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Old 18th July 2004, 03:25 PM   #1
pa is offline pa
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Default Tuning en Stuffing my big TL subwoofer ?

Hi !
I've just finish a straight 15" subwoofer with a 15b100r Beyma driver mounted in a 15.748" diameter sonotube wich is 157.48" long. I think it could be largely more dynamic than it sounds i probably have to shorten and stuff the tube but i don't know how. I think i can use Holofill 2 fiber or whool. but if you have comments on the driver or how long the tube should be it would be cool.
Thanks in advance for your help.
PA

BEYMA 15 b100r THIELE-SMALL PARAMETERS
Fs 25 Hz
Re 5.2 Ohms
Qms 3.32
Qes 0.35
Qts 02
Vas 400 liters
Cms 0.368 mm/N
o 1.75 %
Sd 880 cm
Xmax 3 mm
Vd 280 cm
Le 0.9 mH
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Old 18th July 2004, 04:34 PM   #2
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I'd leave the length as is; with a Qts of only .2 you need it. This driver is not well suited to TL use, but with that low of a Qts you'd be hard pressed to get strong bass from it in any alignment.

The stuffing issue is a matter of measuring the system impedance. With a bare pipe you should see a major spike at the Fb, which in your case will be around 40-50 Hz, and series of minor resonance spikes above that. Gradually add stuffing until those upper spikes disappear. What you'll end up with when it's right is an impedance chart that looks just like a sealed box, with the one spike at the Fb and a smooth line above that until it rises in the high frequencies where the effect of the coil inductance is seen. Response will clean up a lot when you're done, as the midbass resonances will be eliminated and the roll off below Fb will change from the 4th order of a vented box to the 2nd order of a sealed box. Bailey recommended using long fiber wool, but practically speaking plain old polyester pillow stuffing works just fine.
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Old 19th July 2004, 02:26 PM   #3
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I have tried to model the speaker in MJK's worksheets. I tried several things but I don't have an easy solution. I think right now you are getting many peaks and valleys the whole way up the frequency spectrum. You can stuff it to even out the response, but in the end, it looks like the response will be similar to the driver mounted in an infinite baffle. Try between 0.3-0.4 pounds per square foot in the bottom 1/3rd of the tube.
You might want to try different setups with the Mathcad sheets to find a better solution.

Good luck
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Old 19th July 2004, 06:05 PM   #4
qi is offline qi
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According to the AlphaTL spreadsheet that GM kindly posted on this forum, you need to stuff with ~ 1/2 lbs per cubic ft of PolyFill (pillow stuffing) and you'll get a tuning of ~21HZ at that length.

But here is the rub. Based on your driver's specs you REALLY should be using a tube with an inner diameter of 21.75" -- not 15"

BTW, are you sure Qts is .02 and not .2?

Regards
Joe
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