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Discussion Starter #1
Hey guys,

Looking to build (actually get a sub built) to fill a ~8000 cuft room. I was originally looking to build a sealed 18" (thanks Musikpirate for helping with the design) but after much weighing of my skillset and available time I'm gonna outsource this one. After reading the LLT basics thread, I'm pretty excited to go in that direction.

After playing around with winisd, this is what I came up with an Fi Q15 in a 14Hz tuned 5.7cuft box. I already have an EP4000 amp, so running off of one channel (640amps @ 4ohm), it looks like this can put out 107dB @ 14Hz. This would be with a 3/4" X 19.5" slot port. From what little I know it looks like it successfully meets the LLT parameters, though I admit my knowledge is pretty crude.

Anyway, any thoughts? This would be for about 70% HT and 30% Music.


 

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Discussion Starter #3
Thanks, I will try update the model when I get home. I'm curious, though... Most of the info I've read puts 34m/s as a limit - which is not exceeded until ~15Hz. Are the constraints different for slot ports, or is 25 more of a conservative number?

I'm also curious about how audible the chuffing may be over 100+ dB of movie.
 

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Discussion Starter #5
That's true - especially when you're getting the thing built! I will try to figure out some updated box parameters when I get home, though the bigger and bigger this thing gets the harder and harder it will be to get it by my fiancee.

My lowly experience is with HTIB and retail-type subs and I've never actually heard chuffing. So I'm still wondering how audible will it be over the movie audio?
 

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I've heard chuffing...during loud movie segments. It's very annoying and to be avoided with every effort. If a bigger port does no harm and helps avoid chuffing, I'd do it. It was so annoying I unplugged the sub...it wasn't the only one in the system thankfully.
Nice plan otherwise. Should kick!
 

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Discussion Starter #7
Thanks kadijk, that's exactly what I wanted to know. I've played around with winisd, raised the port to 1" X 19.5", upped the box to 7 cuft, and am changing the driver to the Fi SSD15. Looking at the design, cone excursion doesn't exceed the SSD xmax until 12.5 Hz, so the additional x-max from the Q might be wasted.

At max power (650amp) the port air velocity still peaks at 47m/s at 12.5Hz. I'm hoping it won't be a problem at since it is so low, but to be safe I'm thinking about adding a 12Hz HPF (2nd order). This keeps the peak down to < 34m/s. It does add 8ms of group delay, I'm not sure how much that impacts the overall sound.
 

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Discussion Starter #9
Yeah, I figured as much, but I'm doing the MiniDSP/REW thing anyway so I'll keep it in mind if it becomes an issue.

Thanks for the help, guys! Now I just need to get a design for the box ...
 

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Also, don't forget the option of using passive radiators. Don't know much about the woofer, but a couple of pr's may get you the low tune you want, without worry about port noise. May even lead to a smaller overall box. Cost is a drawback, but still worth considering.
 

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Discussion Starter #12 (Edited)
I've thought briefly about going PR, but I know so little about it that I'm not very comfortable with that direction. It seems like there is so much more written up about LLT. I'll try to do a little reading tonight.

Personally, I think passive radiators look bada$$ .

Edit: original post got censored to say the opposite of what I meant to say.
 

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http://audio.claub.net/software/jbabgy/jbagby.html

This is an excel spreadsheet that's slick for modeling PR boxes. Shows you realtime changes as you change box parameters, and also tells you needed mass for desired response. It recalculates changes to PR specs as mass is added. It also lets you sim the response difference when introducing electronic filters. Pretty nice piece of free code for diy'ers.
 
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