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.