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Twelve 12" driver dipole subwoofer build thread
Twelve 12" driver dipole subwoofer build thread
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Old 16th August 2015, 06:26 AM   #11
Zero D is offline Zero D  United Kingdom
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You still haven't said what the drivers are ?

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Old 16th August 2015, 07:52 AM   #12
Zoran is offline Zoran  Serbia
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Charlie excellent work.
Experimental and with courage, true audio spirit.
waiting for the burning in first impressions listening
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Old 16th August 2015, 03:27 PM   #13
CharlieLaub is offline CharlieLaub  United States
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Quote:
Originally Posted by Zero D View Post
You still haven't said what the drivers are ?

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Here's a link to the drivers I am using:
GRS 12SW-4 12" Poly Cone Subwoofer 4 Ohm

Less than $20 each if you buy at least four (why not?). I tested each of them for TS parameters and they were all very close to the MFG specs. In total, the drivers cost less than $250, including shipping.
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Last edited by CharlieLaub; 16th August 2015 at 03:29 PM.
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Old 17th August 2015, 02:48 AM   #14
djn is offline djn  United States
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With a Qts of .98 they should work very well in your OB cab. Can you measure them when they are complete?

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Old 17th August 2015, 04:31 PM   #15
CharlieLaub is offline CharlieLaub  United States
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With a Qts of .98 they should work very well in your OB cab. Can you measure them when they are complete?

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Yes, unless the Q increases a lot due to loading by the cavities these should work very well for this project.

I always try to use measurements, however, in this case I won't be able to make direct free-field or even quasi-free-field measurements. Instead, I will make a near field measurement at the front opening and another at the back opening, and then combine them with the rear response having both inverted polarity and delay that corresponds to the pathlength difference. The result will be the free-field frequency response on axis.
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Old 17th August 2015, 06:04 PM   #16
sreten is offline sreten  United Kingdom
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Hi,

8.5mm of linear Xmax is rather impressive for a
$20 12". They seem excellent for open baffles.

rgds,sreten.
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Old 17th August 2015, 11:58 PM   #17
CharlieLaub is offline CharlieLaub  United States
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I managed to finish the assembly today. After connecting the wiring and doing some initial checks I took some measurements. These are attached (attachment at left). The measurements are of the front (orange) and rear (green) output, with the rear SPL level adjusted to account for the larger open area. I have also moved the front passband response to about 0dB as a guide for the eye since levels are only relative.

There are definitely some resonances present. The "slot" cavity resonance can clearly be seen in the front response, peaking at about 250 Hz. Since this doesn't really start until about 100Hz (the upper edge of the passband) I am not too concerned about it. In the rear response I believe there are two different resonances present. The peak at 250Hz is still there, but below part of it is missing and there is some peaking around 100Hz. I think the lower resonance is being produced by the large rear cavity and this has a null around 200Hz, which cancels part of the cavity resonance. Interesting stuff.

The near field front and rear measurement data can be combined to model the free-field response. I did this in the Active Crossover Designer, and the result is included as the second attachment. The cyan and yellow traces are the front and rear responses, as before, and the dark blue trace is the combined response with the rear response inverted and delayed following the acoustics properties and dimensions of the dipole design. Because of the various resonances the combined response has some peaks and dips, e.g. at 45Hz. I plan to fill this in using EQ and flatten the response overall as part of the crossover. I'm surprised that the low end does not roll off more (from the dipole cancellation) but this is just how the data plays out. In general the response has the characteristic 6dB fall in SPL per octave below about 150-200Hz or so as you would expect.
Attached Images
File Type: png SLUD front and rear responses.PNG (32.3 KB, 757 views)
File Type: png Uncompensated Free-Field Response.PNG (49.1 KB, 675 views)
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Old 18th August 2015, 04:39 AM   #18
CharlieLaub is offline CharlieLaub  United States
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After playing around a little while I think I have figured out a set of EQ filters that I could use to flatten the response and make it useful. It's definitely not the standard 6-dB-per-octave boost on account of the various resonances involved, but it does flatten the bass response nicely. The low end extends to about 25Hz after applying the EQ (according to my model).

See attached pics of models of EQ'd subwoofer response and the EQ filter used to generate it.
Attached Images
File Type: png EQd Free-Field Response.PNG (43.1 KB, 668 views)
File Type: png EQ response.PNG (27.4 KB, 642 views)
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Old 18th August 2015, 04:45 AM   #19
Brian Steele is offline Brian Steele  Grenada
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That's looking pretty good!

What sort of SPL capability to you expect from your beast?
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Old 18th August 2015, 04:45 AM   #20
zettairyouiki is offline zettairyouiki  United States
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What are your impressions? I'm kindof curious what the strong and weak points of this design would be.

Is it more effective than buying a single woofer of comparable price and dropping it in a large ported box? Say something like Dayton Audio RSS460HO-4 18" Reference HO Subwoofer 4 ohm
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