Basically a Giant Sub Woofer.
I built this as part of an audio visual project for Burning Man 2007. Using a tube with the driver mounted inside it resonates much like a church pipe organ within a specific frequency range. Hopefully the bass frequency range.
Called "Rare Angles" it comprises 12' of cardboard Sonotube as used by concreters to form columns. Its cheap, easy to work with and rigid enough to use for this application. You can get it at a concrete supplier.
As soon as I laid eyes on what Holly and team over at positron.org did, I knew it was just a matter of time, and any flimsy excuse to build my own.
This Bass Cannon is designed very basically. I was able to buy a 12' length of 16" sonotube. The driver has to be placed exactly 1/4 the distance down the full length of the tube or 3 feet.
From what I can gather from research, the longer the tube length will raise your resonant frequency. I figured my resonant frequency was about 30Hz. I am probably wrong. I did the calc a while ago so can't be sure how I arrived there.. It wasn't too loud on the "test fire" audio sweep, but a neighbour from 2 doors down, certainly heard it and came running down the street.. "Are you making that noise?!"
Maybe me sitting on my deck with a giant carboard tube and earmuffs on gave me away.
Step 1: Ingredients
1 x sub woofer 12 or 15" depending on the sise of tube you want 15" in this case
1 x length of sonotube slightly larger than your sub woofer diameter 16" in this case
1 x sheet of 3/4" ply
glue / sealant (sikaflex works well)
Circle cutting jig for router
3/4" spade bit
circular saw or jig saw.
Step 2: Cut the Sonotube Into Two Pieces
Do do this accurately I measured with a large sqaure 3' from one of the ends at about 6 points around the circumference of the tube. I then wrapped a piece of masking tape around, making sure one edge of the tape passed through the points I'd just marked. I used this as a guide for the jigsaw. I drilled a starter hole to get the jig saw blade in then went at it! Afterwards I removed the tape. The cut will be kinda fury but that ok, this will be concealed inside the flanges.
I didnt take a photo of this step so I drew it. See the diagram with the light grey lines being my 3'
long marks then the dark band being the tape which i used as a guide to cut the tube squarely.
Step 3: Cut the Sub Woofer Mounting Flanges
I used a circle cutting jig, consisting of a strip of ply with the router mounted on one end. This was nailed to the centre of a board of 1/2" ply which would become the first flange. I began to cut out a the diskusing a 1/4" bit at 10" radius to give a 20" flange disk.
Starting from the outside of the flange in, I cut out the perimeter.
Then cut a 1/4" wide groove 1/4" deep at 8" radius for the top half of the sonotube to sit in. This is where the cut is concealed, and also provides a little extra surface for the adhesive to bond and create an airtight seal.
Then I flipped the disk over and cut a 10mm deep groove at 7.5" (I love changing between metric and imperial) then another 2 at the same depth inside that, which makes a shoulder for the driver to sit in.
The next cut is just inside the last to go all the way through to remove the centre section.
The second flange was identical.
Again I have drawn a cross section of the flange piece.
I then clamped the two flanges together and drilled 4 x 3/4" holes with a spade bit for the bolts to hold the two flanges together. When drilling through ply (or any timer) with a spade bit, keep an eye on the underside and when the point of the bit starts to emerge flip the piece over and finish the hole from the other side. This will prevent tear outs and give a nice clean hole..
Step 4: Install the Sub Woofer.
I screwed the sub woofer securley onto the lower flange, added the second flange ontop then added the second length of sonotube.
I worried that the wind would possibly snap the tube out of the 1/4" grooves so re enforced the tubes with ply triangles made from scraps.
I used marine Sikaflex to glue the tube into the grooves, then screwed the ply rib triangles from inside the sonotube. I used more sikaflex to bead between all the support ribs. It ended extremely strong.
I did have to drill a hole through the lower flange to let the sub woofer cable pass through. This was an oversight. In future I'd run the cable to the sonotube wall and pass through to lugs on the outside.
Step 5: Wire Up and Test.
I made a stand for the whole thing to stand on, but this was an error. The resistance of the bass cannon standing close to the ground created back pressures that introduced some unwanted characteristics.. nasty resonances that caused humming and nasty distortions.
This should always be mounted with plenty of room beyond either end of the tube.
Finally is a pic of the bass cannon in situ on the playa with the remainder of the paraphenalia attached..
See a full description here;