I'm enclosing 3 pictures to help explain this.
Imagine a jet flying along. It's a noisy
jet, so it's emitting sound waves all the time.
These waves travel out from the jet at the
speed of sound. If the jet is traveling slower
than the speed of sound, this will look like the
picture"subsonic.jpeg". Let me explain this
picture. The circles around the jet are the sound
waves. The outer-most circle is sound that was
produced the earliest, since it's expanded the
most since it was emitted. The innermost circle
was produced last, just a moment before the
picture was drawn, since it hasn't gotten very big
yet. (Imagine dropping pebbles in a pond and
watching the ripples. The biggest ripple is from
the pebble you dropped the earliest.) The
important thing to notice about this picture is
that the waves are getting bigger faster than the
jet is going.
See -- right ahead of the jet,
are waves that are going even faster than the jet
itself -- they're going to arrive where the jet is
heading before the jet!
Now look at the
next picture, "sonic.jpeg".
This jet is going
exactly the speed of sound. There are no waves
out ahead of the jet anymore. They're right with
the jet, all built up at the jet's nose. This
should make sense. The jet is moving at the speed
of sound. So are the sound waves.
will all travel along together, at the same
To exceed the speed of sound, the
jet must break the "sound barrier" --
all those waves that are built up right around the
When the jet breaks through, and
flies faster than the speed of sound, it will
appear as in the last picture, "supersonic.jpeg."
Now the jet is going faster than the waves.
There are no waves out in front of the nose like
in the first picture. But look what's happened.
The circles aren't inside of each other like in
the first picture. They're overlapping! The"V"
lines show the shock front that has now formed.
If you're standing on the ground, when that "V"
reaches you, that means all of that sound is
reaching you at the same time. All of the circles
(the sound waves) will reach your ear at the same
time and it will be really LOUD! That's the sonic
FYI, Here's where I got the pictures
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