SOUND, to the home experimenter, has the advantage of being something tangible. Unlike light, it consists entirely of vibrations of air or other material substances. Its pulsations take place forward and backward, not transversely, along a line between the source and the observer. For all its simplicity, however, sound may be made to persounds cancel each other.
Connect two equal lengths of rubber tubing to a glass T tube. Hold their open ends near a watch, and the stem of the T tube to your ear. You will hear the ticking distinctly. Now clip small pieces from one of the pieces of tubing, "listening in" each time as at left. The sound will diminish, becoming faintest when the tubes differ in length just enough for the crest of a sound wave to arrive through one at the same instant that the trough of a sound wave comes from the other, blotting out both.
How radio studios and auditoriums control sound reverberations may be demonstrated with a cardboard carton, cotton batting, and a clock. Set the open carton on its side, stand the clock within it, and find how far away you can hear the ticking. Now pack the space around the clock with cotton, as illustrated below, and you will find that the sound carries to a much shorter distance. By applying more or less absorbing material to the walls, the desired sound reflection is obtained.
Clamp a coin securely to a table. Place another at its left, just touching it. From the right, snap a coin sharply against the clamped one. The center of the anchored coin does not move a fraction of an inch, but its rim vibrates with force enough to fling away the left-hand coin, as pictured above. Likewise, materials transmit sound without being carried along.
form engaging tricks in an amateur laboratory. Only the least of apparatus is required to show how a pair of sounds can destroy each other; studio methods of sound control; strange geometrical pictures drawn by sound; the principles applied in such musical instruments as the trombone and cornet; and other queer facts about sound, as demonstrated in the experiments on these pages.
Select two test tubes, of which one slides smoothly within the other. With a file, cut off the bottom of the inner one, and slip it into the larger tube. Blow across its mouth, and you will hear a musical tone. The pitch will fall as the tubes are drawn apart, and rise as they telescope together. The slide of a trombone, by lengthening or shortening the column of vibrating air, serves a similar purpose. In a cornet, finger-operated valves replace a moving slide, to direct air through the desired length of tubing.
Sound need not come through the air, and enter your ears, for you to hear it. To prove this, fasten a piece of thin wire about your head, as in the picture above, or hold one end of it in your teeth. Attach a phonograph needle to the other end of the wire. You may now play a phonograph record and hear it clearly, even though you place your fingers tightly in your ears. Transmitted through the bones of the head, the sound vibrations agitate the nerves of the inner ear in practically normal fashion. Apparatus based on this "bone conduction" principle has been developed to restore the hearing of persons whose deafness is due to impairment of the delicate mechanism of the outer or middle ears.
Strings of a violin or mandolin vibrate in simple patterns, but bells, cymbals, and other vibrating plates oscillate in complex ways that vary with their shape and the manner of striking them. Fine sand, sprinkled on a square plate of thin metal or glass, traces interesting designs when the plate is fastened to a bottle cork at its center and bowed at one point or another along its side. Touch the edge with one or two fingers, and the pattern may be changed at will. In these "Chladni's figures," as they are known, the sand is shaken away from vibrating parts, and heaps up at "nodes" or stationary parts. One of the many odd designs obtainable is illustrated below.