Question

1. why does the pitch of an ambulance siren seem to decrease as it moves away from you? what causes this?

Explanation:

This is due to the Doppler - effect. When the ambulance moves away, the sound waves are stretched out, increasing the wavelength and decreasing the frequency, which is perceived as a lower pitch.

Answer:

The Doppler - effect causes the pitch of an ambulance siren to seem to decrease as it moves away.