Dr. Gadget's Science Machine
Create air column to make sounds

By Joe Laszlo

Sacred Hearts fifth-grader Krys Melton blows on a homemade slide whistle to hear the different pitches.

A pipe can cause an air column to vibrate. Pipes are found in musical instruments that are wood winds or brass. They are called "closed pipes" because something is placed on one end that causes the air column inside of the pipe to begin vibrating.

A good example is an oboe. It has two reeds in the mouthpiece. When the musician puts the correct tension on the reeds and blows, the reeds vibrate and set the air column in the oboe into vibration.

Let's make a simple straw version of an oboe.

• Get a soda straw and cut it in half. Set one half aside. Get the other half and flatten about half an inch on one end between your teeth. Be careful to not bite down and leave teeth marks in the "reeds." About one-fourth of an inch back from the end, make a cut on each side at an angle toward the end. Put the reeds into your mouth and blow gently.

What will happen if you shorten the straw? Did it change the pitch of the sound? Pitch is how high or how low you hear the sound. Why did this happen?

To answer this question, think about what would happen if you shortened the string on an 'ukulele. The pitch also would go up. If you have short pipes and short strings producing the sound, they vibrate much faster than long strings or pipes. The faster the vibration, the higher the pitch. If you play a flutophone or a recorder, you will notice that the pitch goes up as you uncover the finger holes. The uncovering of the finger holes causes the air column to become shorter, so the pitch goes up.

Try blowing into your straw oboe with a lot of force. What happened to the pitch or tone? It also went up. When you blew harder, your air came out of your lungs with more force, but also with a greater speed. The increased speed caused the reeds to vibrate at a faster rate, and this also caused the pitch to go up.

• Here's another experiment to try: Put your fingers on your neck and make any musical tone with your voice, using the syllable "la." Now "slide" the note and make it become higher. Did you notice what happened?

If you did it in the most relaxed way, your muscles near your stomach became tighter as they forced more air out of your lungs. At the same time you may have noticed that the vibrations in your throat were faster. The air coming out of your lungs caused your vocal cords to vibrate faster, and so the pitch became higher. You also put a certain amount of additional tension on your vocal cords, stretching them. It's like when you "tune up" an ukulele.

• Let's make a slide whistle. Get a piece of a bamboo skewer, some à-inch masking tape and the other half of the straw. Cut a 5-inch piece of tape and carefully wrap it around one end of the skewer. You should be building up the end so that it is like a piston that will fit snugly inside the straw. Insert the piston into the straw.

Wrap about 3 inches of tape around the end you'll blow in; this will make a mouthpiece and protect your lip from the sharp plastic.

Hold the straw vertically in front of your lower lip and blow over the end of the straw to get a tone. Move the piston up and down as you get a tone. Did the pitch change? The piston causes the length of the closed pipe to change. The shorter it is, the higher the pitch because the air is vibrating faster within the straw. With a little practice you can make your slide-whistle warble like a bird.

"Dr. Gadget's Science Machine" is written by Joe Laszlo, a retired science teacher and winner of a Presidential Award for Excellence in Science Teaching. His column alternates in this spot with "Hawai'i Nature Squad."