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Discussion Starter · #1 ·
Cheers to all passive radiator pros! I come to you with a project.

For the past month or so I've been trying to plan out how to build a subwoofer that will perform down to 20hz. I've looked into all types of enclosure designs, and it seems to me that the passive radiator design will work the best. Here's what I have to start:

Two rockford fosgate 12inch car audio subwoofers. One is an 8ohm 200w rfp3812 punch HE. the other is an rfp3412 4ohm 200w punchHE without the magnet.

one 120w subwoofer amplifier.

the T/S parameters for the rfp3412 are the following:

FS 28
Qms 6.237
Vas 86.4
Xmax 13
Sd 660.5
Qes .51
Re 3.69
Le 4.48
BL 17.17
Qts .472
1 watt SPL 87.7

T/S parameters for the rfp 3812 (from what i could find on the internet) are the following:

Frequency Response 28 - 200 Hz
RMS Power Range (Watts) 50-200
Peak Power Handling (Watts) 400
Top Mount Depth (inches) 5 3/4
Bottom Mount Depth (inches) N/A
Cutout Diameter or Length (inches) 11 1/8
Vas (liters) 86.4
Fs (Hz)29
Qts 0.533
Xmax (millimeters)13

I've used WinISD to figure out roughly what box size I need and what weight needs to be added to the PR to achieve my response. With a little EQ boost of 4db at 20hz and 2db at 30hz, in a 4 cubic foot enclosure, and with 350g of weight added to the PR, the graph seems to look pretty beautiful. (I can provide screenshots if requested)

What I really need is a 2nd opinion, and/or some advice regarding this project (is there a better way for me to go about it?) I know that WinISD gives you the "current version can't calculate optimum box parameters of this box type" which is why i'm looking for more educated opinions. Also, I should note, that I'm thinking about a sonotube/passive-radiator design as well, so bear that in mind. I want to make sure that I'm not making foolish mistakes, or overlooking anything, because this is my very first DIY project.

Thank you in advance for your help and time.

Premium Member
19,397 Posts
The Sd on the 3412 should be 460.5 cm, not 660.5. Here are the parameters for the 3812:

Text Diagram Technical drawing Design Parallel

And here is the WinISD project file.

View attachment RFP3812 PR.wpr

You'll see that with 55 watts the PR reaches its limits, assuming it can handle 200 grams in added mass and with a high pass filter at 21 hz. You'd be better off just building a ported sub the 3812. Purple is the PR and orange is ported. The ported has a high pass filter at 22 hz.

Text Technology Line Electronics Electronic device

Premium Member
19,397 Posts
The sub will lose some of it's excursion in a down-firing position, it doesn't have much to begin with. The sub is also 8 ohms, any 4 ohm amp you use will have its power cut in half. You also need a HPF at 22 hz in the above model. Asking this sub to go down to 20 hz is asking a lot. If you really have to use this sub, the best you could do would be 7 cu.ft. tuned to 21 hz with two 4 inch ports. Powered by a Bash 300 amp (150 watts - 8 ohms) with the HPF changed to 20.4 hz and 1 db of boost between 31 - 35 hz. Anything less than 7 cu.ft. and you're giving up low end extension.

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