Edit 26.12.2025: There was a CAD design error in the 2x4" baffle, not allowing for the port recess. This has been fixed now, in both STL and STEP files. Thanks, Jason.

NB: If you are using onshape for CAD, you can find and copy the original document in the public section, under Subwoofer 2x4" and 5.25" or my username Homunculus, make a copy and mod the heck out of it. Would be useful to adapt the baffle for other speaker chassis.

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Small ported subwoofer around 12L volume, can be extended to 18L (or more) if needed. The middle extension is about 6L volume, stacked in between the base and top. I have not built any larger version - so at your own risk.

Looks like a monolith 41x24x24cm, good as a foot rest or altar. Or add in the middle extension to make it even taller and arguably uglier.

This could be used as overkill PC speakers to attract noise complaints and eviction notes. Or to bluetooth in with your phone, with the amplifier version and 2 desktop speakers added. Or to give your tiny bookshelf speakers some much needed bass. Just be aware in case you came here to save some money, you probably get a similar second hand subwoofer cheaper on Facebook Marketplace but without all the curses, sweat and tears of building it yourself and justifying that 3D printer purchase.

STEP files provided for all parts, so changes can be made for other setups and speakers.

I designed 2 versions, one for 2x 4" speaker Dayton Audio TCP 115-8 tuned to 40 Hz as a passive version with terminal, and the other one for HiVi D5.8 tuned to 38 Hz with a cheap plate amplifier ( this speaker is unfortunately discontinued) - pictured is the latter version. The two speaker setup has to be run in parallel, not serial, otherwise the impedance will lead to very low output of the amplifier and the output at 2.83mV will be only around 80dB, instead of 85dB (according to simulation in VituixCAD).

The tuning was done with Dayton Audio DATS 3, I have used the measured Thiele/Small parameters rather than the manufacturer supplied - quite a difference, although I have not broken in the loudspeaker chassis. The DATS sweeps of the finished subwoofers are attached.

The size is dictated by the maximum build version of my Qidi Q1 Pro - 245x245x245mm. I would have gone a lot larger and used other loudspeakers in a larger build volume printer. Maybe one day, after winning the lotto when I get a Bambulab H2C (sigh) smuggled in my garage without my wife noticing.

For the print, this will need around 4kg of filament, with 4 walls and 25% gyroid infill, and probably takes a week print duration of anxious waiting to complete. I have used eSun PLA-CF, but I am sure regular PLA would have done. A 0.6mm hardened nozzle was used, with 0.24mm extrusion, slow down speed for external walls. No brims or raft. Support only needed for the 90 degree overhang of the recess in the base upper part and mid section upper part. Build plate temperature was 60 degrees. Glue stick used, as the PLA-CF does not stick very well to the PEI plate.

Ideally big 3kg spools are used, otherwise there is a small ridge after changing the 1kg spool - at least on my printer. If the print of the larger parts fails at any stage, that's bitter. But you will be rewarded with the view of a man-sized tumble weed of PLA Spaghetti, and it's around $40 down the toilet (I had a power failure after almost finishing the top part). Still cheaper than setting myself up for woodworking with MDF and buy the tools, that stuff costs a fortune now as well and the dust is probably more toxic than the PLA-CF particles.

It was sliced with Orcaslicer, using the fuzzy skin option to match the sidewall appearance to the PEI plate imprint of the top.

The in-build supports for the mid braces need to be removed after printing, they are designed to be breakaway.

Threaded inserts M4 x5.8 (speakers, amplifier, terminal), M5x9.5 (feet). The holes in the print are dimensioned for the CNC Kitchen inserts, see link below. Our friends from China will sell you similar, but usually shorter versions - not recommended.

The feet and gaskets are printed in TPU 95A. For the speaker you might need a gasket, the other gasket for the enclosure is only needed when tuning the box as after tuning the parts need to be glued together and the gasket is obsolete. The Dayton speakers came with their own gasket.

For tuning, there are several port extension lengths provided, however depending on the speaker used and tuning frequency this might need to be adapted. The 5.25 version tuned to 38Hz needs the 90 degree angled 250mm version. The 60mm port diameter with flared ands prevents port chuffing reliably, at least with the power the plate amplifier can produce before distorting itself. Connected to a Fosi BT30D pro, the speaker will exceed it's safe excursion rating before chuffing occurs.

For gluing I used JB Weld Plastic Bonder, this sets in 30 minutes and can be peeled off your fingers better than epoxy. Don't use Superglue as is is to viscous, unless you want leaks which can add some interesting whistling and hisses to the sound. After tuning, first glue the port extension, then after 30 minutes glue the top on the base with some weight or clamping to hold it down. The fitting of the top can be a little tricky with the curved port extension turned on an 30 degree angle for better clearance, but it definitely works.

No damping material is needed, due to the limitation as subwoofer only.

Sound is pretty good, the 5.25 speaker more powerful due to stronger motor, and Xmax 5mm. In the impedance sweeps there are notches around 400-700 Hz, but when using amplifiers subwoofer lowpass this does not matter as there will be not much transmission past 300Hz.

Sources for some of the parts (not affiliate links, you will find this in numerous places):

Amplifier (several versions, next time I would use the Aiyima variant with switchable sub/satellites option) https://www.aliexpress.com/item/4000009985023.html?spm=a2g0o.order_list.order_list_main.235.330d1802oPvx1u

Terminals: https://www.aliexpress.com/item/4000113380991.html?spm=a2g0o.order_list.order_list_main.250.330d1802oPvx1u

Threaded Inserts: https://cnckitchen.store/collections/threaded-inserts