What causes a sonic boom when an aircraft breaks the sound barrier?

-Karthik Gurumurthy

I find the physics behind breaking the sound barrier fascinating. To really understand what happens, you need to first appreciate that sound is a wave with a finite speed. Most of us have experienced this firsthand – when you see lightning in the distance, there’s a delay before you hear the thunder. This delay occurs because sound travels relatively slowly.

At sea level and 22°C, sound moves at about 345 meters per second (or 770 mph). Interestingly, temperature affects this speed – at high altitudes where aircraft fly, around 35,000 feet with temperatures of -54°C, sound slows to approximately 295 meters per second (660 mph).

Because sound waves travel at this limited speed, a moving object can actually catch up to the sound waves it creates. As an object approaches the speed of sound (sonic velocity), something remarkable happens – the sound waves begin piling up in front of it. With sufficient acceleration, the object can burst through this barrier of unstable sound waves and actually outrun its own sound, breaking the sound barrier.

When an aircraft reaches supersonic speeds, it generates steady pressure waves attached to its front (called a bow shock). If you’re on the ground, you don’t hear anything as the aircraft approaches because it’s literally outrunning its own sound. After it passes overhead, those generated waves (Mach waves) radiate toward the ground, and the pressure difference across them creates that distinctive thunderclap we call a sonic boom.

I think it’s one of the most dramatic demonstrations of physics in action that we regularly experience in the modern world.