from salt & balloons
Science Standards by State
of objects, or sometimes whole objects are sensitive to vibrations of some
sort. Perhaps you have heard something in your house vibrate when
a certain truck passes. Or maybe a part of something at home vibrates
when a certain music is played. This is known as sympathetic vibration,
and although it may be annoying, it is seldom serious. But serious
accidents have occurred when sympathetic vibration upset a delicate bit
of important machines. Even so, sympathetic vibration is fun to observe
and play with if it’s done safely.
A good way to observe and study such vibration is to cut the bottom half from a large rubber balloon and stretch it across the mouth of a large, open can. Pull the rubber tight (maybe have one partner pull with you), and put rubber bands around the can to hold the ballon in place. Then you can sprinkle table salt on the balloon as indicators of how this rubber diaphragm moves.
Make or get a small megaphone, or cup your hands around your mouth to direct the sound. Then, without disturbing other people, sing “Ah” at the salt-covered diaphragm (rubber sheet). Start with a low note, and gradually raise the pitch, higher and higher, until you see the salt particles begin to “dance.” When they seem to dance violently, keep that pitch as best you can, until the salt has formed a pattern on the diaphragm. When the salt no longer seems to be dancing, but has stopped in a certain pattern, look closely at the pattern. Where there is no salt, the rubber was moving up and down, tossing the salt to one side or the other. Gradually, it gathered in lines, forming what are known as “Chladni figures.” These lines of salt are places where the rubber did not move up and down. The salt-free spaces between the lines are where the salt got tossed aside.
Studying Chladni figures is one way in which guitar makers determine the sound qualities of the instruments they make. Studying Chladni figures is also a way for makers of musical instruments, aircraft, and delicate machinery to study the vibrations that occur and how to change or eliminate them.
If you can, set your balloon-covered jar near a radio or TV set, sprinkle some more salt on it, then watch what patterns develop as certain music is played. Look for differences in patterns of the salt from very low notes, and from higher notes.
SOME QUESTIONS TO THINK ABOUT: