Fleta Tallent, age 12, of Franklin, N. C., for the question:
How is sound produced?
Most of the sounds we hear come through the air, carried by tiny molecules of gas. To grasp how this happens, lets scale these small particles up to the size of baseballs. Lets imagine a couple of hundred of these enlarged molecules in the living room. Normally they fly to and fro in haphazard directions, crashing into each other and careening off the walls, the floor and the ceiling. Actually, a gas molecule of the air has several million of these traffic accidents every minute.
Now let's start a sound by beating a drum. On our enlarged scale, of course, the drum would be as big as a house. Lets imagine that the floor of the room is the head of the drum. When we whack a drum, the surface begins to vibrate, to throb back and forth with little pulses. If you whack a drum and put your finger on the surface, you can feel these throbbing vibrations. On our enlarged scale, the surface of the big drum would vibrate back and forth several feet.
Imagine what this would do to our flying baseball molecules. Those near the floor, the surface of the drum, would also get whacked and start to vibrate. They in turn would whack into the baseballs nearest to them. Soon all the baseballs in the room would be vibrating back and forth to the walls and ceiling. This, of course, is a very simplified description of how sound travels through the air. It is carried by countless trillions of vibrating molecules. And the vibrations cause the sound to travel in waves, outwards in all directions.
On the way, those vibrating molecules may reach a piece of skin very like the head of a very small drum and set it vibrating. This little drum is your ear drum and it acts as a receiving station for the sound vibrations. It is attached to nerves which flash the vibrating sound signals to your brain.
Sound travels through ordinary air at about 1100 feet a second. It travels faster through liquids and still faster through solids. Sound waves travel through water at about three miles a second and through steel at about five miles a second. Heat speeds up sound and cold slows it down. The gas, the liquid or the solid through which the sound travels is called the medium. And sound cannot travel without being carried by some medium.
We can prove this with an experiment. Letts place an alarm clock under a glass dome, then draw out all the air. The clock is now in an almost vacuum. It will go on tacking and the bell may ring but we shall not hear it. For there is no medium for carrying the vibrating sound waves. If we let the air back into the glass dome, the sound will start again. When it reaches our ears, the sound picture will be complete. We hear it when the ear nerves flash the pattern of the vibrations to the brain.