Photo: kristykay22 (flickr)
You probably imagine that your ears are rather passive.
They sit there quietly on the sides of your head like two fleshy microphones, waiting for sounds to come to them. Actually, this isn’t the whole story. Your ears can play a surprisingly active role too.
Your inner ear has sound sensitive cells called hair cells. No, these aren’t the hair follicles that sprout the real hair out of your grandfather’s ears, these are microscopic structures deep within your inner ear. When sound waves hit these cells, they detect the vibrations and send a message to your brain. Strangely, these cells can also start vibrating all on their own. Why do they do this?
They do it to help extend the range of your hearing. Generally speaking, your hair cells only vibrate when they hear a very quiet noise coming in. If a sound like rustling leaves or a whisper enters your ear, those hair cells start vibrating in a way that mimics the incoming sound waves. In this way, your hair cells act like an amplifier for very quiet noises. It’s like singing along loudly with a very quiet singer so that everyone in the room can hear.
If it weren’t for this amplification, we wouldn’t be able to hear sounds as quiet as whispers. Indeed, when older people lose their hearing, they often lose this amplification process first. This means they can’t hear quiet sounds, but loud noises sound just the same. That’s why an older person might say “Speak up!” when you whisper, but “There’s no need to shout!” when you switch to a louder voice.
NOTE: Humans have remarkably sensitive hearing. The softest sounds we hear are ten billion times quieter than the loudest. This tremendous range comes, in part, from the fact that those hair cells can vibrate on their own.