The conical angle of a shock wave narrows down as a supersonic aircraft increases its speed.
Work Step by Step
This is discussed on page 367 and shown in Figure 19.20. Here is an alternative proof: an internet search for the term "Mach angle" shows that the sine of the cone's half-angle gives the ratio of the speed of sound to the speed of the plane. As the plane flies faster, the ratio decreases, so the half-angle must also be decreasing.