Mach Numbers and the Sound Barrier

This is a value commonly used to describe the speed of an aircraft.

It’s not a constant value. It represents the speed of sound in whatever air is being used at the time. The speed of sound changes with elevation and air density.

Also, “speed of sound” is kind of a misnomer. What’s really being described is the speed of a pressure wave traveling through air. This is what humans perceive as sound, but it’s not sound until it enters our ear and activates our eardrum.

What’s happening when flying is that the sound of the aircraft is being cast off as waves in every direction. If we fly faster and faster, we can actually catch up to the waves in front of us. As this happens, sometimes bad things result – airflow over the wings can be disrupted, causing the buffeting of the plane. As we go even faster, we can break through these shockwaves (“break the sound barrier”).

A sonic boom happens when these sound waves catch up to a human’s ear. A plane flying fast enough breaks through its own sound waves – it’s literally flying faster than the sound it’s making. A very fast plane will fly past you silently, and you will hear a boom when its sound waves finally pass over you.

Mach 1 is about 760 mpg at sea level.