Here is a fun school project that can help us learn about sound… the really big ear. It can be used to listen to very faint sounds from a very far away. Two of them can even be used to communicate across a crowded room, but without using radio waves. People in the middle usually cannot hear the conversation, but the two persons at the ear's focal points can hear each other quite well.

   Big ears can be fairly inexpensive and easy to build, and lots of fun to play with. Basically the "ear" is a parabolic reflector. So the bigger it is, the more faint sounds it will pick up. The listener simply sits with his ear at the focal point, or one can place an ordinary microphone there, with the sound being electronically amplified in the ordinary manner.

   Sound spreads from its source in an inverse-square manner. That is, if you double the distance between you and the source, the amount of sound energy you receive will be only one fourth as much. That means that sounds get really weak, really quick as you move away from the source.

   To reverse that effect, the big ear collects energy over a large area, and bounces it into a smaller area at its focus.

   The big ear can be made of epoxy, or even concrete. Epoxy is easier to move around, but concrete is less expensive. There are two important qualities needed to make an effective parabolic reflector for sound. First is the correct shape, and second the material. The harder the material, the better it will reflect sound. That is why concrete is a good choice. But it needn't be very thick, a couple of centimeters or so should work fine. It just needs to be mechanically strong enough so that it can be moved around without warping or breaking.

   The easiest way to produce the correct shape, is to use a pre-manufactured parabolic. Satellite dishes are an excellent choice. Just be careful not to bend it while using it to make your master mold.

   You could use the dish directly, but they have been made for reflecting microwaves rather than sound, and you need something more solid. Besides, once you have made a master mold from the dish, you can then cast as many big ears as you wish. The class might even come up with some games that can be played using a group of them.

   To make the master mold, lay the dish on the ground face up. Coat it with Vaseline Petroleum Jelly so the cement won't stick to the dish. Mix portland cement, sand, and natural fiber by volume, one part cement, two parts sand, and 3 parts fiber. The fiber can be ground up coconut leaves, or even grass at least 4 or 5 cm long.

   Mix in water, just enough so that the mixture can be trawled easily as one would for putting stucco on a wall. Coat the dish with this mixture to several cm thick. Tap on the back side of the dish with a piece of wood being careful not to dent the dish, but just hard enough to cause the wet cement and sand to fill any air pockets next to the dish.

   Let it set for a week, so it gets good and hard. Then pull the cement mold off of the dish, clean up the dish, and return it to service. Clean up the mold, and then use it to cast big ears, in much the same manner used to make the mold itself. Cast in bolts that can be used to secure a fixture for holding a microphone at the focal point if desired. And legs, so the ear can be set on edge.

   Wouldn't it be fun to set up a science park, where the displays are made by school kids for teaching old and young alike?

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   Now that's exciting, isn't it!