12PWK 2010 - Evaluation

This year’s task in 12PWK was to design and make a pair of fully-functional speakers.

We began by drawing 10 ‘conceptual designs’. Of these we selected ONE design which we would create. I chose a ‘giant headphones’ design where there would be two speakers joined together in the shape of over-head headphones.

I began making a 1/1 scale wooden template of the so-called headphones using MDF. This template was not for use with the actual cabinets, but just to express our designs on a 1/1 scale. Once this was completed, we made ½ scale mock-ups using old political billboards.

However after I made this up, Terry announced there would be a possible 6 level 4 credits for anyone using recycled materials. So I went down to the Happy Valley tip shop and picked up two identical microwave/grill ovens. These would be my new design.

I began preparing the microwaves by stripping all the electronics from the insides. This involved removing all the cables and glass panel out front. Once it was fully stripped, I put the microwaves back together.

Now it was time to make a wooden template for the front of the microwaves, where the speakers would sit. This is where I encountered my first problem. There was no wood in the workshop that would fit flush on the front, so Terry suggested gluing a few sheets together to make up the width. So I cut out 4 identical pieces of ply, so I could glue two sheets together. I did this using PVA glue, clamping them together for the weekend to dry.

Once it had set, I had to cut out three holes for the woofer (110mm), tweeter (70mm) & port (60mm). This was easy, as the drill press and circle cutter did most of the work. Once this was completed, Terry told me to remove the elements from the microwaves as they served no purpose. I did so, using a small hacksaw to remove both the top and bottom elements from each microwave.

Before I could drill the template on, I had to soundproof the microwave using foam. So I cut out 12 pieces of foam, and lined all the internals with them. I used PVA to stick the foam down everywhere except the wooden template, which had the foam stapled to it.

Once it was completely insulated it was time to drill the templates on. I marked out the holes on the template and microwaves, and drew pilot holes for the screws. Terry insisted I counter-sunc the holes, so the screws would sit flush on the wood. I then screwed the templates onto the microwaves. I then cut crosses in the foam so the speakers and ports could fit in.

I then had to put the terminals for the wires onto the cabinet. I drilled a hole next to an existing hole in the back, making room for the two prongs on the terminal. I screwed the terminals on with two screws each.

This is where I encountered another problem. Where most other peoples ports were very simple (a perfect circle), my microwaves had metal covering the top half of the port. This meant I had to cut a section out of each port with the band saw so it would fit. On my first attempt, I cut it too short so both ports were useless. However my second attempt worked because I cut it slightly larger, and then sanded them back. To fix the ports to my cabinets, I made some braces using aluminium. Terry cut out the 4 aluminium pieces in the metalwork room next door, then I drilled two holes in each brace then finally bent them in half. I joined these to the ports using the rivet gun. I then screwed on the ports to the cabinets.

Last thing to do was to lacquer the front wood. I masked up everything else using newspaper and masking tape. Terry sprayed them with clear lacquer in the spray room. I decided I wanted the wood to look darker, so Terry sprayed two coats.

Finally the cabinets were completed. It was time to move on to the crossovers. I drew these up on freehand and 2D-design. The crossovers were cut out by the Roland CX300 cam machine. However the cam machine didn’t make the tracks deep enough, so I was forced to scribe the circuit out. After, I drilled a small hole in the centre of every circle on the crossovers. These were for the components.

Before I could solder on the components, I needed to sort the wiring out. There were 6 wires per crossover. Each of these wires had to be stripped at both ends and tinned before use. I soldered these, along with the components onto the crossovers. Now that the crossover was complete I had to test it out with a tweeter and woofer.

This is where I encountered another problem. Because when I tested the crossovers out, the first worked fine, but the second crackled loudly. Terry explained this was a fault in the tracking, and that I needed to re-scribe the circuitry. I did as he said, and the crossover started to work correctly.

It was then time to solder the wires to the speakers and terminals. I started by drilling a hole through the back of each microwave for the wires to go through. I then soldered the two wires to the terminals. Before I could put the woofer in, I needed to stick the crossovers to a brace inside the microwave. I did so, the brace sitting in the top-right corner in each microwave by itself.

I got two 5 ¼” woofers, one for each cabinet. I soldered the two wires (positive & negative) to the appropriate terminal on the woofer. I then drilled 4 pilot holes and screwed the woofers on. However I struck another problem here. The hole was too small, so I had to sand and file down the sides until the woofers would fit flush. This took a surprising amount of time – probably around a week.

For my tweeter, Terry announced I was getting top-end ribbon tweeters for getting the ‘first in class’ award. However these were a completely different shape than the old circle tweeters. So I had to make two bases which would sit on top of the existing hole. I used the scroll saw to cut the tweeter shape out of the base. I had to sand the base for a while, so the tweeter would fit correctly. Once the bases were complete, I screwed them on to the wood.

Lastly, I had to cut out the tweeter shape out of the cabinet. This is where another problem arose. Terry suggested chiseling out the shape, but while doing so, the wood chipped. This was fixed using special wood bog.

Terry then sawed out the remaining wood, paying special attention not to worsen the small chip while the bog was still drying. I sanded the shape so the tweeters would fit, then soldered and screwed the tweeters in.

I thought the speakers were finished, but when I tested them out, I noticed a final problem. The speakers were rattling.  I thought this had something to do with the crossovers, but when Terry heard, he realised it was the back metal that was rattling. As the back wouldn’t be seen when in use, I just removed the backing. Last thing to do was to re-attach the terminals to the new back. I drilled two holes in the back for the prongs and re-soldered the wires. I then screwed the terminals on.

The speakers were finally finished!