Bryan Osckowski and I collaborated in building this two player Mbira. Its a two octave instrument with one side tuned normally. The other side is slightly detuned, so when the tuned note and the corresponded detuned note are plucked together the phenomenon of "beating" occurs. The wavering between these notes is set to mimic the ratios between octaves (1:2). So an 'A' (440 Hz) and its corresponding out of tune 'A' (449 Hz) which causes 9 beats per second, but the next 'A' up (880 Hz), has a corresponding out of tune 'A' at 898 Hz causing 18 beats per second. Its an attempt to harmonize pitch and rhythm.
If we play the same melody on either side of the instrument simultaneously, the slight tuning differences would cause beating, which we thought of as percussion. So the BPMs of this percussion doubles up when going to a higher octave. It could have been done easier with a drum machine.
Before we started we first consulted Bill Wesely, owner and inventor of Array Instruments, who makes similar sized quality Mbiras. We picked up some valuable tips on materials and resonance chamber design.
Inside the resonance chamber.
Smoothing out the tines.