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Traveling Faster than Sound

by Ron Kurtus (15 March 2008)

As an object moves through air, it must push some of the air out of the way. But also, the object creates sound waves as it moves through the air.

As a moving source of sound, the object causes the Doppler Effect. When that object reaches the speed of sound, air cannot readily move out of the way and a shock wave is formed.

When the object is moving faster than sound, the resulting sounds travel behind the object, creating a sonic boom.

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Moving in air

When an object such as an airplane moves through the air, it increases the frequency of the sound waves ahead of it, due to its velocity. This is the Doppler Effect, where the pitch of the sound is heard as higher when the object is moving toward you and the pitch is lower, as the object moves away.

(See Doppler Effect for more information.)

Doppler effect

Doppler Effect

At speed of sound

When the moving object or airplane reaches the speed of sound, it catches up to the sound waves it is creating, and they bunch up at the front end of the object, forming a shock wave. This is a reason that it is difficult for an airplane to break the sound barrier.

Shock waves form when object is at speed of sound

Shock waves form when object is at speed of sound

Supersonic speeds

When an aircraft travels at supersonic speeds or is moving faster than sound, it leaves the sound waves it makes behind it. These waves fan out and cause a sonic boom.

Object creates a sonic boom

Object creates a sonic boom

Summary

An object moving through air not only must push some of the air out of the way, but it also creates sound waves. This moving source of sound causes the Doppler Effect. When the object reaches the speed of sound, air cannot readily move out of the way and a shock wave is formed. When the object is moving faster than sound, the resulting sounds travel behind the object, creating a sonic boom.


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