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Discussion Starter · #1 ·
I am building a box for an Image Dynamics ID12 D4. My current system has a great top end down to the woofers- nice and punchy all around... but I want to add low "rumbly" frequencies. I have an amp that matches my sub's RMS, and had a 1.7 cu ft box that sounded great but doesn't fit in the trunk of my car. It looked terrible too- my first foray into fabricaton and fiberglass, and I built it about 11 years ago. TS Parameters are hard to find on the Internet, but here they are:
Fs: 20.9 Hz
Qes .397
Qms 4.172
Qts .363
Vas 5.92 cu ft / 168L
Xmax 17.0mm
Sd 545cm2
Spl 88.6db
Driver displacement is .07 Cu Ft
12.25"OD
11" ID
6" mounting depth

The Mfr. recommends large 1.5 cu ft, Medium 1.25 cu ft and small .8 cu ft. sealed enclosures and also ported enclosures:
Large 2.0 Cu. Ft. (NOTE: GROSS VOLUME- Includes port)
port freq. 30Hz
port dimensions 4"ID x 15"
-3db In Car / Free Space: 18Hz / 30 Hz

Since I am looking for mainly low lows, I am planning to build an angled sub box for the largest ported size.
Can I just take the volume of the round port and convert it for a rectangular one? The dimensions of the port as I calculated them would be 1"X12"X15.7.
the inner dimensions of the box are for the base: 17"X 20.65"X 12". The top of the box will be inside 12" X 12" X 20.65" The port will be the back wall of the box with a leftover piece of 3/4" MDF that's 15.7 inches long (Really 15.75, but I'm rounding the corners of the port piece to cut back on turbulence). So basically a slotted port with a simple vertical wall to define it.

The volume of the box and the round port information was supplied by the MFR, and I calculated the linear dimensions and rectangular port size. This far into the box design, I could probably switch to the round port- I'm sort of torn. Any advice? Let me know if my numbers seem out of whack. I'm also unsure of the port issue, because I thought that round ports were calculated with a resonant frequency in mind, but that doesn't seem to factor in when doing the conversions recommended elsewhere. This is going into my trunk, and the car is a sedan, if that might factor in... I appreciate anything that might help me with this project.
Thanks!
 

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A 4" vent tube has 12.57 sq in. area, and a 1" x 12" would be 12 sq. in. However, at some point the vent width to height ratio becomes too large for the vent to function properly or predictably. FWIW, I've read recommendations that anywhere from 6:1 to 9:1 is as high as one should go, which suggests that there might be issues with your 12:1 ratio.

Also, I've read that you need to add half the internal width of the port to it's length if the beginning of the port starts along an enclosure wall.

Here's a great DIY subwoofer resource on JL website: https://jlaudio.zendesk.com/hc/en-us/sections/200876670-Enclosure-Design

Hopefully some experts will chime in.
 

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Discussion Starter · #3 ·
Started on a decision... I didn’t realize there were limitations on port length for slots but I did find with a minor volume adjustment that the round port will fit per mfr spec (amazing how easy it was to fit a 4”ID pvc pipe and figure out its volume plus the bracing to support it!) I thought this was going to be a real bear of a job, but the calculations even seem simple with what I have. Know anyone who needs a 12” sealed sub box? First one I built was 1/2” too tall for my trunk- that is why I’m now building the angled enclosure!
 

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just change the dimentio of the slot port. make it deeper and less tall. grinder is spot on about the limitations. you dont want a port that is sliver thin and super tall to get your port volume. also you do want 1/2 the length of the smaller port dimension worth of clearance from the nearest wall. (this also goes for clearance outside the port too.)
 

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Discussion Starter · #5 ·
Thanks for the advice! The way the clearances are in the box with the driver orientation, the slot port will not work, but as it turns out the round port does. I just tweaked a few numbers, reran some calculations, and all I have to change is the width. External clearance is not an issue on that dimension- it is roughly a 2’ wide box, port on the side, centered on the back seat. The dimension I have to watch is the vertical clearance. The previous box I made was 15.5” tall and you cannot squeeze anything taller than 14.75 in there. That’s the reason for the angled design... but the angle means that a vertical slotted pot conflicts with the driver installation depth at the bottom. I have just enough room at the top for the round port plus a support for it. Since the port length is 15”, and .75 of that is inside the exterior wall of the box, the width of the box allows the clearance inside for the port... amazing how much math is involved to build a box with two holes and a tube!
 
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