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Parts
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.
TRY THIS:
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:
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Could a passing truck vibrate the
rubber diaphragm on the can?
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How could you tell?
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Do some of your parents’ friends
have especially deep voices? Look at the patterns (the Chladni figures)
they create when they talk.
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How do those compare with ones produced
when a high-pitched lady talks?
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How about the pattern produced when
a dog barks?
There are so many interesting ways
of examining and recording vibrations around you!
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