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Old 8th January 2008, 04:41 PM   #11
txsmoke is offline txsmoke  United States
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Since you have it, it's worth trying. MDF is cheap. I modeled it in WinISD and came up with a pretty nice response with a 3 ft^3 box tuned to 25hz and crossed over at 80 hz with a 2nd order lowpass (typical setting in a AV receiver). Just exceeded Xmax with 100 watts at 35 hz. And reached 105db at 25hz. That's a 4" x 15" port. Group delay is a little high, but for HT, it's not that critical. -3db points were about 22hz and 100 hz.

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Old 8th January 2008, 06:40 PM   #12
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Eureka!
This WinISD is fantastic and I assume Iíve done something right because Iím seeing curves and power limits a lot like txsmoke above; both in shape (when modeled with similar box and port size) and power limits at the xmax.

It appears that the speaker likes a lot of volume (relative to the JBL spec sheet designs), and I think Iím going to limit my design to about 60L for size constraints and a tuning frequency of around 30hz. This makes the graph look like a higher than 0.707 ďQĒ, but Iím assuming thatís ok since the frequency response hump is quite small like in txsmokeís plot above (to a controls person it looks like overshoot, donít know what youíd correctly call it).

Thanks all for helping me get started.
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Old 8th January 2008, 07:31 PM   #13
GM is offline GM  United States
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FYI, WinISD has some issues, though the 'pro' version is somewhat better. Other than Akabak, of the freeware I've tried, Unibox is the best/most comprehensive along with the other software listed at the FRD Consortium: http://www.pvconsultants.com/audio/frdgroup.htm

'Overshoot' = 'ringing' = under-damped (what we normally call it) response, so you're apparently interpreting the plot correctly.

GM
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Old 8th January 2008, 07:33 PM   #14
txsmoke is offline txsmoke  United States
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Boxes for cars are typically smaller since the need for low end extension isn't as great because the car's cabin adds gain the lower the frequency goes. Rooms do too, but the contribution typically doesn't start until much lower- and may not come into play in very large rooms.

So the reasoning for smaller car boxes is that vehicles have around 8-12db/octave gain starting around 60hz or so (varies car-to-car), so a smaller box rolls the sub off at the same rate giving a (summed) flat response and increases power handling due to the stiffer 'air spring' a smaller box exerts on a driver.

Mike
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