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What is Shock Compression?

Shock waves are the primary reason you hear what's called a sonic boom. But there's more to a sonic boom than just shock waves. Here are the basics:

  •  Object moving at supersonic speed (airplane)
  •  Medium through which sound can travel (air)
  •  Shock waves

Picture an airplane flying through the air. As the airplane moves, it pushes air molecules out of its way, continuously creating waves of compressed and uncompressed air. These air pressure waves move away from the airplane in all directions at the speed of sound. (Imagine ripples that form by dropping a pebble in a pond.)

Figure 1. F-18 at Mach = 1.4,
altitude = 35,000 ft,
NASA Dryden, pilot: Ed Schneider.

As an airplane flies faster than the speed of sound, it "pushes" on the sound waves in front of it. But sound waves obey the speed limit-they can't travel faster than the speed of sound. So the waves pile up against each other as they are created and compress, forming shock waves.. These "piled up" waves are similar to a "bow wave" that piles up at the front of a boat as it moves through water. The greatest shock waves are at the tip and tail of the plane and their intensity is dictated by the speed of the object and its shape.

This NASA photograph shows the shock waves created by a plane in flight (The "rings" in the photograph are camera artifacts and are not part of the shock waves)

The shock waves will move out and back from the plane, towards the ground. There is a sudden change in pressure when the shock wave hits your eardrum. You hear this as a loud sonic boom.

Schlieren photography allows the visualization of density changes, and therefore shock waves, in fluid flow. Schlieren techniques have been used for decades in laboratory wind tunnels to visualize supersonic flow about model aircraft, but not full scale aircraft until recently. The Schlieren photograph below is a laboratory image that shows the density gradients in the fluid more clearly.




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