View Full Version : Ported Box Calculations

06-25-2006, 01:10 PM
I am interested in putting my subs (dual 12" Assassin's) in a ported box. I have no way to make the cuts myself, so I'm looking at this pre-fab:


With the advertised internal volume (3.47 cu.ft.) and port displacement (0.195 cu.ft) (and guessing the port dimensions of 12.75" x 2" x 13.214") WinISD gives me a tuning frequency of 34 Hz, and a plot that looks very much to my liking. I get a peak of 120.8dB @ 54.5Hz and f3 of 34.35Hz

But I did some calculating myself and after removing the width of the MDF, the insets on the ends and port displacement I get an internal volume of only 2.78 cu.ft. :crap: This gives me tuning frequency of 38Hz, a peak of 121.4dB @ 60Hz and f3 of 38.6Hz.

Can someone check my calculations? (other settings were 500W power @ 2 ohms, 22Hz highpass (no overexcursion) and 90Hz lowpass)

And if the box is 2.78 cu.ft. and tuned to 38Hz, is that going to be OK for daily listening? I like mostly rap and want it to be loud, but I don't want it to sound like total crap on other types of music.

Right now I have them in 1.25 cu.ft. each, sealed, and I want more SPL! The 38Hz tuning gives me a similar peak frequency and f3 as the sealed box, but the response curve is more pointed and less round with less extension below f3 (f10 of 29Hz (ported @ 38Hz) vs. 27Hz (sealed)).

Here's a screenshot of the SPL graphs on WinISD:


blue - sealed
white - advertised volume
grey - my calculated volume

06-26-2006, 01:27 AM
Well, I've decided that this box is definitely better for my application.


It's about 3.46 cu.ft. net volume but the stock port is tuned to 41.6Hz!


I played with WinISD and it looks like I can fill in some port area and get a much lower tuning frequency without port noise!

If I put 0.75" MDF all along the side and bottom wall of the port this will take the port dimensions from 2" x 14.5" down to 1.25" x 13.75" while keeping it the same length. This changes the tuning frequency to 32.6Hz!!!! PERFECT

And according to WinISD the port air velocity never gets above 12 m/s (I think it is due to the small box volume that I can get away with 34 in^2 of port area for 2, 12" subs)

If no one sees any major flaws in this plan, I'll probably buy this prefab sometime soon Hopefully I can still fit my golf clubs in my trunk with this box!