Sonic booms are the noise heard when an object travels at or faster than the speed of sound.

As an object travels through the air it pushes air out of the way creating a shockwave — like the wake of a boat. If you were traveling fast in a plane you can imagine that as the shockwaves move away from you at the speed of sound (around 340 m/s), because you're traveling too, the ones in front of you compressed together a little.

As you reach the speed of sound, you're now traveling as fast as the shockwaves and they all bunch up forming a strong pressure wave.

An object traveling faster than the speed of sound leaves its shockwaves behind it in a cone-shape called a Mach cone after Ernst Mach.

To an observer on the ground the sudden change in pressure as the pressure wave passes is experienced as a loud boom.

There's quite a lot of subtlety to sonic booms and we're still researching technology to reduce them to make faster than sound travel less, well, noisy.

Sonic boom

If you’ve ever been close enough to lightning when it strikes you may be familiar with the startling high frequency thunderclap or crack it makes. But the high frequency soundwaves of the clap are attenuated by their travel through the air away from the source so that from a kilometre or so away all you hear is the growling of the thunder rumbling away as it clatters and rebounds off everywhere it hits.

I learned about this from Randall Munroe’s fun book How to: Absurd scientific advice for common real-world problems.

Thunderclap or rumble

The Doppler effect is the change in frequency of a wave as its source moves relative to the observer. The same effect that causes the change in pitch of a siren as it drives past is also used to estimate blood flow with ultrasound, measure the speed of a passing car, and even determine the motion of the stars. Doppler shift-based satellite navigation was also the first operational use of a system that eventually led to GPS.

When a siren or similar approaches I try to imagine the crunching up of the sound waves — the vehicle seemingly chasing after its own sound — and the stretching out of the waves as it heads past and into the distance. At least it makes it less painful on the ears.

The Doppler Effect

Funny how it can take nearly 40 years to learn that something as commonplace as the stereotypical owl sound is made, not by one owl, but as a duet between the male and female tawny owls.

Also see: how owls’ necks turn so far around, or why do birds migrate so far.

The classic twit-twoo is actually a duet

7 minutes well spent watching Julian Treasure at TED.

3 ways to listen better

I shall always remember when I was finally taught that the trick is to dip your finger in the wine first.

Sketchplanations tagged with sound