What happens when a headphone plays a second sound?

A jet engine is roaring. You press one button and the headphone answers with a sound of its own — a second wave, right on top of the first. The headphones aren't thick enough to be a wall. So what does adding more sound do to the roar? Let's find out, then try to break the trick.

1Two things to know about sound

Sound is a wave, and waves add up

You need just two ideas. Watch each one move:

Sound wiggles

Sound is a wave that pushes the air up and down, up and down. A loud roar is a big, tall wiggle.

Waves add together

Play two waves at once and they stack point by point into one new wave. Up plus up = bigger. Up plus down = smaller.

2The two ways two waves can meet

Stacking, or cancelling

The stacking case

Two waves that match

When both waves go up at the same time and down at the same time, every push lands on another push. They add into one taller wave — louder.

The cancelling case

Two waves that are mirror twins

When one wave goes up exactly while the other goes down exactly, each push meets an equal pull. They flatten each other out. Scientists call this interference.

3Your turn — make the roar

Crank up the engine

This is the roar sneaking into your ear. Slide it up and watch the wave grow tall and loud. (No headphone helping yet.)

The engine roarthe noise reaching your ear
Engine roar: a steady rumble
SOFTJET ENGINE!

4Now turn on the headphone

The headphone plays a second wave

A tiny microphone hears the roar. Then the headphone plays its own wave — and you get to slide it into the exact upside-down mirror of the roar. Before you do, make a guess.

Guess before you slide it

The headphone plays a wave that's the exact upside-down copy of the roar, at the very same time. When the two waves meet at your ear, what do you hear?