The Doppler effect is where the frequency of waves sound different because of an approaching or disappearing sound. For example, a fire trucks siren is blaring as they drive down cooper. While driving down cooper getting closer to the school, the sound waves come toward the school and have a higher frequency. And as the fire truck passes the school, the sound waves move away from the school causing a different pitch in sound. The Doppler effect also determines the speed in moving cars. Radar guns measure the different frequencies between the emitted waves from the gun that bounce off the moving car. This determines the speed in which the car is traveling.