JBL 1220d driver Box design - hep needed

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I have a JBL 12" car subwoofer (Double coil) with a 350 watt plate amp currently in a 17' x17" x19" cabinet and it works ok , but I want to build a better box.

Using Win ISD program, it came up with a 31 x 18 x13 box for a 4th order bandbass design. Using that design the specs read FLAT 20hz to 50hz -6db all the way (My amp has enough gain to bring the volume up)then a nice roll off.

But looking at that design, the woofer is inside the box and only having a 4" port. Where is the output? To me that doesnt make sense, like it will only creat Boom not reproduce music. Am i correct? Can i use this design for music (Not HT) or should i just stick with a vented design open woofer cabinet

TM: 7.71
Sensitivity 93db

Or should i not wast my time and buy a home subwoofer

From what I've heard, bandpass and music don't go together. Without knowing the model number or Xmax of the sub, it models well in 4 cubic feet net volume tuned to 22 hz. The cabinet you have it in now is 2.4 cubic feet net volume and I'll guess sealed since you didn't mention a tuning frequency. If this sub has a decent Xmax, there will be a big difference in low end response between the two cabinets.


Joined 2003
Re: Yes those are it.

The vent will play music, but what simple box programs don't show is all the 'music' the vent also produces since it's a 1/2 WL resonator and due to the physics of the situation it must be large/long to keep it from compressing excessively, so will need to be stuffed like a TL, reducing its gain BW.

A vented cab has the same vent size problem, ergo the same stuffing solution is required to get a high SQ.

The fact that a large/long BP looks good in the desired BW tells us it's a candidate for horn loading, so either a simple folded tapped pipe or horn (TP, TH) seems a good choice if size isn't a major concern.

Since THs are the current rave, a quick sim yielded ~123 dB peak average/m/20 Hz/144 W corner loaded in 304.92 L net, so very low distortion in practice. Not too shabby for a 12". With a little EQ to flatten its pipe harmonics it's nominally flat to ~113 Hz if desired, otherwise just roll it off with a steep XO:



  • jbl gto 1220d 14.14-113.19 hz conic th.gif
    jbl gto 1220d 14.14-113.19 hz conic th.gif
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TH Cabinet size

That looks like the design for me. 304 liters looks like 74 high 17 wide and 18 deep. But I cant figure out the inside baffle dim. I also need to confirm. That I mount the woofer on the inside baffle and cut the opening on the front of the cabinet right in front of the woofer Baffle?

Can you help give me box dimentions and did I understand the concept?

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