An airplane moving with the velocity of the sound generates the shock wave.

A source of acoustic waves (airplane) moves with subsonic velocity. As a result the wavelength of the sound in front of airplane will be shorter than the wavelength of the sound behind it (Doppler effect).

If an airplane moves with supersonic velocity, then the sound waves arising behind it will superpose with each other and form in space a conic surface of high pressure. This is a shock wave. The angle of this cone depends upon the ratio of the speeds of the airplane and the sound (Mach number).

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