The Measure Inharmonicity button begins the process of measuring the inharmonicity of the selected note.
Before tapping this button, you should get ready to play that note with only one string sounding. Mute any other strings of that unison to ensure the inharmonicity reading is as reliable as possible.
After tapping Measure Inharmonicity, the software first waits for the beginning of the note. At this time a status field on the screen will say "Measuring, waiting for trigger". VoiceOver will speak this status immediately without your having to select it. The trigger is the point where a sudden rise in microphone volume is detected, indicating that you have played a note. If the software hears some sound before you play the note, it might begin analyzing the sound too early, resulting in an inaccurate inharmonicity reading. The detection of the trigger is based on a threshold volume level that constantly adapts to the ambient microphone level. If no sound is present for a long time, this threshold could become very low, making the trigger detection ultra-sensitive. If this becomes a problem you can raise the threshold volume level by playing the note once before tapping on the "Measure Inharmonicity" button, and then play it again after tapping on that button. If you hear VoiceOver report "Measuring listening" before you play the note, just wait six seconds for that measurement to complete and then discard it because it is not reliable.
When you play the note and the beginning of the note is detected, the software will analyze the sound for up to six seconds. The VoiceOver will say "Measuring listening" to indicate that the inharmonicity analysis has begun. There will also be a fast ticking sound when the software is waiting for you to play the note and a slower ticking sound during the analysis period. The analysis time is six seconds for lower notes and progressively shorter for higher notes that do not sustain very long. When the analysis period is over, a new display will be shown, giving the results of the inharmonicity measurement.
The ultimate end product of the inharmonicity analysis is called the Inharmonicity Constant. This result is spoken by VoiceOver as soon as the listening is done. If you want to accept this measurement, activate the "Save" button on the results screen. Multiple measurements of the same note are averaged together to create a tuning. The current average inharmonicity constant for the measured note is also displayed on the results screen, as well as a button to clear any previous measurements for this note.
You can also use the two-fingered Zee escape gesture to save the reading and return to tuning.
You can measure inharmonicity for any 5 or 6 notes to make a custom tuning for a particular piano, but normally we recommend that you measure C1, C2, C3, C4, and C5. You can measure more notes or different notes if you like, but it probably will not make any difference in the tuning, since most pianos have a uniform pattern to their inharmonicity, and measuring just these few notes gives the software a good enough estimate of the inharmonicity for all 88 notes.