At the start of Back to the Future, we find Marty in Doc’s lab, standing in front of a six-foot-tall speaker with a guitar in his hands. One strum on the strings sends him flying back into a bookshelf, emerging covered in debris. This scene was the boyhood inspiration for two University of Wisconsin students and their attempt to recreate that speaker.
Kyle Hanson and Dan Ludois undertook this project as a labor of love while working on their PhD theses. They fabricated the entire coil driver by hand, including winding three layers of 18awg wire on a lathe. Then with the help of some friends, they built a monstrous 8′ x 8′ speaker cabinet.
The speaker cone was made from polycarbonate and attached to the cabinet using foam insulation. Once completed, the crew had some interesting notes on the speaker’s performance:
Testing the speaker was a FUN time. Electrically, the speaker was driven with a 20kW PWM voltage sourced inverter (intended for motor drives and microgrids) which was powered by a 400V DC power supply. At low frequencies (~10 Hz) peak coil currents were 100A. We connected the amplifier to a number of audio sources including iPods, bass guitars, and Matlab audio scripts. The mechanical sweet spot of operation was 5Hz to 50Hz, that’s where the speaker put out appreciable audio power. Above those frequencies the inertia of the cone was simply too large. We found the resonant frequency of the building to be approximately 7 Hz, as nodes cropped up around the building resulting in numerous complaints, headaches, and all-around team giddiness. The power limitations occurred in two places, cone rigidity and mass, and the thermal capability of the voice coil. At low frequencies the cone experienced structural modes that wanted to tear it apart (we affectionately refer to them as “taco” modes due to their shape) while the high currents in the voice coil cause it to overheat. Regardless it performed well, and we a had great time doing it. Plus, if we ever decide to improve it, we know what to do. Below is a video of a “chirp” function generated in Matlab and fed into the amplifier. The audio of the video does not do any justice for the in person experience. In the video we can clearly feel vibrations in our chest and gut, an eerie feeling…
For more info and full documentation, check out Dan Ludois’ site.