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Discussion Starter #1
I found Andrew Robinson's article interesting about using a second sub as a mid sub.

http://www.andrew-robinson-online.com/take-her-down-using-two-subwoofers-to-achieve-low-frequency-bliss/

So I got to messing around with a plot. I would like some input on the theory and my driver selection for a DIY mid sub. But first let me say Im using one side of a FBQ2496 and a Behringer EP2500 driving a CSS SDX15 in a 4.3 cu.ft box tuned to around 18Hz. Im wondering if I crossed my mains at 80hz to a mid sub then allow it to naturally roll off to the SDX15 at 40hz. By the time I reach 30hz there would be a -3db change and falls off quick after that.

The driver Im thinking about using is the Dayton Ultimax 15.

http://www.parts-express.com/pe/showdetl.cfm?partnumber=295-514

Dayton notes in a sealed 3.1 cu.ft box this would have a f3 of 35hz and a qtc of .707. My plot in WinISD shows a qtc of .78 in a sealed 4.5 cu.ft box. So I don't know what to believe. I think this driver would work as a mid sub. Does anyone have any input on this?

Here is my WinISD plot in a the 4.5 cu.ft box, qtc of .78. Thanks in advance.

 

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It sounds like it should work to me. I know that some expertly designed setups (such as Hsu subwoofers) include a "mid sub." A sealed design makes perfect sense since you don't really need the mid sub to dig low, and improved transient response (and perceived SQ) is better with sealed subs also.

My only thought is that you'd want to get your crossover points exact by testing them in-room. As in, plan for a mid sub solid down to 30-60 Hz, then tray various crossover points in between while taking measurements. Having the LF spectrum broken up will have a big impact on room interaction, especially depending on where your subs are located with respect to listening location.

There's been quite a lot of discussion about the ultimax subs and the suggested box sizes. If I remember right you'll need to include stuffing (lower the "Qa" WinISD box parameter) in order to duplicate the Dayton recommendation.
 

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You can use the Ultimax but it really is not the right tool for the job. The Pendragon Subwoofer uses pro style drivers which is why it is efficient for mid-bass and not low. A pro style sub is what you should be looking at instead of the Ultimax.

I have not looked into the best mid-bass pro subs in awhile but this was what a lot of people used before: http://www.parts-express.com/pe/showdetl.cfm?partnumber=290-590
You can tune it to around 35hz ported with a HPF and it will eat up the Ultimax 15" from about 40hz on up with less power.

Here is a comparison so you can see. The Ultimax 15 is getting 600 watts and the Kappalite 12" is getting 400 watts with a 30hz HPF.

 

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Discussion Starter #4
Thanks everyone for your responses. At this point its just a idea. Im still learning all of this. I really wish the SDX15 was still being made so I could add a second sub to match the one I have.

Thanks again for your time.
 

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The CSS SDX15 models very similar to the Dayton Ultimax 15" so you might not notice a difference in the 2 if you wanted a second.
 

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Discussion Starter #6 (Edited)
Funny you said this. I was just plotting a Ultimax in the same size box as my SDX15. Just need to put both in the same graph and look at it. This might be the best thing to do is to add a second sub and ditch the mid idea for now. Thanks again!!!.
 

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I think you would be happy with either option. Extending your headroom and eliminating nulls is absolutely worth adding a second sub, imo, as dynamic passages are so much clearer.

I have dual bass bins that handle the frequencies you are talking about and I really like how they perform. I would do it differently again, using a mid-bass driver instead of subwoofer, so I could extend the higher end a bit.
 
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