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At supersonic flow, that is when an object is flying faster than the speed of sound, strange things happen. To explain it let us consider a humming bird that can hover or move with a speed V. The speed of sound (in the medium) is c.
The humming bird issues one chirp every second
M = 0 (incompressible)
The speed of the source of sound (humming bird) V = 0
The speed of sound = c [ft/s]
The listener is stationary
He will hear one chirp per second
M < 1
The speed of the source of sound (humming bird) V = c/2
The speed of sound = c [ft/s]
The listener is stationary
He will hear more than one chirp per second (increase in frequency - source moving to the observer)
Doppler effect
M = 1
The speed of the source of sound (humming bird) V = c
The speed of sound = c [ft/s]
The listener is stationary
He will hear the bird only when it is over his head
Sound Barrier
M >1
The speed of the source of sound (humming bird) V = 1.5c
The speed of sound = c [ft/s]
The listener is stationary
He will hear the bird long after it has past him
The angle shown is called the Mach cone. The sharp line can also be considered as a shock line.
The pressure difference across the barrier can break window panes, eardrums etc.
You can sneak up on the listener by traveling at supersonic speed
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