Jeannie Ann MacKenzie, age 9, of Plymouth, Mass., for her question:
Where do sounds go when they disappear?
A sound is loudest where it begins. It spreads out and out like a growing bubble, getting fainter and fainter as it goes. Finally it fades away into silence. We are lucky that sounds fade away. Otherwise our ears would be deafened. We would hear every sound in the world at once.
Sound travels through air, water and solids. It will not travel through a vacuum ‑ which is an empty space without even air. This is because sound needs tiny particles in order to travel. Everything in the world is made of these little particles called atoms and molecules.
Most of the sounds we hear travel through the air. Air also, is made of molecules, too small for our eyes to see. Billions of these mites dash around in every thimbleful of ordinary air. These little mites carry the sounds that reach our ears.
Lets hear what happens when we beat a drum. The drum head is a skin, or membrane, stretched tight. When we hit it with a drumstick it starts to jump up and down. It shivers with trembling vibrations. If you touch it with your finger you feel the throbbing vibrations.
These vibrations start the sound. For the drum membrane is banging up and down against the air molecules. With each vibration they get a whack. Billions of little molecules are sent pelting away from the drum. These fellows crash into the molecules behind them. And these fellows pelt into the neighbors behind them. The vibrating whacks make the sound. On and on it goes, spreading like a bubble.
The first molecules hit their neighbors with a terrific whack. The neighbors hit their neighbor’s but with a little less force. The sound these fellows make is a little less. Hence, the vibrating force is strongest at the drum head. It grows weaker as it goes away from the drum. Finally the vibrating fades away altogether and sound fades into silence.
Outdoors, the drum sounds reach out wider and wider. The big bubble of sound reaches a long way before it finally fades away‑ Indoors, the drum sounds reach out until they touch the walls. It is not so easy for the vibrating air molecules to jostle the molecules in a solid wall. Most of them can go no further.
However, these molecules are still throbbing with vibrations from the drum. Instead of jostling their outside neighbors they jostle their inside neighbors. The sound bounces off the wall and comes back into the room.
This bounce is called an echo. The sound travels out until it meets a smooth, solid wall. There it turns around and comes back. Indoor echoes fill the whole room with sound. The drum sounds louder indoors than outdoors. When the drum beating stops, these echoes carry on for awhile, getting softer and softer. Finally, all the vibration is spent and even the echoes fade into silence.