The phonautograph makes it possible to see the actual shape of a sound wave. Sound is produced when objects vibrate. As an object moves back and forth, the vibrations create sound waves by compressing the surrounding air.
The stylus vibrates due to sound waves coming through the ear. As the smoked glass moves under the stylus, the vibrations of the stylus etch out the sound wave pattern on the glass. Bell's phonautograph showed that even very complex sounds, such as human speech, produced sound wave patterns that were single wavy lines.
