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My car sub box

I have built many sub boxes before but this will be the first with the most pics in the build stage.

The speaker I will be using is the Polk Audio MM840dvc. The box will be using a round flared port exiting the side. I chose to port it on the side to reduce boominess as with the previous box. This will be my SQ box.

We start by preparing all the tools needed. (Coconuts for refreshment )

Tunable port

My old but faithful cordless drill that has been converted. (Used to be 6V internal battery, rewound to 12V, handle shortened by 2 inches and powered by an external Li-poly battery pack or alligator clips to the car battery)

Doing the first cut. A 'C' aluminum channel acts as a ruler and an adjustable square to ensure consistent panel width. C clamps hold the aluminum channel in place when drawing the line with a pencil.

Ensure the cut is at the side of the saw blade so that saw kerf does not subtract from the planned panel width. (I'm left-handed so the blade is cut from the left side of the line)

All required panels cut to size.

I bought a drill + countersink combination bit about 3 years ago and never got to use it because I haven't made a sub box since. Now's the time to put it to use.

I modified it so that it would fit the quick release chuck from the screwdriver bit set. While spinning in the drill chuck, a dremel with a diamond bit ground the shaft. Worked like a charm.


Quick release chuck mounted on the drill chuck.

Drilled the holes. The combination bit took one change less. I used to use a drill -> countersink bit -> screw driver bit. Now it's drill-> screwdriver.

Screwed in.

Four sides are done. (my companion is sleeping)

Cutting the hole for the port. I didn't have a compass handy so time to improvise. A wire wrapped around a screw works well enough.....

Same process was done on the woofer cutouts. I planned the box dimensions for a 3 inch port. But it was not available so I used a 4 incher. luckily, the MM sub is somewhat slim mount.

The port lenght can be adjusted by adding/removing segments. Here's a sample 33Hz tuning

37Hz tuning.....

40Hz tuning.....

MDF dust piled up after sanding the enclosure edges.

Completed enclosure minus the carpet.

Port exits the passenger side. (there's my sleeping companion again)

Now there's more room for cargo...

Side port. (braided hose is for the air compressor behind the sub box)

Alternate box position.

The sub sounds nicer in this box. It's currently tuned to 37Hz and the bass is tight but can boom depending on the music. I have not tried other tunings yet as it sounds nice as it is. I'll fine tune it when I have listened to it some more. I'll also have it wrapped in black carpet hopefully next week.

06 Mar 2011: I decided to flush mount the port.

Then finally wrapped in carpet. Here's how it looks like now

Now it looks as good as it sounds.

30 Mar 2011: After tuning the whole setup again, I decided to add stuffing on the port side of the box. Filling the area around the port with polyfill.

My thinking is that the chamber around the port is a long tube with a sealed end. This would result to a kind of pipe resonance. Adding a lot of stuffing should damp this resonance.

The results were positive. It was not apparent when I first built and installed the subs but there was indeed less resonance on the upper bass. As a result, the subs seem to disappear at the back of the vehicle. The subs can no longer be localized and it seems as if the deep bass is coming from the front doors. I'm very happy with the result!

Page created and copyright R.Quan © 30 Mar 2011.