time reversal

the stories of four physicists separated by the whims of fate

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Hi-Fi Speakers

May 4th, 2009 by william

Me with first speaker front panel

Since Spring Break and my trip to Zion (from which I will finish posting pictures sometime), I have dropped completely off the map in a mad rush to build a set of hi-fi speakers and a vacuum tube amp. Well, actually my friend Matthias and I have been trying to build two sets of each. This probably seems like a strange thing to spend time on in grad school, but who cares? We live in the middle of a giant corn field and have to keep ourselves entertained somehow.

All of this started because Matthias has to take eight classes to get his masters before heading back to Germany, so he decided to take a physics of music class to fill out his load. The final project for the class requires everyone to build something related to music, test the hell out of it, and write a vaguely scientific paper describing what you did. His idea was to build a vacuum tube amp.

He told he his idea, and I told him to build two. Figuring the only way to convince him of this was to throw something into the mix, I offered to build two sets of speakers. What good is one without the other? Right?

We haven’t had a free weekend since. There was a good month of researching different components, designs, and deciding what we thought would work best. In the world of audiophile equipment, there is no upper limit to how much things can cost (some people will purchase $100 RCA jacks). In the end we decided to build ported loudspeakers based around 6 inch aluminum woofers and 1.25 inch silk dome tweeters based on the MIT IAP Speaker Design course.

Gluing speaker components together.

Our single largest time sink was building the speaker enclosures. We started with one 4’x8’x3/4″ piece of fiberboard which I deemed was just large enough for all four enclosure. After one evening of cutting all the pieces out on the bandsaw, we had to figure some way to route the grooves for gluing the pieces together. Neither of us had a router or access to one, so we milled everything. This has the advantage that every cut was carefully planned and withing a few thousands of where I wanted it to be. But, it took forever!

Gluing all the pieces together wasn’t a huge deal–everything fit perfectly and we found some large clamps in a nuclear physics lab that helped immensely. We built the cross-overs, wired everything up, and then the testing began. One of the major benefits of a ported enclosure (a box completely sealed except for an open-ended, resonating pipe) is that one can tune the length of the port to change the bass response of the enclosure, and often one can imitate the bass response of a much large cavity.

The speaker are soldered, sanded, and primed. Their bass response is truly impressive (go port tuning!) and their frequency response is almost flat to 14kHz. Things go a little crazy above that, but the tweeters only cost $20 a piece, so I cannot complain too much. In the power spectrum it is not possible to identify the crossover point, and since the drivers are so close to each other on the enclosure, one cannot hear where the transition happens either. In short, they sound pretty awesome. We are still working on the tube amps. One of them will likely be done this weekend. The other will be done at some indeterminate point in the future after some science happens.

Any suggestions as to what color is should paint my speakers? I’m thinking Pantone 201.

Testing directionality of speakers.

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  • 1 josh h. Jul 3, 2009 at 6:26 pm

    is that the most ghetto anechoic chamber ever? because that looks like the most ghetto anechoic chamber ever.