How can adding sound make things quieter?

Sound waves can add and subtract

Sound involves changes in air pressure. Two sounds combine where they meet. If one pressure change opposes another at the right time and place, they can partially cancel. This is destructive interference.

The headphones use microphones, processing electronics and speakers to create that opposing sound. The cancellation is aimed at the listening area near the ear; the original source continues making noise.

Read the source: Bose: What Is Active Noise Cancellation?

Why is an engine hum easier than a nearby conversation?

Noise cancellation is usually strongest for lower-frequency, steady sounds such as engine or train noise. Sudden or changing sounds, and higher-frequency sound, are harder to reduce in the same way.

That is why “noise cancelling” does not mean “complete silence.” How the headphones fit, their design and the sound around you all affect the result.

Read the source: Sony: What Is Noise Cancellation and What Can I Expect?

Active cancellation and passive isolation do different jobs

Passive isolation is the physical reduction of outside sound by the ear tips or ear cups. Active cancellation uses powered electronics to generate a cancelling signal. Many headphones combine both.

ANC can operate without music on models that support that mode: the cancelling signal is separate from the music you choose to play. The electronics still need power.

Read the source: Bose: Do Noise Cancelling Headphones Work Without Music?

Try explaining the surprising part

Why can adding a second sound make the first one less audible? Explain it using the pressure change at the ear, rather than imagining the headphones turning down the outside source.

In a Clevaa lesson, ask to see two opposing wave patterns and their combined result. Then ask what changes if the timing is slightly wrong. That follow-up connects a simple drawing to the engineering challenge of making cancellation work.