The pitch of a sound is related to the frequency of the sound and the source that generates the sound. It is not directly proportional, but it is directly related in some manner.
As the pitch increases, the voice or sound becomes more shrill. As the pitch decreases the sound becomes coarser.
Let the bicycle move uniformly at v. Let the radius of the wheel be R. Let there be N spokes in the wheel.
f = frequency of the spokes touching the plastic ruler scale
= N v / (2 π R)
Every time the spokes touch the scale, the scale vibrates. As the speed of the bicycle decreases, the sound slowly wanes down.