How Spatial Position Affects Piano Tuning

Over a given time window of measurement, each partial has 3 parameters: Frequency, Amplitude, Phase
A mic position affects all 3 parameters, and most importantly for tuning, the frequency is affected spatially by a spectral tilt induced by the soundboard resonances in the acoustic wave.
The position of a string motion sensor on the wire does not affect frequency accuracy, but only affects amplitude and phase.
The purpose of this experiment:
To show the spectrograms of the partial frequency drifts as observed from each pickup.

[@rickclark1372]

Those are top grade mics placed in good positions. Imagine how much wilder the deviations would be where ETD mics end up getting positioned for a tuning- which is to say for good line-of-sight between the tuner and the screen, not good audio. Not to mention whatever weird characteristics cheap built-in device mics have.