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Krampf #464 Filling Sounds
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KRA-@aol.com
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Jun 19, 2006 15:26 PDT
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Robert Krampf's Experiment of the Week
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This Week's Experiment - #464 Filling Sounds
Greetings from Ft. Lauderdale, Florida. This week, we are busily taping and
editing on the FCAT science videos. We are having a blast, and I am
learning a lot about making videos. The technology sure has changed since the last
time I tried this, about 20 years ago.
This week's experiment comes from the sound effects that we are adding to the
video. The microphones are directional, which means that they pick up lots
of sound from one direction, such as the sound of my voice, and very little
sound from other directions, such as cars and barking dogs. Because of that,
you lose many of the small, everyday sounds that you need to make the video
seem complete, so you have to create sounds to take their place.
At one point, I was filling a glass with water, and without the familiar
sound of water filling the glass, it just did not work. Get a tall, narrow glass
and fill it with water. Listen to the sound that it makes as the water
nears the top. It is a very familiar sound, but why does the tone change like
that?
Think about what is changing? The level of the water. As the glass gets
fuller, the pitch gets higher. OK, so why does that happen?
You may remember from past experiments that sound is caused by vibration.
The speed of that vibration determines the pitch. If something vibrates
slower, the pitch will be lower. If it vibrates faster, the pitch will be higher.
So we need to figure out what is vibrating to make the sound. It could be
the glass, the water or the air. How can we figure out which? Let's start
with the glass. It is easy to make the glass vibrate. Tap it gently with a
spoon and you will hear a nice tone. Next, hold the glass firmly with your ha
nd, near the top, and tap it again. Hear how your fingers deaden the sound?
They are keeping the glass from vibrating as much, so the sound changes.
Try filling the glass with water as you hold the glass firmly. The sound is
still the same. That tells us that the sound we hear is not caused by the
glass vibrating.
OK, so maybe it is the water in the glass that is producing that rising
pitch. How can we test that? If the change is due to the increasing amount of
water, then the same thing should work in a bowl or some other container. Try
filling a wide, open bowl. No, you do not get the rising pitch.
So, if it is not the glass or the water, it must be the air. Inside the
glass, you have a column of air. As you pour water into the glass, the surface
of the water does vibrate, producing some sound, but that vibration is the
same as the glass is filled. The vibration of the water causes the column of
air inside the glass to vibrate.
Now, if you happen to have a guitar or some other stringed instrument around,
look at the strings. The thicker strings make the lower notes, and the
thinner strings make higher notes. Less mass lets the thin strings vibrate
faster, even at the same tension.
As the water level rises, there is less air in the column. It is the same
as making a guitar string thinner. There is less mass to vibrate, and the
pitch goes up.
The same thing should work with other liquids, such as a nice, cold cola.
But, if you put a scoop of ice cream in the glass, the resulting (quite tasty)
foam will deaden much of the sound. That is an experiment well worth trying.
Have a wonder filled week.
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From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
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