This is about distinguish each note overtones.
It is not easy but it can be done.
A string has many modes of vibration. One mode is the string having max vibration at the center. Another is the string having 0 vibration at the center, etc. See bibliography.
When we play a middle C on the piano for example we hear one note but in fact this note is composed of many decaying sinewaves in harmonic relationship. Harmonic relationship does not mean x2, x3, x4, x5 ect in frequency as this sounds very bad, like a cheap sounding calculator. But if the frequency is slightly higher than x2, x3, x4, etc compared to the fundamental frequency it sounds great to human ear brain. And this is how great instruments such as good pianos, guitars, pipe organ are scalled.
How to hear each mode of vibration individually?
The middle C note is pressed on a good well tuned piano without releasing the key.
We wait until the note decays.
Then we play other notes on the keyboard and releasing them, one at a time, until we find the ones that resonate with one of the modes of vibration of middle C (C4).
Once we have heard each one individual mode of the C3 we will be able to distiguishing it together with the other modes when we normally play the C4.
It is like being introduced to many members of a party. Then we know them and can recognize them.
Bibliography:
Waves - Frank S. Crawford, Jr. - Berkley Physics Cource
Musical Acoustics - Donald Hall
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.431.8104&rep=rep1&type=pdf
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