DIY

MUSIC

ELASTIC STRING BASS

Optically amplified rubber band twang.

By Len Keeler

I’m always inventing new demonstrations for school. To show how inductive pickups work, I once built a comically large guitar that I strung with steel cable. Later I decided that optical sensing is more versatile, since it works with strings made from any material, and it’s also actually easier.

Photograph by Matt Blum

So I came up with this rubber-band bass. Plug it into a standard guitar or bass amplifier, and you can play amazingly low frequencies and cool sounds. Each rubber band sits between a paired infrared LED and receiver, and as it vibrates, it varies the amount of light detected. Each string’s signal is then amplified and mixed with the signals from other strings.

Rubber bands sound very different from steel or nylon strings. Their tone is rich in harmonics, and the high frequencies damp out fast. Rubber’s high elasticity also means you can generate unusually low notes out of short lengths of band.

Because the amplifier requires both positive and negative voltages, I power the guitar using two 9V batteries, which are switched with a single dual-pole toggle. A red LED indicates when power is on.

My original version had 4 elastic bands, one much longer than the rest. For simplicity, this article shows how to build a single-string version, which you can easily extend to accommodate multiple strings.

1. Plan the overall layout. Figure out how you’ll fit the circuit board, components, and batteries into your guitar body (Figure A, page 139). My sandwich maker’s interior measured 4"× 4"× 1¼", so I had to trim the board a bit (Figure B). I used a saw, but you can also score a line with a file or Dremel and snap the board along the line. If you’re building your own guitar body, leave extra room for wires and components; it’s easy to underestimate.

Make: 137