So ND Striking a gong initiates vibrations that create a compression sound wave which travels outward and eventually fades away IN Matthew Wakem Digital Vision Getty Images 12
MAKING SOUND the speaker cone one can feel its vibrations as a kind of tickling sensation in the finger tips If one touches one's throat while singing a low note one can feel the vibrations of the vocal cords A common experiment in phys ics classes is to strike a tuning fork and dip the end of it in water The vibrating fork splashes the water and sets up little waves that are easy to see Sound waves are often compared with water waves but are actually a very dif ferent sort of wave What they are can be seen by considering what happens when an object vibrates in the air Suppose some one strikes a gong As the gong vibrates it alternately bends outward and inward very rapidly This movement pushes and pulls at the air next to the surface of the metal Air is made up of tiny molecules billions of them to every cubic inch or cubic centime ter Therefore when the metal gong bends outward it crowds together those air mol ecules that are close to its surface These molecules push outward against other mol ecules and they in turn push against still others Thus a compression wave is set into motion The wave travels outward from the gong becoming weaker and weaker until it dies away 13
SoUND A single sound wave such as this does not actually produce a sound of course As the gong continues to vibrate each outward bending of the metal sets up a new compres sion wave Between each pair of compression waves is an area in which the molecules of air are spread apart more widely than normal Such a wave of rarefaction corresponds to a moment in which the gong is bent inward pulling instead of pushing the molecules The whole series of compression and rarefaction waves traveling outward from the gong make up what is heard as sound The sound waves travel in all directions from their source REFLECTING AND FOCUSING SoUND WAVES Like light waves sound waves can be reflected and focused An echo is simply a reflection of Sound A flat surface like that of a cliff or wall reflects sound better than an irregular sur face which tends to break up sound waves Echoes are useful in many ways In a fog a ship's captain can often tell whether he is near a hilly shoreline by listening for echoes of the ship's whistle Underwater sonar equipment uses echoes of a supersonic signal similarly to detect submarines The device automatically 14
MAKING SOUND Fireworks exploding over the band shell of the Hollywood Bowl in Los Angeles while an orchestra performs The curved construction of the shell focuses sound coming from the ampitheater stage AP Images times the echo from the submarine s hull and computes the distance Depth finders use echoes from the ocean bottom to measure the depth of the water A band shell focuses the sound of the band in just the same way an automobile headlight reflector focuses light The head light reflector and many band shells are in 15
