes, having a large number of them will help with volume, and lower distortion, but will not really help much in terms of getting down to an honest 20Hz.
4) If 20Hz is your goal, expect to need EQ to get there. How much will depend on the driver, cabinet, and a host of other factors.
5) A transmission line enclosure will get you somewhat below Fs, but they're huge compared to other cabinet designs (infinite baffle, reflex, etc.).
6) True dipole bass to 20Hz implies either a really, really wide baffle or lots of EQ. Or both.
7) I'd recommend trying to find an actual response curve for these drivers. If you can't find one, don't buy them--they don't want you to know how far off the specs are. If you can find one, I'll bet you a wooden nickel that you'll find the true -3dB point to be no lower than 45-50Hz. Don't be surprised is it's even higher...say, 70-80Hz.
Good luck.

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Old 17th December 2005, 06:34 PM   #4
Rudolf is offline Rudolf  Germany
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Exipnos,

before trying to get very low bass from a dipole, please read this:
http://www.musicanddesign.com/roomgain.html
Itīs not by accident that almost all experienced dipole builders recommend sealed bass below 40 Hz.

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Old 18th December 2005, 09:24 AM   #5
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Thanks for the comments.

Okay I just read the room gain article and came away more confused! Its tough to be a newbie

In the second part of the article it had some figures that described frequency response for various rooms. It said that monopole and dipole had similar results down to the rooms nominal fundamental. In the case of the large room this frequency was 28hz. They also described that after this point the dipole started to decay rapidly while the monopole works well and was able to pressurize the room.

Is it true then that you could use dipole down to the rooms fundamental frequency and then use monopole after that? But why have the monopole go up to 40hz if dipole effect is fine until say 28hz? Is the 40hz a general rule to fit many room types? Or is the above just theory and that practise has shown that 40hz is optimal?

My room is 7m long 4.5m wide and 3m height. How do I calculate that rooms nominal fundamental?
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Old 18th December 2005, 01:09 PM   #6
Rudolf is offline Rudolf  Germany
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Exipnos,
its your fault daring to build a OB line array as a newby.
So donīt expect mercy from me.

For a dipole the relevant room resonance is that along the listening axis. If you are listening along the long room axis, the distance wall-to-wall is 7 m. This corresponds to a resonance wavelenght of 14 m, which equals 24.5 Hz. If you are listeninig along the short room axis (4.5 m), the lowest room mode in this axis will be 38 Hz (=9 m).

If you look at the diagrams in the Kreskovsky article you will notice that room gain sets in way above the lowest room mode. So 40 Hz is just a ballpark number that fits average rooms.
You could even make the transition from dipole to monopole such that the lowest room mode is best subdued.
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Old 18th December 2005, 04:41 PM   #7
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Show no mercy. Just help me design the best system

I keep on getting confused. Long or short room axis! I think I'm listening on the long room axis. Screen is positioned on the short side and I have 2 rows of seeting. So thats 24.5hz. Thats pretty close to my desired 20hz.

Room gain. Okay I see that the blue line is higher then the yellow dipole line from about 60hz. Do I need to worry about the room gain issue if I can get enough SPL by increase of amplification and additional woofers?

I've saw a thread on a Swedish forum about a dipole subwoofer system using the cheap drivers. It seems to be quite potent.

http://www.hififorum.nu/forum/topic....97&whichpage=1
On this page is his measurements.

http://www.hififorum.nu/forum/topic....97&whichpage=4

But these meauresments seem to have "stuff" happening around 100-150hz. Maybe it won't be a good solution to mate with the NSB line?

Exipnos
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Old 18th December 2005, 05:18 PM   #8
Rudolf is offline Rudolf  Germany
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Quote:
Originally posted by exipnos
Do I need to worry about the room gain issue if I can get enough SPL by increase of amplification and additional woofers?
You need not worry, but you will need WALLS of woofers to overcome the exponentially increasing demand of dipoles for more displacement the lower you go.
http://www.linkwitzlab.com/faq.htm#Q14 will give you an idea.

Quote:
I think I'm listening on the long room axis. Screen is positioned on the short side and I have 2 rows of seeting. So thats 24.5hz. Thats pretty close to my desired 20hz.
Yep.

Quote:
But these meauresments seem to have "stuff" happening around 100-150hz. Maybe it won't be a good solution to mate with the NSB line?
That "stuff" is the quarter wavelenght resonance of the w-baffle they are using. You will have to care for that unless you cut off rather low with a high order lowpass. It seems you are voting for the latter.
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Old 18th December 2005, 06:07 PM   #9
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I downloaded linkwitz max_spl spreadsheet and I used some of my values. This is the SPL up to 141hz. Third column is for my 4 woofer dipole.

25 97 83
35 103 92
50 109 102
71 115 111
100 121 120
141 127 129

If I use 7 identical set of woofers then I get the following data

25 114 100
35 120 109
50 126 118
71 132 127
100 138 136
141 144 146

Is it valid to just calculate the area for all the woofers and not consider that they are spread out in 7 locations in the room. Probably not since there will be cancellations. But can I get something close to these numbers? Are they good enough or should I still just go with monopole subwoofer.

If I can get decent bass down to my rooms resonance then I might be happy with that and not spend more funds on monopole woofer. Yes it will not get me the really low end stuff but I can probably live with the compromise.

You think it will be a problem finding woofers (cheap) that will play well up to the NSB's 150hz level and also give me low bass?
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