Source: How It Works Magazine
What I did:
I was reading one of my recently bought "How It Works" magazine when I found an interesting article about noise-cancelling headphones.
What I learnt+summary:
There are two ways that noise cancelling headphones can block noise. The simplest way, passive noise reduction, works by blocking sound waves with layers of high-density foam. This is good for masking High frequency sounds like an explosion, but these headphones will struggle at stopping noise at a lower frequency.
The second way is known as active noise cancelling. These also have special materials, but they go one step further, by creating their own sound waves. Inside the earpiece is a small microphone that detects ambient sounds and feeds it to a digital processor, which analyzes the sound wave's composition, and creates another sound wave that is the complete opposite of the wave detected. When these anti-sound waves are then played back, actively blocking the sound wave, as the name suggests. This is also known as destructive interference.