text: The analogy of surface waves on the water 
  these phenomena. Since a 
  may help clarify 
  surface wave is simply the propagation of a 
  pressure disturbance, a ship moving at a speed 
  much less than the wave speed will not form 
  a “bow wave.” 
  As the. ship’s speed nears 
  the wave pro$agation speed the bow wave 
  will 
  form and become stronger as speed is 
  increased beyond the wave speed. 
  
  it should become apparent 
  that all compressibility effects depend upon 
  the relationship of airspeed to the speed of 
  sound. The term used to describe this rela- 
  tionship 
  is the Mach number, M, and this 
  term is the ratio of the true airspeed to the 
  speed of sound. 
  
  At  this point 
  
  ,-I 
  M=; 
  
  velocity and pressure changes occur which 
  create pressure disturbances in the airflow sur- 
  rounding the object. Of course, these pressure 
  the air 
  disturbances are propagated through 
  at the speed of sound. If the object is travel- 
  ling at low speed the pressure disturbances are 
  propagated ahead of the object and the airflow 
  immediately ahead of the object is influenced 
  by the pressure field on the object. Actually, 
  these pressure disturbances are transmitted in 
  in all 
  all directions and extend indefinitely 