When an ambulance passes by with its siren blaring, why does the siren’s pitch sound higher when it is approaching you than when it is moving away? Originally proposed by Austrian physicist Christian Doppler, the **Doppler effect** is a phenomenon that explains this apparent change in pitch.

#### The Doppler Effect

Sound travels through the air in waves. Each sound has a **frequency**—the number of waves that pass a given point each second—measured in hertz (Hz). The **pitch** of a sound is your ears’ interpretation of the sound’s frequency; the higher the frequency, the higher the pitch you perceive.

The sound of an ambulance siren has a frequency that is subject to the Doppler effect because there is relative motion between the source (the ambulance) and the observer (you). This means that the original frequency emanated by the source—the actual pitch of the siren—is not what the observer experiences; you hear a different pitch.

The observed frequency is *higher* than the original when the distance between the source and the observer is decreasing (whether it be because the source is moving or because the observer is). In other words, the observed frequency is *higher* when the relative motion between the source and the observer is toward each other. On the other hand, the observed frequency is *lower* than the original when the distance between the source and the observer is increasing—when the relative motion is away from each other.

Read more about the Doppler effect here: http://www.physicsclassroom.com/class/waves/u10l3d.cfm

#### Explore More

Below is the Doppler equation. If a source emanates a wave with frequency , it will appear as wave with frequency to an observer, as defined by this equation:

where is the velocity of waves in the medium, is the velocity of the receiver or observer relative to the medium (positive if the receiver is moving away from the source and negative if moving towards the source), and is the velocity of the source relative to the medium (positive if moving away and negative if moving towards).

When an ambulance approaches you, you record the pitch of its siren at 3166 Hz. When it moves away, the pitch is 2850 Hz. If you’re stationary and the speed of the ambulance is constant, how fast is it moving? Take the velocity of sound to be 343 m/s.