Doppler Effect

The Doppler Effect phenomenon is observed anytime the sound source (or the listener) is moving relative to the other.

When the sound source and the listener are traveling toward each other, the listener hears an upward shift in frequency. When they pass and travel away from each other, the listener hears the frequency shift down in pitch. This effect can be observed when you are standing on the side of the road and an emergency vehicle passes with its siren turned on. You would notice a distinct drop in the frequency of the siren after the vehicle passes.

Let's say you are a certain distance from a sound source with no relative move toward or away from each other. In this case, you simply hear the frequency of the sound being created. When you travel toward the source, you encounter the sound compression/rarefaction waves more quickly, so the pitch appears to increase. When you pass the sound source and are moving away, you now encounter fewer compression cycles in a given period of time and, therefore, the pitch decreases.

Doppler Bats

Bats generate short bursts of ultrasonic sounds to navigate through echolocation. Some bats, known as Doppler bats, are also able to detect the speed and direction of a moving object by monitoring pitch changes in the reflected sound.

Going Supersonic - Doppler Shock Waves

The Doppler Effect becomes much more pronounced when an airplane travels at close to the speed of sound. The sound source is traveling so fast that the pitch heard by an observer appears to go higher and higher, eventually approaching infinity. Air molecules cannot support such high frequency waves, so the pressure waves break apart creating the sonic boom that we hear on the ground. The plane itself will experience buffeting as it breaks the sound barrier and encounters its own sound pressure waves.

When the airplane travels faster than the speed of sound, it creates a continuous huge pressure wave like a cone emanating from the nose of the plane, followed by a huge cone shaped rarefaction created by the tail of the airplane. This sound is very loud and unpleasant for people and can often cause real damage to plants and buildings on the ground.