This one is straight from Frank:
The upcoming pianoscope 3 will be able to use the multiple microphones that already exist on iPhones and iPads to do pitch analysis for even better and more consistent results in the high treble. For even more fun, you could also use an external microphone array or an audio interface with multiple inputs.
Currently, I use a Korg CM-300 contact mic that I'm very happy with.
I'm very excited to see Frank continue to innovate in piano tuning technology. pianoscope has been an absolute game changer for me and the tuning stability of my pianos.