You probably heard the same note with slight changes in the timbre (resulting in a brighter or darker sound). They are all C2 played by a piano but we have filtered some harmonics from sounds 2, 3 and 4 in the following way:

Sound	Filtered Harmonics
1	Original sound with all harmonics
2	Harmonic 1 (C2)
3	Harmonics 1 and 2 (C2 and C3)
4	Harmonics 1 to 3 (C2, C3 and G3)

What is an harmonic? What is the harmonic series?

Here is the spectrum analysis of each sound (click over the images to listen to each sound). Each peak in the graphic represents one of the harmonic values in the harmonic series:

1 - original sound with all harmonics

2 - first harmonic C2 filtered

3 - harmonics C2 and C3 filtered

4 - harmonics C2, C3 and G3 filtered

Spectrum analysis generated with Audacity

Why do we keep hearing a C2 even when this and other harmonics are filtered out?

© 2017 José Rodríguez Alvira. Published by teoria.com