Dual Instrument

experimental 2-player musical instrument

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part of Gamified Digital Instrument Project—GHP 56 Engineering Project for Jason Cho, Malik W., and Sam T.

Dual Instrument was born with a pseudophilosophical mind-wandering on music.

What is music?
We broke it down into the two fundamental (physical) components: [pitch] and [rhythm].

What is an instrument?
We thought of it as a machine that translates human labor into a musical tone. A human-music interface.

Wait, what was music?
[pitch] and [rhythm]. Actually, the combination of the two components, well-synthesized and beautifully rendered... By more than one person and one instrument.

Aha, so music is collaboration.
Usually, musical collaboration is done with multiple individuals each with their own instrument (of same or different type), most likely playing different parts at once.

Then, we asked: what would happen if collaboration is done within one instrument?

Working Principle

  • On one side, the first player rotates the knobs which move the belt-driven "moving frets. " These control the effective length of vibrating strings (distance between the fret and the bridge), therefore the natural resonance frequency and the pitch.
  • The second player hits on the switches, hitting the string with a piano-hammer like mechanism. 
  • The transparent knob casing contains an Arduino which decodes a MIDI file and outputs LED signals at varying points of the knob, directing the signal to the knob player (and potentially the hammer player), acting as an additional communication layer. 

General Construction

  • Plywood - rails, top, body (box)
  • 3D Printed PLA Plastic - knobs, "moving frets," bearing mounts, bridge
  • 608 Bearings & M3/M4 fasteners were used throughout.
  • Violin Strings - The maximum fret-bridge length is actually 30 cm, totally violin range. 
  • (Acrylic) - wanted to use them, but did not have any of them at hand.

Components Breakdown

The instrument is divided into three main parts:

  • Top
  • Piano
  • Drum

Total part count excluding the fasteners is: 137

All the parts were designed with the manufacturing method (laser cutting and 3D printing) in mind, so are mostly flat with equal thickness or have little protruding features (to print without support).

Top Plate Pattern

The creation of the pattern was totally coincidental. We looked for a good pattern, but with very little success (also thanks to how unoptimized Fusion 360 is). We traced a random "curvy pattern" image off of Google, and I felt I was to lazy to trace every single curve. Playing around with linear and circular patterns, the resultant pattern came up, and we were more than satisfied. Maybe except for our instructor who had to sit through the cut for hours that night.

Design Process

Explore the design

First few concepts on paper


The use of Blender to detail out the geometries of individual components in Blender was so helpful in doing CAD later on. 

CAD - v1
CAD - v2

Lack of part organization and sole focus on speed cost us a lot later on (with feature rebuild issues), so we had to redo the most of the CAD before proceeding. (We also did not use Parts-Assembly structure but the Component feature, also not so helpful.) 


dual instrument 2.0 v116.step

STEP file for the instrument!

step - 29.53 MB - 12/31/2019 at 00:40


dual instrument 2.0 v115.f3z

Fusion 360 design file for the instrument!

f3z - 7.51 MB - 12/31/2019 at 00:38


  • 8 × 608 Bearings
  • 20 × M3/M4 Fasteners
  • 1 × Violin Strings Set

  • Now What?

    Jason Cho12/30/2019 at 23:44 0 comments

    I am trying to build another prototype, this time using intended materials (plywood and acrylic sheets, instead of MDF all-around!) and also revive the electronics part of the instrument. 

    Admittedly, the original concept is not very enlightening; we still could not answer the question, "What purpose does this actually serve?" Still, this is a start. 

  • First Prototype

    Jason Cho12/30/2019 at 23:41 0 comments

    We constructed our first prototype and presented it in the STEM Exposition thing on last Thursday of GHP. 

    The precious programs and electronics were taken off of the project because of the tight deadline, which we later regretted, realizing the project did not have to be presented in its refined state or anything. 

    Lessons learned:

    • The design does not account fasteners into account very well, and tolerances were quesitonable
    • The hammer mechanism does not work with the strings. 
      • As a temporary fix, we just plucked the strings with a nickel (coin).
      • More research is required... may be that we could not use soft material on contact point, but more likely just that violin strings (with a lot less tension than piano strings) don't fit the mechanism.
    • The sound is really small. Needs active amplification or just better acoustic design
    • The top plate bent a bit due to the string's tension. The tension can be mitigated with lower string height, but the construction material (MDF instead of plywood) also may have contributed in it. 

    June 2019

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Enjoy this project?



Mike Szczys wrote 01/06/2020 at 22:26 point

Very cool! I'd love to hear what it sounds like. Do you have any demo clips?

  Are you sure? yes | no

Jason Cho wrote 01/06/2020 at 22:52 point

Thank you!

Unfortunately, I don't have the prototype right now, so I will have to build one again to show you how it sounds. 

The sound is pretty similar to that of a violin, except it has the diminishing sustain (don't know if I'm wording this right) like an acoustic guitar. Also, the hammering mechanism turned out to not work with the string very well (very small sound), so we just plucked the string with our fingers when we were presenting it. We were also thinking to modify the hammer part to pluck the string on the upstroke using some sort of a compliant (flexible) mechanism, so the sound will ultimately be that of a violin-guitar hybrid.

  Are you sure? yes | no

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