 Have you ever wondered what causes the siren of a speeding ambulance to seemingly change pitch as it zooms by you on the street? Actually, there is no change in pitch of the siren itself. What you are experiencing is a perceived sound shift due to a phenomenon known as the Doppler Effect. The Doppler Effect is defined as the apparent change in frequency of a wave caused by the relative motion between the wave's source and the observer. The Doppler Effect can be observed with any wave producing source, waves of water created by a bobbing bug moving across the surface of a pond or sound waves of a siren from an approaching ambulance. The Doppler phenomenon occurs when the object emitting waves moves toward or away from a fixed point or an observer. In front of the object, the waves are compressed. Behind it, the waves are spread out. Let's look at the Doppler Effect as it relates to the sound waves produced by the siren of a moving ambulance. A stationary ambulance produces a consistent siren at a steady pitch. The sound waves are uniform from all sides of the siren. As the ambulance moves forward, it continues to produce a uniform sound wave, yet the sound waves in front of it start to bunch up, increasing the wave's frequency. To an observer, the siren of the oncoming ambulance is perceived at a higher pitch. As the ambulance passes by and moves away, the sound waves of the siren are stretched out. Thus, the frequency of the wave is diminished, resulting in a perceived lowering of the pitch from the observer's point of view. The observed effect is that when the ambulance approaches and then departs, an observer will hear the siren shift from a high pitch to a low pitch.