Build notes from Stefan Powell’s presentation at Frequency Freaks.

Howdy! So here are some notes from my presentation at Frequency Freaks.

The origin of the idea came from thinking about Aeolian harps, which I’ve had an interest in since I was young. An Aeolian harp is a musical instrument played by the wind: a wooden box with a sounding board and strings stretched across two bridges, placed in a slightly opened window so the wind can blow across the strings and produce sound. The idea of a sound machine that plays in non-deterministic ways has been something I’ve wanted to make for a long time. I was toying with the idea of wind chimes without any percussive sound, and was doing stuff with ribbons outside.

Those ribbons would play once in a while in strong wind, but left me wondering: is there some other “frictionless” way to vibrate a string? I knew that electromagnets need a lot of power, but a strong permanent magnet might not need much mechanical energy to excite a string. So my friend, JP Timmerman, modelled and printed me an 8-slot wheel that I could arrange 8x3mm magnets in.

There was a lot of EM noise from the motor coming through the bass pickup, so I had JP do a 16-slot tapered wheel so I could move the motor away from the strings. That’s Pure Data talking firmata to an Arduino to control PWM on a MOSFET to do variable-speed control on a little DC motor.

At this point, everything seemed to work, but I wasn’t finding what I liked in terms of sound. I was hearing the bass and the effects. This was all just a couple of days before the Toronto Sound Festival. At the festival I lurked around but didn’t see anything that was in my groove. While there, I went on Craigslist, found a humbucker pickup, went back to my workshop and made a new version.

I was going to Winnipeg and staying at a rental house that had a grand piano, so I built a rig that I could clamp into the piano and use the wheel to drive a set of strings. I used that through effects pedals to create Test Flight.

In Winnipeg, the guitar pickup did not work very well. The soundboard of the piano was like hundreds of antennae making a horrible racket. I wasn’t expecting much at that point, but I decided to place my handheld recorder on a little tripod directly on the soundboard, which of course picks up every footstep in the house. Anyway, I was super thrilled with one of those recordings. That recording is November Crows.

I don’t have a piano, and they are a helluva thing to lug on the TTC, so I took a 2x4 and some scraps of wood and cobbled together the duochord, and I’ve been playing with it for a couple of months now. Carrying a five-foot-long 2x4 instrument on the TTC is a remarkable catalyst for generating interesting questions from strangers.

Here is a recording that my daughter called The String’s Song, made using that duochord, and that is roughly what I performed for my first set at Frequency Freaks. That’s the duochord tuned to G#1 and C2. On my second set I changed to G1 and C2.

Here’s a link to the high carbon steel wire I used. I also ordered the tuning pegs from Amazon, but I wouldn’t recommend them, as they are a bit flimsy for the task.

The motor speed controller I used was a DC speed controller from Creatron that uses PWM. This works sooo much better than the Arduino control I was using. If there is a more simple, affordable, plentiful, easy-to-access solution for motor speed control, I’d like to try it. I have not had ANY success with just adjusting voltage with various types of potentiometers.

I found a fella online who built a similar magnetic wheel about four years ago (video on Vimeo). His appears to use a brushless DC motor salvaged from a hard drive, speed-controlled using a hobby brushless speed controller and an Arduino. I think he’s using just one magnet. I like his term “Rotary Magnetic Bow.” That kinda nails it.

Those who are into the mathy stuff might find Matt Baker’s post on the balanced centrifuge problem interesting.

I have a favourite arrangement of magnets that I really don’t fully understand. It lets me shift into different harmonics in interesting ways without a lot of high-pitched ringing. I’m sure someone could figure out a good explanation of the interaction between the pattern and the vibration of the strings. The RPM of the motor for those Frequency Freaks performances was around 1000 to 2000 RPM. There’s also a nice five-magnet arrangement on a 24-slot wheel, although the orientation of the magnets on my 24-slot wheel is not ideal, so I haven’t spent much time working with it.

Like I said at the workshop, I don’t really know where I’m going with all this, but I’m going to keep scratching the itch. What I’d like to see:

• an amplified electromagnet unit to feed a signal into strings
• dual pickups for stereo separation
• workshops to build and experiment with these types of devices
• a CV-controlled motor speed controller
• this technique used for site-based installations
• a nicely crafted, completely mechanical acoustic version

I recorded a piece while I was in Costa Rica:

For that recording, I mounted the wheel on an articulating arm which was tied to the tree separately.

JP posted the wheel file on Thingiverse: https://www.thingiverse.com/thing:4228053.

Do you have a humbucker pickup? Just solder up a jack to the humbucker. I added a volume pot to my first one but never dialed it down, so I skipped that part for the recent pair I made.