why does the noise from a plane in the sky happen suddenly?

Question

Noticed it today, rather than a gradual build up of noise you just suddenly hear an airplane like it appeared out of nowhere?

In: Physics

Answer 1

It is a gradual buildup, but planes move FAST, so the gradualness happens fast. Commercial jets tend to fly at around 75% to 85% of the speed of sound. That means the sound from the plane isn’t all that far ahead of the plane itself. So the sound builds quickly, and then – bam! – the plane is almost right above you as the sound pressure peaks.

If you ever hear a slower moving prop plane approach, the sound buildup is much slower as the plane itself is moving slower and the sound wave gets pretty far in front of the approaching plane.