Piano Tuning Assistant (formerly Piano Temperament Assistant) helps a student of piano tuning visualize the partials and beat rates that are important for tuning a piano by ear. Although more temperaments may be added in a later version, currently all note values assume standard Equal Temperament.
Select two keys in the piano keyboards at the bottom of the screen and see a simulated visualization of the component partials, both on the keyboard and on the graph view. If a partial is detuned from the actual equal temperament note value, a small line marks the amount detuned flat or sharp.
Yellow keys mark where two partials coincide close enough to have a useful beat frequency (less than 20 hz). The beat frequency is also shown in Hertz above the corresponding partials. Tap near a beat in the graph view and the program will play a simulated beat at the exact rate and pitch.
For example, press F3 & A3 (F & A below middle C) and you will see that there are two beats: one near A5 at 6.93 hz and another near A6 at 13.86 hz. If you slowly press F3 and A3 down on an actual piano keyboard without allowing the hammers to strike the notes but releasing the dampers, you can strike A5 to hear the 6.93 hz beat, and strike A6 to hear the faster and weaker 13.86 hz beat.
SOME USAGE NOTES:
If you need to see the help screen, swipe up in the display area above the keyboards. However, you have to swipe in a blank area where there aren't any coincident partials (two close peaks with text listing the frequency and beat rates above).
When there are coincident partials, touch the screen near the partials. Coincident partials will show the frequency of both partials and the computed beat rate in text above the simulated peaks. Press near the text to hear the beat rate. Press again to stop. You can actually press any coincident partial to stop the beat, so if you have a beat playing and change the notes, to stop the sound just press near any set of coincident partials.