When the speed of objet is faster than speed of sound, it is called supersonic. Usually, it can be said as mach. For example, if the speed of sound and object 340 m/s and 680 m/s respectively, so it is called mach 2.
When the object has speed faster than sound of speed, it will be followed by shock wave. What is shock wave?
Sound is wave, when source emit sound, it will spread to any direction (figure a). If the source moves with low speed, we will get Doppler effects. If the speed is increased it will be like figure c. When the speed of object is increased until faster than speed of sound, as we can see at figure d, shock wave is formed.
Shock wave (Giancoli: Physics Principles with Applications)
The analogy of this phenomenon is when boat makes waves because its speed is faster than water waves.When aeroplane approach the speed of sound, sound barrier is in front of it. To be faster than speed of sound, aeroplane needs extra thrust to through the sound barrier. Once sound barrier is broken by aeroplane, sonic boom is formed. Sonic boom is produced only when the aeroplane breaks the sound barrier.
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