When playing high notes on piano (above F6) lower notes can be heard. Why is this?

Question

I can only hear this effect on F6 or above, but if I use the sustain pedal, i can hear it from G5. The higher the note the easier it is to hear the low notes.

Answer 1

We call this sympathetic resonance and it happens when two strings are related via harmonics.

This is made especially obvious when we consider the harmonic series:

Begin by pushing down the G right above middle C without having it sound. Then put down the sustain pedal and play middle C; you may hear an upper pitch, which we'll talk about soon. Now try playing the C an octave below middle C while still keeping that G sustained; you should now hear that G loud and clear!

This is because that G is within the harmonic series of the second C that you played (the one below middle C) but not within the harmonic series of middle C. (Although, if you heard a higher pitch sounding when you played middle C, you actually heard a G above the one you were sustaining, because that higher G is within middle C's harmonic series.)

For a more advanced example, try silently holding down the B♭ below middle C. Then play the C below middle C, the C two octaves below middle C, and the C three octaves below middle C. The B♭ will be faint, but you should hear it when you play that lowest C.

If your piano is consistently sounding these lower pitches to a distracting degree, you may need to make sure that your dampers aren't faulty.