Voigt Pipe Composite Speakers

This is a build of the original voigt pipe plan, published on the lowther club of norway site, http://www.parow.no/Lowther/voigtpip.html
Because i live in a rural area, and had no means to acquire suitable silencing material to quiet resonances I decided to attempt the build from steel, in the belief that it would be stiff enough to be acoustically dead.

9" anglegrinder with discs, welder, drill press or magnetic drill. hand drill also necessary, vice grips and a straight edge to cut against to prevent the blade wandering.

8ft by 4ft 18mm plywood, void free, birch ply ideally, but anything will do

8ft by 4ft 6mm mild steel sheet, or plate wide enough to fit the driver. this is enough to build a pair of cabinets

66 bolts and nuts, 19mm head, 2 inch long thread, but size is not crucial. sealed nuts are not necessary.

19mm ratchet, long socket and 19mm spanner

5 tubes of clear silicone and silicone gun

assorted drill bits to match the bolts

stuffing material, i used sheep wool, but whatever you can scrounge up will do.

hacksaw blade or keyhole saw

100 wood screws 4cm long

The first failed build was made from entirely mild steel, from a 6mm thick 8ft by 4ft sheet. when struck with a hammer, the cabinet would ring
for 12 seconds, like a tuning fork. after i installed speakers, when i switched off the drive, the cabinet would continue to hum for a few seconds. after careful analysis i realized that the standing waves were bouncing between opposing sides, and that gave me the idea to make each opposing side out of a different material, because the opposite sides would resonate at different frequencies, and the entire cavity would not couple well to either, leading to a strong suppression of ringing. This assumption was proven to be correct after completion. When the hammer test was repeated, as the steel was struck it was acoustically dead, similar to how heavy wood https://www.instructables.com/editInstructable/edit/EGEPMWNKQ6MO0AL/step/1#sounds when struck.

I expect that i would have better results with thicker steel, say 12mm but I used 6mm since i already had the steel bought. I'm sure it also would be cheaper to use a smaller amount of plate instead of an entire sheet, as well as being easier to lift. the composite design is quite conservative on the amount of steel used.

It would save a lot of cutting discs to use a 4-5cm wide 6mm plate for the bolt holes, but since all i had was a mag drill i drilled the
bolt holes along the edge of the sheet, where there was sufficient area to stick the magnet and cut them off after. i made sure to grind down the holes on each side afterwards, to ensure a flush airtight mating to the wood. although you might be tempted to only weld one side, it is advisable to weld both sides to have close to a 90 degree join. The holes were evenly spaced, perhaps 12cm apart. I was not accurate with hole spacing, nor when i cut out the wooden speaker baffle. i did not have a router to make a round hole for the driver, so using an old pulley as a guide, i drew a circle around it in pen and drilled holes around the perimeter. then i pushed a hacksaw blade through one hole and sawed a cut from one hole to the next, by gripping it in my hand. a keyhole saw would be useful here. The steel tends to warp near the narrow end, therefore it is best to cut the timber so that the triangular side overlaps the front. this is so that when the front plywood is bolted down, the warpage it experiences can be traced as a curve on the triangular panel, ensuring a flush fit. I like to clamp the front plywood down with a vicegrip at the warp point, then use a pen to draw a circle inside each bolt hole. drill the bolt holes in the plywood, then use a grinder to remove any burrs.

before screwing both wood panels together, use silicone on both mating surfaces. leave the bolts loose so that you can lean on the plywood to ensure a 90 degree join.

Do not forget to fabricate a short steel bracket at the bottom to stop the triangular panel from vibrating at the front. it's best to point those two bolts out from the inside to reduce the space taken inside. i ground the plywood speaker baffle top and bottom to a 45 degree angle to make it easier to fill the groove with silicone.

The speaker sounds quite good, i did not notice any frequency comb which is universal in voigt pipes, but as this is my first build, i would not be able to identify what it sounds like. at first i used no stuffing and was pleased with the results, then i used too much and it sounded deadened, so i only used a little at the moment. i may end up removing it altogether. i used a wood screw to hold the stuffing in place. The cabinet does not ring at all, but you can hear how the steel speaks. there is a wooden and steel timbre from both materials. some pure tones like in cher's believe makes the audio a bit louder as the steel starts to resonate, but again it does not ring. it would be interesting to see if that effect went away with thicker steel but my budget has ran out, unless i could sell these to fund another version.

I would be interested in hearing from someone with a better ear who recreated this project. as far as i know this is one of a kind. I got the idea for a composite from a commercial voigt pipe site which used mdf and plywood, but not in opposing planes.

I used visaton bg20 full range drivers, which are a wee bit too large, had to grind 6mm away from the inside of the plywood corner. i ensured any air leaks were plugged with silicone. strangely i did not find any bass notch in this build but maybe i have overlooked it somehow. if you were to use these drivers i would suggest making the internal dimensions 21cm wide.