Expert: Paul Soderman - 6/4/2011

Question
QUESTION: Hi,
I know what is sonic boom and how does it occur but,my question is why does it occurs only above the speed of sound.Why does the air in front of the aircraft coagulate it front of it only when it travels above the speed of sound not below that speed..

ANSWER: A wing flying subsonically in still air will create a pressure field around the wing.  The field is a result of a pressure disturbance at the wing that radiates outward at the speed of sound.  Air ahead of the wing feels the pressure gradient in the field and responds by moving and flowing around the wing smoothly more or less.  At supersonic speeds, the pressure disturbance cannot radiate upstream faster than the aircraft flies so the air encounters the pressure field moving with the aircraft as a sudden jump.  That jump or discontinuity in pressure is the shock wave.  The angle of the shock wave relative to the flight path depends on the Mach number of the aircraft.
Paul

---------- FOLLOW-UP ----------

QUESTION: This very well explains how sonic boom works.But i want to know something more about sonic boom....
It is clear that sound does not produce sonic boom,air produces sonic boom and sound is the result of sonic boom,so it means that initially sound has no role to play neither its speed.
Now my question is why does sonic boom takes place at mach 1 only why not at (lets say) .8 mach.Is it a coincidence that the pressure waves cannot move out of the way of the aircraft only above speed of sound as we know that sound plays no role in producing the sonic boom...

Answer
Imagine a body flying right to left.  Now fix a coordinate system to the body and picture the flow moving left to right. At subsonic speeds, say M = 0.8, the pressure disturbances from the body are free to propagate away in all directions in this coordinate system.  The wave fronts have a velocity equal to the vector sum of propagation velocity (speed of sound) plus the local flow velocity.  Only when the body reaches supersonic speeds does the propagation speed of the waves and the local flow speed become equal and opposite in some direction (mostly upstream direction) and the waves coalesce into a discontinuity or shock wave.  That shock wave is attached to the body (or close to it) and travels with the body.

Now put a coordinate system on the ground and let the body fly overhead supersonically. A person on the ground hears the shock wave pass over his ear and experiences a boom. At subsonic speeds, the person hears only the weak pressure disturbance waves from the body if he is close enough, but no boom because there is no shock wave. A book such as  Elements of Gasdynamics by Liepmann and Roshko may make this clearer.  Perhaps you are getting confused by the fact that pressure waves and flow fields propagate as vector velocities. It is only when the flow speed is equal to or greater than the wave speed (Mach one) can we get a shock wave.
Paul