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Old 23rd December 2005, 12:47 AM   #1
TroelsM is offline TroelsM  Denmark
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Default Very Big Woofer in Very Small Box

Hi!

I finally got some pictures of one of my sub-woofers. It's based on Rod Elliot's idea :http://sound.westhost.com/project48.htm but I never got around to finish it. The few tests I have done, with some simple electronics to straighten out the frequency response, are very promising.

Here's a link to a few pictures: http://kofoedmejer.squarespace.com/d...alleryId=22250

Have you guys any experience with this kind of sub? Any thoughts? I found it to be very "controlled" but not that "fun"... It can sure make some very clean tones, but when it comes to playing music I'm not so convinced...

Regards TroelsM
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Old 23rd December 2005, 07:09 AM   #2
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Though I don't know the specifications of your 18" woofer, most of the older ones with high sensitivity have T/S parameters that suit them for large ported boxes. I used to compare an 18" radio shack woofer with the Dayton 5 1/4 from Parts Express. If you built both of them into optimum ported boxes, they both had about the same f3. If you were to put that 18" woofer in a smallish sealed box, it's f3 would go up to around 100 Hz. At 6 dB per octave, it would take 4 times the power to produce 50Hz as it would 100Hz and above. 25 Hz would take 16 times the power to match the >100Hz frequencies.

The possible hidden gotcha here is the XMAX. Most older PA woofers didn't have any (1 to 2.5 mm or less than .1"). Trying to use the 18" cone area to make up for limited XMAX will probably get you to easy listening volume levels, but if you try to get to show off volumes or frequencies, distortion you hadn't counted on may show up.

If you use too small a box for that 18" woofer, it will drive the Qtc up high, and box damping will suffer which could make the bass muddy.

I suggest getting a box design program and run the Theil/Small parameters through it and see what you get. I use Loudsp, which is an old DOS character level program, but it still works with XP, and it gets the job done simply. You can probably find it or some other available for a free download.

If you need to get more volume in the box, filling with fiberglass or long fiber wool will make the enclosure perform like a larger enclosure. Also a variovent will make it look larger. They are like a port that has dense fiberglass in it to dampen any resonances, but let the box leak.

These things were shown to me by Vance Dickason's Loudspeaker Design Cookbook.

Good luck with your project.
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Old 23rd December 2005, 08:06 AM   #3
rinox is offline rinox  Italy
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There' s something wrong in that box like Mr. AnotherCarl said, I will suggest you to make some simulation with a specific prediction software like Speaker Workshop you can find at: http://www.audua.com...... it's free and it's good enough for your needed.
Hope this will help :-)
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Old 23rd December 2005, 01:35 PM   #4
TroelsM is offline TroelsM  Denmark
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Of cause there's a potential problem with the X-Max. But My woofer is not that old!.. I know about those old-school woofers with very limited X-Max but I don't think this one has that problem. I don't have the data-sheet but a guess would be X-max better than +/- 8mm.

The woofer has a die-cast frame, BIG magnet and 4" glass-fiber voice-coil.

It's probably right that I need 16 times the power at 25Hz, but that's not a big problem! it makes a lot of "noise" at 10W and the peak-capacity is better than 400W.

All this said, I won't argue that some sims´ and measurements would be the right way to go. What I'm asking is: "Is it worth it? What are your opinions on this approach"

Regards TroelsM
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