The A4 Numbers
The “A4 numbers” are the locations of the 2nd, 4th, and 8th partials of A4 when A4 is tuned to A440 (1st Partial).
Robert Conrad, Registered Piano Tuner/Technician, Tucson, AZ
The “A4 numbers” are the locations of the 2nd, 4th, and 8th partials of A4 when A4 is tuned to A440 (1st Partial).