The pitch of a sound is related to its frequency, which is the number of vibrations (or cycles) per second. When you speed up or slow down a sound, you are changing the rate at which these vibrations occur, thus altering the frequency of the sound wave.
When you speed up a sound, you are compressing the waveform, which means that more cycles of the wave occur in a given amount of time. This results in an increase in frequency, making the pitch higher. On the other hand, when you slow down a sound, you are stretching the waveform, so fewer cycles occur in the same amount of time, leading to a decrease in frequency and a lower pitch.
Imagine a simple sound wave with a certain number of peaks and troughs occurring in one second. If you speed up the playback of this sound, more peaks and troughs will pass by your ear in one second, leading to a higher frequency and a higher pitch. Conversely, if you slow down the playback, fewer peaks and troughs will pass by your ear in one second, resulting in a lower frequency and a lower pitch.
This phenomenon can be observed with various sounds, including musical notes, voices, and any other sound with a distinct pitch. It's essential to note that the speed at which you change the playback affects how much the pitch is altered. If you double the speed, the pitch will generally increase by an octave (a doubling of the frequency). If you halve the speed, the pitch will decrease by an octave (a halving of the frequency).