Speaker Design: October 2010

FINAL EVALUATION

DESCRIPTION OF PROCESS

It was put forth the idea by Terry, my teacher, to make an electrostatic speaker. Electrostatic speakers had not been made in this class before so we would be the first to do so. At this time I didn’t know what electrostatic speakers were so I researched it to gain a brief knowledge of what it was and the theory of how it works. I decided to accept the challenge and try to make a pair of electrostatic speakers along with Sam, another student. Firstly I had to determine the shape of cabinet I would create; it was recommended to me to use a trapezoid shaped box for better sound quality.

With the basic shape decided I then had to work out the dimensions of the cabinet. I based this on the size of tweeters and woofers I was going to use. I made a front template with circular holes cut out for a woofer, tweeter and port. The template was oblong with a height of 400mm and width of 300mm. With the front plate dimensions I just had to work out the rest of the plates by working back to the back plate. I found this easy to work out each of the dimensions so it did not take very long.

With my dimensions I then drew out my front, back, side, top and bottom plates for my pair of speakers onto 18mm MDF. I cut out the separate plates with the ban saw and also circular saw and then sanded them down smooth with the belt sander. Since I had a trapezoid I had to work out one particular angle on the side plates so that they would sit flush with the front and back. If I were to angle it too much I would have to start again so I took extra time to accurately sand to the correct angle.

I then had to put the plates together to build my cabinets. I used PVA glue and wood screws to put together the plates one by one. Before screwing on my back plates I inserted and fixed a layer of carpet underlay on all interior faces of the cabinets. This is to cancel any unwanted frequencies for better sound quality. I then put in a brace from the front to back to stop any flexing of the wood. Then I attached the back plate to finish assembling. With the cabinets assembled I then had to prepare for paint. I filled all holes and nicks with builders bog to create flat surfaces. I sanded the cabinets smooth with a mixture of the belt sander, electric hand sander and hand sanding. I finished sanding with 400 grade sandpaper for a smooth finish.

I then sprayed primer on the cabinets. I had to spray 3 coats with sanding between each one. 3 coats insured the whole cabinet was covered so the final coat would stick well. I then sprayed two coats of my final colour. I chose to use black as I thought it would go well with the electrostatic part which was yet to come. With the 2 black coats my cabinets were finished.

I then designed the crossovers on the computer and cut them out onto PCB board with a CAM machine. This was quite easy and only took a few lessons. I drilled holes for the components and wires I had to solder on. Soldering took another couple of lessons but once finished I tested them and they both seemed to work fine. I then strapped in my crossovers to the cabinets. I soldered the woofer, tweeter and input to the crossover wires and fixed them with screws to my cabinet.

I set my speakers up to the test rig. One speaker had better quality than the other so I took out the poor crossover and scribed the cuts so there could be no unwanted electrical connections. I fixed ports to give a clearer sound. This all worked so now I had two functional speakers.

I now had to add the electrostatic parts to it which would give it an even crisper sound. To workout dimensions I drew out a 1:1 scale drawing of the perforated metal I was to cut for this. I cut out large pieces of perforated metal, one for each speaker. I used tin slips to create my desired shape. I then had to work out how to fix the metal to the front plate of my speakers. I created spacers to do so by cutting aluminum tubing to the correct length; I used these as spacers. I then screwed the metal to the front plates with the spacers in between them.

It was time to test out if the electrostatic idea worked or not. I hooked up my speakers to the test rig and the speakers were noticeably clearer which what was supposed to happen. I had finished my electrostatic speakers and was able to take them home finally.

I did complete my brief reasonably well as my speakers worked with electrostatic. I didn't correctly calculate the mass of the cabinet to work out how far apart my woofer and tweeter should be but that was compensated by correct tuning of the port. The electrostatic made my speakers sound better as the frequencies were being dispersed at a wider range. So other than correctly spacing the woofer and tweeter I completed all parts of the brief and have completed a positive technological outcome by having running electrostatic speakers.

PROBLEMS ENCOUNTERED

There were a few problems encountered throughout the building process but all were overcome eventually. The first was that I did not understand what kind of speakers I was even making. Electrostatic speakers were completely new to me so I had to take some time to research and learn a bit before I could start making my own. Another issue was that the ports which I bought for my speakers were much too big so they could not be used. For this I went to a plumbing store a bought some PVC pipe to use as port. This worked just as well; it just didn’t look as good aesthetically. Also while in the painting process my partner and I had not cleaned the spray gun out fully so then the paint dried inside it and blocked it. We had to wait for about a lesson before this was cleaned out. One of my crossovers did not work properly the first test but this was easily fixed by using a scriber to make my tracking more defined. My input plugs wouldn’t easily fit in my small input holes so I had to rasp out the holes slightly before fitting them. All my problems encountered were reasonably minor and each only set me back in time briefly; I was lucky no major problems occurred.