Question

What causes the pitch to change of a siren to change as it goes by you? How is frequency related to pitch?

Answer 1

The Doppler Effect causes a siren to change pitch as it goes by you.

Explanation:

The Doppler Effect is the apparent change in the frequency of a wave due to the relative motion between the source and the observer. When a emergency vehicle speeds by you, the pitch of its siren first increases and then decreases. This is because as the source of sound moves towards you, the waves get closer together, resulting in an increase in pitch, then as the siren moves away the waves spread out resulting in a drop in the pitch.
Visualization (look on left)

Frequency is directly related to pitch - a higher frequency results in a higher pitch and vice versa. Pitch is defined by frequency.