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Old 25th August 2008, 03:16 PM   #1
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Default Eliminating the backwave on a dipole

Does anybody have any experience or any ideas as to how you would go about eliminating as much of the backwave from a dipole as possible. I know this will minimize some of the benefits of a dipole, but I want to be able to put the units as close to a rear wall as possible. I am hoping to cover the midbass from where my tapped horns are not flat anymore (they are flat up to 60hz) up to 150-200hz or so.
With the drivers I have on hand, the only option of reasonable size is dipole since they have a weak motor and a qts of .7ish.
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Old 25th August 2008, 07:15 PM   #2
MaVo is offline MaVo  Germany
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Lots of absorbtion material like rock wool or open cell foam, but still, at 60hz thats rly alot of absorbers you need to damp this frequency.
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Old 25th August 2008, 07:59 PM   #3
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try doing nothing and putting them within a foot of the wall. perhaps the long bass wavelength will sum close enough to in-phase.
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Old 25th August 2008, 11:59 PM   #4
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Try scattering the backwave instead.
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Old 26th August 2008, 08:30 PM   #5
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Here is what I'm planning to do. The tapped horns will cover from 20-60hz and the OB arrays will cover from 60 up to around 150 or so. The OB array will act as a speaker stand for my mains.
http://i516.photobucket.com/albums/u...BassDipole.jpg
Does anybody with experience with OB designs see anything wrong with this design?
Here are the t/s paramaters:
PARAMETERS

Re 7.20 [ ]
Fs 44.26 [Hz]
F1 35.48 [Hz]
F2 56.23 [Hz]
Zm 53.74 []
D 208.00 [mm]
Qms 5.80
Qes 0.90
Qts 0.78
Bl 6.76 [N/A]
L1K 1.47 [mH]
L10K 0.72 [mH]
Ms 20.47 [g]
Vas 101.85 [l]
dBSpl 92.34 [dB]
Cms 0.63 [mm/N]
Ma 48.00 [g]
FsMa 24.20 [Hz]

I was thinking about using some Wood Hardner on the cones of these to make them stiffer than they already are. This could also add a little bit more mass thus increasing the qts and lowering the fs along with the sensitivity some.
here is a link to a thread about the wood hardener:
A solution to MDF expansion on joints, translams etc.
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Old 26th August 2008, 08:40 PM   #6
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Don't bother trying to use absorption. The wavelengths are far too long and you would only be wasting your money.
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Old 26th August 2008, 08:51 PM   #7
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Dipole is not the only option. Look up Linkwitz Transform.
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Old 26th August 2008, 09:01 PM   #8
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Yeah, I have looked into the linkwitz transform before, but I really lose a ton of sensitivity. My subs will do 124db each at 20hz. There will be 8 of those 10's per side and they start out at 92 db each. I have tried 4 in a large OB before and it sounded very good. Would the lin. transform lower my total output potential at 40-60hz?

edit: I just messed around a bit in WinISD and came up with a 5.5 cubic foot box will get me around 114.5 db at 60hz w 200 watts with a linkwitz and will be good up to 115 hz or so. With a huge box im getting around 117db at 60hz w 50 watts and it is good up to at least 200hz. How soon does the Dipole start rolling off assuming that the 'wings' will go back a foot and a half or so?
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Old 26th August 2008, 09:29 PM   #9
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As SPL is simply a function of excursion and cone area, it's possible with the right box that the LT box may go louder than the open back sub as it can control the cone better and there is no rear wave cancelling stuff out. The LT should make no difference to "sensitivity" or SPL in other parts of the bandwidth of that box, however the amount of power required compared to open back will be greatly increased. I presume that's what you mean about sensitivity.
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Old 26th August 2008, 09:32 PM   #10
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The speaker can only handle 30 watts if I am correct, and there is only 3mm of xmax. The 30 watts is the real limiting factor for the lt.
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