Hear a pin drop. Hear some car keys jingle. Hear a bird chirping outside.waves between 20Hz and 20,000Hz. We use sound waves to communicate,to sense danger (I.e. a tree falling) or even to drive a car. Our ears help us detect sounds coming from many directions. Humans can tell if a pin droppig is in front,behind or on either side of them. This is all due to air pressure and our ability to sense changes around us. If a pin drops to our left,the sound wave from the pin fell on the left side. Then how do our ears actually work?