Tuning a piano by ear is a skill that requires training, experience, and a good sense of pitch. If the middle octave of a piano, from middle C to the B above middle C, is perfectly in tune, it can serve as a reference for tuning the rest of the piano.
However, even if the middle octave is in tune, it does not guarantee that the entire piano will sound good when tuned by ear. The tuning of a piano involves adjusting the tension of the strings, and slight variations in tension can cause the pitch to drift as you move across the keyboard.
Tuning a piano is a complex process that requires attention to detail and precise adjustments. Professional piano tuners often use electronic tuning devices to assist them in achieving accurate and consistent results. These devices can detect the frequencies of the strings and help the tuner make precise adjustments to bring the piano into proper tune.
While it is possible for an experienced piano tuner to achieve satisfactory results by ear, relying solely on the middle octave as a reference may not guarantee a perfectly tuned piano throughout. It is generally recommended to use electronic tuning devices or software to ensure the best possible tuning outcome.