If you play piano, you might have heard of the Stopper Tuning system, developed by Bernhard Stopper some 30 years ago - it's called 'Pure Twelfth Equal Temprament' - and not being a piano player myself I can't hope to explain it, so I'd advise some research, and maybe check out this video here
Anyway, Bernhard has been busy since and developed some tuning apps (check out his website for more info and free versions) and while he was doing so, noticed that some of the visual displays he was working with could also indicate the optimum location for placing microphones when recording or playing live. The TUNIC Unisono 'Sweet Spot Indicator' can help you find just that - it runs on mobile devices and can show exactly where the microphone should be placed for best results.
More info at Piano Stopper
Friktion strings, iZotope RX-8, Pendulate, Jupiter 6, FSS MTX8