1456 
VIII, THE RELATION TO THE CRITICAL ANGLE FOR 
REFLECTION AT A RIGID SURFACE 
The conditions for the beginning of the surface attenua- 
tion of the initial shockwave are due to the failure of the Meyor 
corner solution. The treatment of the reflections of a shockwave 
at a rigid boundary is similar: the condition has to be found 
when the reflected shockwave just begins to travel along the 
initial shockfront; that is, when the usual assumption of inci- 
dent and reflected wave are no more correct. 
The difference in solving the two problems arises from 
the fact that for a surface reflection the propagation velocity 
of the reflected wave can be assumed to be the sound velocity of 
the medium behind the incident wave. For the reflection from a 
rigid wall the pressure behind the reflected wave and therefore 
its velocity of propagation is unknown. For this reason an 
additional condition has to be applied: the flow behind the 
reflected wave must be parallel to the boundary. 
The problem can be solved in using the following picture. 
The incident wave (1) with an amplitude @ = P27 anda 
velocity D has an angle of incidence @ , the reflected wave 
(2) with an amplitude @ ’ © P,-f2. anda velocity D' in 
the medium behind the incident wave has an angle @ vetween 
wavefront and a normal to the boundary. The initial conditions 
are 7% , (SF and flow velocity u, = 0. Behind the incident 
shockfront there exists joe ; 2 and flow velocity u in direction 
28 
