#acoustics

If you have ever heard a helicopter approaching from far away, you probably recognized its rhythm immediately.

Whup... whup... whup...

The sound is so distinctive that you can often identify the aircraft before you can see it.

One day, I heard exactly that rhythm in the distance and felt certain that a helicopter was approaching. But as I walked closer, I discovered that the sound was not coming from the sky at all. It was coming from a large pneumatic breaker tearing up a road.

That raised a question.

Why can those two machines sound so remarkably similar?

The rhythm of a pneumatic breaker is easy to understand. A piston repeatedly drives a tool into the pavement, producing a sequence of impacts.

A helicopter rotor seems different. Its blades rotate continuously in one direction. Even if we imagine an idealized rotor whose blades never flap or change pitch, they would still be moving smoothly rather than striking the air through an obvious back-and-forth motion.

So why does the resulting sound arrive in beats?

“A Blade Passes, So You Hear a Pulse”

A common explanation says that every time a rotor blade passes, it produces a pulse of sound.

This is basically correct, but by itself it can feel incomplete.

Suppose we draw an imaginary line through one point on the rotor’s circular path. A blade crosses that line several times per second—but why should crossing that particular line create a sound? We could have placed the line anywhere around the circle.