My Racecar-Inspired Speakers

As some of you may know, I graduated from Michigan Technological University with a B.A. in Sound Design. One of the most satisfying courses there is called “transducer theory,” wherein we learn about how speakers work. The coursework then has us use our knowledge to build a set of speakers, which can either be to our own spec (which we can then keep if funded with our own money) or to the spec required by an on-campus location (wherein the school funds the build). I opted for the self-directed option.

The project began, like many, by deciphering exactly what my needs were for the final product. My conclusions here were simple: I needed to be able to fit my entire setup in my desk unit, I needed full range coverage (20Hz to 20KHz) with +-3dB variance, and I needed to not spend more than $1,000USD on the materials. Because I was designing the system to fit to my desk unit, I had very specific size restrictions which would guide my final design.

Using the Winspeakerz speaker design app and thumbing through Madisound and Parts Express catalogues, I had experimented with many box designs. But it was when I came across the Eton 25SD-1 tweeter that my design really began to take shape. The Eton features a wonderfully flat response through its range, plus its response allowed for amazingly low frequency output for such a tweeter. With it I was able to make my crossover point unusually low, at about 1KHz. I then selected the HiVi M8a as a good pairing with the tweeter. It was a bit on the budget end and didn’t go as low as I wanted, but moving up to a beefier driver would have resulted in going over budget.

Here’s the M8a Winspeakerz theoretical output in a 1 cu-ft box port-tuned to 30Hz:

I learned quickly that I was never going to achieve the bass response I desired with my space restrictions in a 2.0 setup, so I committed to a detached subwoofer in 2.1 arrangement. Using the biggest box I could comfortably apply, I committed to a Peerless XXLS 12″ driver.

Winspeakerz with the Peerless in a 4 cu-ft box port-tuned to 25Hz:

I then was ready to model the setup, which I did in Google Sketchup because I could afford free.

Here you see my desk, to scale, with my final model of the speakers including ports but not including drivers. I included an internal bracing structure that would reinforce panel rigidity for a more consistent reflection from the panels themselves, as well as to improve port response.

From there it was time for construction. All of the boxes came out of 2 5×5′ sheets of 13 ply baltic birch, which is great for speakers as the wood is very rigid (and heavy and thick, too.) Always remember to measure thrice cut once. As I may have drilled some holes in wrong places a couple times.

At this point I was also able to start designing the internal electronics that make up the crossover network in the mains. (The sub would get a dedicated amp with crossover capability.)

Given the tweeter’s low-hitting spec I initially aimed for a 4th-order Butterworth-type crossover centered at 900Hz on both sides.

I was using Fuzzmeasure for all spectral analysis. Initial tests showed that on the tweeter side I had actually set too low, and was getting some clipping. Graphically I could see I also had another 100Hz I could comfortably move up and have no issue.

(here is just the speakers w/crossovers in the boxes, a baseline test if you will)

(and here they are working together, after having moved the tweeter up the 100Hz)

From there it was a matter of tuning the box (via use of fiberglass to dampen and “make the box act bigger” as well as adjusting port length) and adjusting the electronics (via adding a Zobel to turn down the highest of frequencies in the tweeter and tweaking some of the peaks from the woofer) and repeating the box tuning in the sub.

Sub with ground-plane testing:

here is the difference between the initial (green) and the final (yellow)

and the sub final (yellow and blue are init (gain knob bumped) and red the final)

From there it was time to decide what sort of finish I wanted on the final product. I decided I wanted to do racing stripes, because why not? I also thought it would be cool to have the underlying “stripe” be the wood grain. I used a very light stain just to add some depth to the color of the grain, and then with some scrap started testing the finish procedures (since I’ve never tried painting stripes on anything before).

Came out pretty good if I do say so myself.

Time for the real thing:

Which came out really well I think.

Check out that cell phone pocket:

Off-axis response tests (15° increments from 0 (red) to 45 (purple) — sweet spot is right at 15° which means when the speakers are set up on my desk they’ll only need to be slightly tipped in, maximizing space

And finally the first reality test; I put my new speaker up against the Sony MB350H I had previously been using for this role:

I was invited to present my project at the Kennedy Center American College for the Theater Arts (region 3) regional show, which was a huge honor. (I was there presenting a sound design at the same time as well, which was amazing!)

Finally, they made their way home to fill the space they were designed to be in.

As for my budget, NAILED IT.

I learned a lot over the course of this project. Sure, lots of the things I learned were about the innerworkings of speakers and also their construction techniques. But more importantly I learned about my own workflow habits, about what motivates me to keep a project going, and about the importance or organization, planning, and scheduling. I logged a total of 78 hours between construction and testing on this project, which had to be wrapped around shop openings, other school work, and facility availability, with a start->finish duration of 10 weeks. I learned how to set short-term goals, keep a log, and plan ahead for problems that inevitably creep up. The fact that I completed the project slightly ahead of schedule says I did an excellent job at keeping on task. When I look back on this project, I think it is the most proud of myself I have ever been in its completion.

There are a ton more pictures right here if you want to see more, and there’s a page on the department wiki that has more detailed final specs plus plays host to the awesome design proposal.

It’s not cars, I know, but when I’m listening to cars and writing, this is what I’m listening to them on.