When an airplane flies, it pushes the air molecules in front of it creating a compression wave. As the airplane travels faster, the air molecules are pushed together further and further forming more densely packed compression waves. In the early days of modern aviation, planes approaching the speed of sound were battered by these compression waves bad enough that they could be torn apart. This led to the notion that there was a “sound barrier” that prevented flight at speeds faster than sound. Although these problems were eventually overcome with better airplane design (which allows some planes nowadays to fly at several times the speed of sound), the name “sound barrier” stuck. Thus when an object accelerates past the speed of sound, many people refer to it as breaking the sound barrier.
When the airplane hits the speed of sound (770 miles per hour) the compression waves merge with one another and create a shock wave which people on the ground hear as a very loud noise called a sonic boom. But you don’t have to fly a plane to generate a sonic boom. This can be done with a whip. The sound that is produced when a whip cracks is the sonic boom produced when the tip of the whip exceeds for an instant the speed of sound. However, using a whip properly requires some practice. As it turns out you can use a regular towel to generate a sonic boom. Most people do this by wetting the towel, rolling it up, and snapping it like a whip, which again requires some preparation and practice. But you can also do it like I demonstrate in the video. Hold a dry towel close to the edges, and flip it upwards and then downwards very fast with a curved motion. The edge of the towel will break the sound barrier and generate a loud crack (sonic boom). I have included a section in the video slowed down to 240 frames per second to better visualize the acceleration of the edge of the towel and the generation of the sonic boom.
If you try this, please be mindful of safety. Flip the towel above your head away from your eyes (tilt your head down, don’t look at the towel while you are flipping it), and wear some protection in case the edge of the towel comes in contact with your head.
Have you ever wondered what would happen if you shoot a ball from a truck travelling in the opposite direction in such a way that the speed of the truck matches the speed of the ball? That is what the Mythbuster folks did in this video.