Resources for THEORY AND PRACTICE OF PIANO TUNING
Headphones are necessary for all these demonstrations.
Make sure your volume is turned down before you listen to these clips, and adjust up carefully to a comfortable level.
For ear training: A set of demonstration thirds C4 to E4 (from a Steinway grand piano) with variable beat rates, together with the corresponding isolated partial. The beat rate, as is often the case, is not constant, and here the rate given is the average. The theoretically correct rate for A 440 pitch is of course 10.8 Hz.