I recommend you take your mic and move it to at least 10 different spots on a concert grand piano all over the soundboard, akin to the paper by Fazioli in 2017 and their contact mic experiments. Every spot is noticeably different.
I started my music school training as a teenager by becoming a recording engineer and I put myself through music school that way. Years later I went from being a music professor to getting my advanced degrees in engineering. Of course I now have a fundamental understanding of the situation.
Without the physics training, a recording engineer will find out the hard way about small mic position changes. Now, this is so well documented in the literature, of course. Using the same mic for your ETD, and doing lots of positional changes, listen to the sound subjectively as though you are professionally recording the piano, not just using the tuning app. If the recording sounds subjectively 'good' it is probably in an ideal spot. It is 'hit and miss'. My experience with the technical analysis that I present in my classes is that small position changes will affect the tuning by 1 cent or more, worst case.
The ETD makers quietly acknowledge this problem. They don't want to expose it because it undermines their salesmanship of the app.
This problem led me to invent the PianoSens device. It is not just a 'guitar like pickup.' As an EE, I solved many problems doing this design.
I place the sensor < 1/8th string length to pick up all the partials > 8 partials, and making small movements of several centimeters in this 'region' and it has literally no discernible variance, which is loically to be expected. You can see small amplitude differences, of course, but not much. Tuning variances are now about 0.1 cents, not 1 cent.
Steven Norsworthy
Professional Engineer, Professional Musician
RF2BITS.com
PianoSens.com
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Steven Norsworthy
Cardiff By The Sea CA
(619) 964-0101
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