(side point: for those unfamiliar with IAP, it’s MIT’s way of saying “do whatever the hell you want for a month and maybe even learn something if you want.” I’m TAing an introductory electronics class that focuses on building cool stuff like little EKGs that kids can take home)
Anyways, I thought I’d spend at least a day this month actually learning something. This class spent a few hours going over the theory of how speakers work and then we built some simple speaker “cabinets” to house the transducers that Bose uses in their 201-series desktop speakers. The cabinet? Well, it’s a piece of PVC pipe. 😛
(yes, I installed one of them upside down and yes it’s annoying me)
Looks pretty weird, right? Well, the sound’s surprisingly good! I didn’t expect it to be nearly that good for the materials we’re using. Basically, we installed the woofer and the tweeter onto the PVC and then ran wires to the back. Then, using a resistor and a cap, we created a damped first-order high-pass filter to prevent the tweeter from being driven by low frequencies. Finally, we threw some fluff into the PVC to dampen some resonances. Here’s a shot of the extremely advanced crossover and the fluff.
It wasn’t very technically difficult to build these speakers. Anybody could really do it so long as they had a drill and some PVC; you don’t need much more than that. I was hoping that we’d actually build the speaker cones, but those were all pre-built (they had a pre-assembled one to show how they work).
These cones can be driven with up to 100W or so (though the Bose site claims that the speakers that they’re usually in can be driven to 120). I’ve got a second set of outputs on my stereo amplifier, so I might try to hook them up to those outputs. They also gave us a simple 15W amplifier kit based on the TPA3122D2 chip from TI.
When Jordan and I were testing them out, we managed to find a really strong resonant frequency of the PVC pipe. When a certain note hits, the pipe resonates and makes a really gross sound, louder than everything else. Good cabinet design is really crucial to making things sound good. Except for that frequency, we didn’t find any other glaringly bad parts of the speakers’ sound.
All this made me wish all the more that MIT had an acoustics class. Bose himself used to teach MIT’s acoustics class, but nowadays, there’s nothing. I’ve always had a sort of fascination with sound, but never really got motivated to build acoustics-related projects. Maybe I’ll work on modifying these speakers to sound better and see where things go from there.