Why does toning up a piano make it sound similar to that a string instrument
The human ear, in collaboration with the brain, recognises what it is that is being perceived (as well as where it is coming from) within the first few ms of the sound waveform.
This &quot;what is perceived&quot; (is it a piano? is it an organ? is it a poggo-stick?) is called Timbre.
This modification of the attack phase is sometimes used deliberately.
Here, it is not the delay that is effecting the modification. Notice his hand on the volume knob when the guitar now starts sounding like a violin. It is clearer from 01:43 onwards in the &quot;Without echo&quot; part of the video. Here, he is killing the guitar's distorted &quot;twang&quot; as the string is excited and &quot;brings it in&quot; just a little after that so that you get more of the tone and less of the &quot;twang&quot;. It then sounds like a violin. You might also notice on a piano, if you simply play the waveform after the attack, it sounds more like a decaying pipe-organ rather than a piano.
By &quot;speeding up&quot; the sound, you are distorting those crucial first few ms that the ear/brain are using to recognise the instrument. For example, the instrument now has a sharper attack than a piano. The reason it begins sounding &quot;stringy&quot; is another byproduct of the speeding up as the combination of sinusoids (and their relative phase) might now be tending towards a sawtooth wave that is typical of stringed instruments.
If you are looking to pitch shift but keep the duration of the sample the same, you need to be looking at techniques that attempt to preserve both the amplitude and phase spectrum of a sound.
Librosa implements this with pitch_shift() but other implementations are also available.
Hope this helps.