The pitch of a sound — whether it sounds high or low — is decided by the number of vibrations per second of the air transmitting the sound. The more vibrations per second, the higher the pitch. The fewer the vibrations, the lower the pitch. When an ambulance’s siren approaches you, the air waves get pushed together by the speed of the ambulance and the number of vibrations per second that reach your ear steadily goes up. This makes the siren sound louder and higher in pitch. Once the ambulance passes you by, the air waves get further apart and the number of vibrations per second goes down. This makes the siren sound quieter and lower in pitch. So the sound changes from a higher pitch to a lower pitch based on where you are standing. This change in pitch is called the Doppler effect.