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. 
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. 
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 
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. 
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. 
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-