Does an aircraft moving faster than the speed of sound
produce a sonic
boom continually as it moves overhead or only as it breaks the sound
barrier? What causes the sonic boom?
When an aircraft is moving faster than sound a shock wave is formed on the
leading edges of the aircraft. This shock wave is there as long as the
aircraft is moving faster than sound. We only hear the sonic boom when this
shock wave passes over us.
To see why the shock wave forms, draw an aircraft on a piece of paper (make
it about 3 inches long and facing the nearest edge of the paper. Now let's
imagine sound coming from the front edge of the airplane while it is
traveling at twice the speed of sound. Use a compass to draw circles
representing the soundwave radiating in a sphere from the point where it is
generated. The first circle will be 1/2" in radius and 1" behind the nose
of the airplane (in the time the airplane traveled 1" the sound travelled
half that distance -- i.e., the plane is travelling twice the speed of
sound). The next circle will be drawn 1" in radius, 2" behind the nose of
the airplane. Then:
Radius Distance behind nose
You'll notice that if you draw a line that is tangent to all the circles it
will emanate from the nose of the airplane. This line represents the shock
wave moving out from the nose of the plane. Where it crosses the ground is
where the sonic boom is heard. All the sound waves
The aircraft makes a sonic boom as it passes overhead, so long as it is
moving faster than sound. Sound waves emitted forward get compressed into a
conic shock wave. When that cone crosses your ears, you hear a boom.
A visible example of a shock wave is the "V" that extends behind a
speedboat. The boat moves faster than the water waves. Like sound from a
supersonic jet, the forward waves get compressed into a single pulse.
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Update: June 2012