I had the awesome opportunity of taking the EE423 Speaker Design class offered at USC while I was going to school there. The class is taught by engineers from JBL, with some help from people like Tom Holman (the TH in THX guy!), who happens to be a researcher at USC. I learned a lot about speaker design, and ended up with a pretty awesome pair of bookshelf monitor speakers.
The speakers use Vifa TC14SG49-04 5.5″ midwoofers and Peerless 53 NDT Neodymium 1″ tweeters. The inital boxes were prototyped in cardboard.
Despite looking absolutely terrible, using 3 layers of sandwiched cardboard actually works extremely well in terms of audio response.
Here you can see a diagram showing the 200-10 khz response (tweeter is thin line, midwoofer is thicker blue line). It worked much better than I had thought it would.
Here is the same test with the final wooden boxes. You can see the extremely similar performance.
One of the best things I learned in this class is just how insensitive your ear actually is. As you can see from the response charts, there are a couple of 6+ dB dips in the response of the drivers, and I was unable to tell the difference. It’s still possible to fix a lot of that by using a “crossover network,” a circuit that “splits” the incoming signal from the amplifier, and sends the appropriate signal to the matching driver. High frequencies go to the tweeter and lower frequencies go to the midwoofer.
This did a pretty good job smoothing things out.
In the above graph, you can see the crossover network in action.
Final response chart for the speakers
To conclude, the speakers sound amazing (and look pretty awesome, thanks to some help from my friend/lab partner Tyson Yaberg and his dad). You can see the final 20 hz – 20,000 hz response chart for the speakers. Thanks to an excellent crossover network, the response is remarkably flat (way flatter than my ears are able to discern), and they perform very well over all. And I only spent about $200 on them! Hooray for awesome, cheap speakers!