CHAPTER 1 MAKING SoUND E very kind of sound is produced by vibra tion The sound source may be a violin an automobile horn or a barking dog Whateveritis some part ofit is vibrating while it is producing sound The vibrations from the source disturb the air in such a way that sound waves are produced These waves travel out in all directions expanding in a balloonlike fash ion from the source of the sound If the waves happen to reach someone s ear they set up vibrations that are perceived as sound Sound then depends on three things There must be a vibrating source to set up sound waves a medium such as air to carry the waves and a receiver to detect them Sound waves cannot travel through a vacuum There is an age old question concern ing the definition of sound If a tree falls in a forest far from any sound detector such as a human ear or a microphone does the tree s crash make any noise The answer of course depends on how sound is defined If it is thought of as the waves that are carried by the air the answer is yes wherever there IO
MAKING SOUND sound waves Sound waves travel through the air from a source to a receiver Encyclopaedia Britannica Inc are sound waves there is sound However if sound is defined subjectively as a sensation in the car for example the answer must be no In that case sound does not exist unless there is a receiver present to detect it The two definitions are equally correct How SoUND Is PRODUCED AND CARRIED It is easy to detect the vibrations of many sources of sound A radio loudspeaker for example vibrates strongly especially when the volume is turned up If one lightly touches II
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
