Greetings friends,
Another little bit of a learning project here. I've been thinking about improving the sound quality for my home "theatre", and a logical step was to get a subwoofer. Unfortunately, any sub of decent quality or with the frequency response I want will cost at least a few kilobucks, so I decided "why not just make one myself?" Well here we go!
As always, I started by scouring the internet for ideas. What shape should it be? How big does it need to be? What drivers produce the right frequencies for a sub? After reading and weighing to pros and cons of various designs, I settled on the SonoSub design. It essentially consists of a simple cylinder into which a driver and vent tube are installed, and blam-o, you've got a sub (at least in principle). Let's dissect the process!
Lucky for me, I got to stand on the shoulders of those who have done this before. Two pieces of software exist to help in designing the sub, SonoSub and WinISD. WinISD allows me to enter the specifications of the driver I select and will calculate the theoretical frequency response for a subwoofer under certain conditions. SonoSub allows me to enter the parameters from WinISD and calculate the dimensions for the actual subwoofer housing.
I decided to get a 12" driver, which seemed a good size without getting ridiculous. This would also fit nicely into an 18" tube, which became one of the independent variables in the calculations. Information for the driver I decided to purchase can be found here. I entered the specs of the driver into WinISD and started playing with the performance specs I wanted from the sub. As a collector of classical music, I have several recordings that play frequencies below 20 Hz, so at the very least I wanted frequency response a few Hz below that. Given the capabilities of the driver, an 18 Hz tuning frequency with a tube volume of roughly 190 litres have me a frequency response I was happy with.
Assuming an amplifier with a low-pass filter, the size and tuned
vent gave a really nice frequency response.
I entered the parameters from WinISD into the SonoSub software, and played with the numbers until I had an arrangement that I liked. I wanted to keep the dimensions easy to work with, so I decided that a 48" tube length (excluding caps) would be simple.
I started off by buying the 4-foot section of 18" SonoTube - they're cardboard molds used to pour concrete columns. Since it has a lot of rigidity and comes in large diameters, it's perfectly designed to be the casing for a sub. Luckily, the supplier cut the tube to length for me so I didn't need to fuss with trying to make perpendicular cuts on a ridiculously big tube.
To line the tube to provide some sound dampening, I got some 11 mm thick carpet padding from Home Depot, and used spray adhesive to line the inside of the tube. Because caps will go into the ends of the tube eventually, I cut the padding slightly short so the tube can accommodate the caps. Peeking my head inside and singing some low tones showed that the dampening was quite effective.
Onward to cutting the caps, into one of which the driver will be mounted, and into the other the tuning port. I foolishly first tried to cut circles freehand with my router, but was quite unhappy with the result. Simply put, a lot of extra sanding a shaping would have been required, an amount of effort I was looking to avoid. I then fashioned a very simple circle jig using a yard stick. Bolting one end onto the router and using a centered screw as my pivot point, this surprisingly made ridiculously easy work out of cutting circles. Though I unfortunately wasted 2 MDF boards in my freehand attempt and had to get replacements, in the end the simple jig was very worthwhile.
Both caps consisted of two rounds cut out of MDF, the outer round having a diameter of 19" and the inner 18". I cut appropriately-sized holes to accommodate the driver and the 4" port, and rounded over all edges to give it a pleasant look. I glued the pieces together and applied a few coats of shellac to seal the MDF. Below you'll see the cap for the port and the driver sitting in its cap.
The driver nestled neatly into the (bottom) cap and is held in place with 8 screws. Binding posts needed to be installed on the outside to connect speaker wire to the driver within the subwoofer, so I drilled additional holes for these. I didn't bother to make the holes inside look perfect, as they'd never be seen when all is said and done.
The posts were very easy to install, and are tightened into place with a few nuts. From the outside they're quite innocuous and will accommodate either speaker wire or banana plugs. A short length of speaker wire made the interior connection clean and easy!
I pushed the 4" ABS pipe into the port hole on the top cap, and set both caps onto the tube to give it a try... Currently the caps are just pushed in and not glued into place yet, since the exterior of the tube is not yet in its final state.
Wow, this thing really packs a punch. It's almost unbelievable how much sound this can produce for what it cost to build. The sound completely fills the room, and the low frequencies are incredibly rich! An incredibly successful project to say the least! Unfortunately no recording I could post here would do the sound justice, so you'll just have to come hear it for yourselves!
As far as aesthetics, the caps will ultimately be painted and the tube will be covered either with carpet or some other soft dampening material so that the sub will be more inconspicuous. When weather permits, I'll spray paint the caps outside and I'll update this post with some new pictures showing the final look. For now, I'm going to sit back and enjoy it!
UPDATE - 2020.40.15
I finally decided it's time to dress this sub, and my wife had some time to give me a hand with the sewing! Here's how I did it.
I bought a regular moving blanket/pad, that people usually use to protect furniture when moving. Blue on one side, black on the other. We'll be going with black!
To start, we sewed and cut the blanket to rough dimensions to see what we're working with. Another quick stich makes a tube out of the blanket and we're very close to a perfect fit. It only took a few more tweaks to have a decently tight fit, and the overhanging fabric will be tucked into the tube by the caps.
The caps were pretty difficult to press into place. Ultimately I had to take some clamps and gradually tighten while the caps slid into place. At least it won't be coming apart anytime soon!
It looks great, sounds great, and we'll be enjoying it for a long while! The caps will stay light-colored; I think it looks pretty nice!
Thanks for reading,
Juju
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