Hello all, I'm just looking for a quick sanity check on a proposed build.
For a little background, here were some of the constraints:
Footprint of 26.75" by 37.7" so it can support an Altec 817a cabinet. It does bring the benefit of bringing the HF horn of the system (Altec 291) near ear level and centered from the floor and wall. I could place the subs to the side but it would require moving furniture and reconfiguring the listening area.
Designed for a pair of JBL 2242h. I know its not the best "sub", if you can even call it that, but I have been building systems from public surplus parts I have acquired because they do bring good performance for the money. 100% for music listening so no need for HT level of low bass, the 817a "extend" down to 80hz according to Altec but that is more of a -6 db point, -3 db is probably 140hz. So looking for a response around 30 hz to 140 hz for the sub. I have a mini dsp and a fully active system so I have some variability on setup.
So with those constraints, I was targeting something similar to the JBL 4645C (8CF, tuned to 25 hz, +6db boost at 25hz q=2, 2nd order highpass at 25hz). Wanted to put slotted ports on the sides with a 2x4 in the middle to add bracing (so 4 ports total), and the port running the full height of the enclosure. That wasn't looking promising so I found a compromise using 3/4" MDF on the sides for 6 ports. This leaves a 9CF box with all the same parameters for the 4645C. I know the shortcomings of the design would be the 2242 will not be able to hit max power handling before running out of Xmax and setting the delay for the system will be an issue. But within Xmax gives around 114db (iirc) at 1m without boundary gain or gain from the second speaker, so more than enough output for me.
I plan on lining the cabinet with rockwool, except the back 45 degree wall that shares with the port (would the rockwool interfere with the port?). The dimensions didn't print, but the port end is 4.25" away from the back wall. Using 2x4 to brace the cabinet between the ports and 1/2" dowel rods from front to back.
I have designed sealed and ported subs with tube ports, but the is a first for a slotted port design. So, I'm just checking to see if there are any critical flaws.
For a little background, here were some of the constraints:
Footprint of 26.75" by 37.7" so it can support an Altec 817a cabinet. It does bring the benefit of bringing the HF horn of the system (Altec 291) near ear level and centered from the floor and wall. I could place the subs to the side but it would require moving furniture and reconfiguring the listening area.
Designed for a pair of JBL 2242h. I know its not the best "sub", if you can even call it that, but I have been building systems from public surplus parts I have acquired because they do bring good performance for the money. 100% for music listening so no need for HT level of low bass, the 817a "extend" down to 80hz according to Altec but that is more of a -6 db point, -3 db is probably 140hz. So looking for a response around 30 hz to 140 hz for the sub. I have a mini dsp and a fully active system so I have some variability on setup.
So with those constraints, I was targeting something similar to the JBL 4645C (8CF, tuned to 25 hz, +6db boost at 25hz q=2, 2nd order highpass at 25hz). Wanted to put slotted ports on the sides with a 2x4 in the middle to add bracing (so 4 ports total), and the port running the full height of the enclosure. That wasn't looking promising so I found a compromise using 3/4" MDF on the sides for 6 ports. This leaves a 9CF box with all the same parameters for the 4645C. I know the shortcomings of the design would be the 2242 will not be able to hit max power handling before running out of Xmax and setting the delay for the system will be an issue. But within Xmax gives around 114db (iirc) at 1m without boundary gain or gain from the second speaker, so more than enough output for me.
I plan on lining the cabinet with rockwool, except the back 45 degree wall that shares with the port (would the rockwool interfere with the port?). The dimensions didn't print, but the port end is 4.25" away from the back wall. Using 2x4 to brace the cabinet between the ports and 1/2" dowel rods from front to back.
I have designed sealed and ported subs with tube ports, but the is a first for a slotted port design. So, I'm just checking to see if there are any critical flaws.
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Well while reading the unorthodox 2x18 build, I noticed that WINisd cannot be forced to use a K=2.227, so I guess it's back to the drawing board and time to use a new box software.
It's fairly easy to put the port calculation into a spreadsheet to use the alternative end correction value, here's my effort if you want.
Damping material increases the virtual volume of the cabinet and has to be taken into account when deciding Fb and vent length.
Lining the sub with rockwool will reduce output slightly, with a crossover of 140 Hz, probably no need to at all, and lining one side and the bottom would be plenty if you did find "ringing" unacceptable.
No "critical flaws" visible, other than without increasing the port end correction factor “K” value from a standard value of 0.732 for a circular port as in the JBL 4645C, your ports will be too long.
In addition to the “K” value differences, the high aspect ratio of the ports may also make them tune lower than expected. Going over a 2/1 ratio increases the apparent length, though I have not found the math for that...
Thank you all for the responses. I went back to Unibox for the first time in a decade or so, I do believe that using the K=2.227 and the equivalent diameter that yields the same area as the slot will result the correct port length. However with the K value given, it ended the port right at the chamfer (of course...) So, still looking at other alternatives
Thanks for the information, I'll make sure to check out the spreadsheet!
I'll make sure to look into that, I might switch over to a closer to 2:1 ratio,. At least I wouldn't need to iterate designs to get port and depth of the enclosure to match.
I adjusted Qa when running WINisd initially (assumed 60), but good to know that fill may not be required at all. Especially since I'm starting to run above my planned box volume.