NAVWEPS OD-8OT-RO  APP,LICATlON OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  be reflected and attenuated to some extent  depending on the character of the reflecting  surface. Of course, if this attached wave form  is carried,across  a populated area at the surface,  th.- population will  experience the pressure  waves as a sonic boom.  by the local flow changes at these surfaces.  Of course, the strength of the shock waves  and the pressure jump through the wave  decreases  rapidly with distance away from the  airplane. While the pressure jump through  the shock wave decreases  with distance away  from the surface, it does not disappear com-  pletely and a measurable-but very small-  pressure will exist at a considerable distance  from the airplane.  The intensity of the boom will depend on  many different factors. The characteristics of  the airplane generating the shock waves will  be of some importance since a large, high drag,  high gross weight airplane in flight at high  Mach number will  be transferring a greater  energy to the air mass. Flight altitude will  have an important bearing on boom intensity  since at high altitude the pressure  jump across  a given wave form is much less. In addition,  at high altitude a greater distance exists be-  tween the generating source of the pressure  disturbance and the ground level and the  strength of the wave will have a greater dis-  tance in which to decay. The ordinary vari-  ation of temperature and density plus the  natural turbulence of atmosphere will tend to  reflect or dissipate the shock wave generated  at high altitude.  However, in a stable, quies-  cent atmosphere, the pressure wave from the  airplane in high supersonic flight at high alti-  tude may be of an audible magnitude at lateral  distances as great as 10 to 30 miles. Thus,  supersonic flight over or adjacent to populated  areas will produce a sonic boom.  Sound is transmitted through the air as a  series of very weak pressure waves.  In the  ordinary range of audible frequencies, the  threshold of audibility for intensity of sound  is for pressure waves with an approximate  R.M.S. value of pressure as low as 0.0000002  psf. Within this same range of frequencies,  the threshold of feeling for intensity of sound  is for pressure values with an approximate  R.M.S. value of pressure of 0.2 to 0.5 psf.  Continuous sound at the threshold of feeling  is of the intensity to cause painful hearing.  Thus, the shock waves generated by an air-  plane in supersonic flight are capable of creat-  ing audible sound and, in the extreme case, can  be of a magnitude to cause considerable dis-  turbance. Pressure jumps of 0.02 to 0.3 psf  have been recorded during the passage of an  airplane in supersonic flight.  As a result,  the sonic “booms”  are the pressure waves  generated by the shock waves formed on the  airplane in supersonic flight.  Actually, it is not necessary  for any  airplane  to fly supersonic over or adjacent to a popu-  lated area to create a sonic boom. This  possibility is shown by the second illustration  of figure 6.14 where an airplane decelerates  to  subsonic from a supersonic dive.  As the air-  plane slows to subsonic from supersonic  speed,  the airplane will release the leading bow and  tail waves which formed as the airplane accel-  erated from subsonic to supersonic  speed. The  release of these shock waves is analogous to  the case where a surface ship slows to below  the wave propagation speed and releases  the  bow wave which then travels out ahead of  The source of sonic booms is illustrated by  figure 6.14. When the airplane is in level  supersonic flight, a pattern of shock waves is  developed which is much dependent on the  configuration and flight Mach number of the  airplane. At a considerable distance from the  airplane, these shock waves tend to combine  along two common fronts and extend away  from the airplane in a sort of conical surface.  The waves decrease  in strength with distance  away from the airplane but the pressure jump  remains of an audible intensity for a consider-  able distance from the airplane. If the wave  extends to the ground or water surface, it will 