Science | Science Experiment of the Week 6 | 265 - Straw Trombone

Whistle, Pan, Pipes, Sound Instruments, Tone, Air, Water - Using straws to make sound instruments.

This Week's Experiment - #265 Straw Trombone

These experiments are from Robert Krampf - The Happy Scientist

This week's experiment has been in my file for quite some time, waiting for just the right occasion. I happen to be getting over the flu, on top of having just finished the six hour drive to Naples for this week's shows. That combination seemed just right for dipping into the holding file and pulling out an idea. This idea got into the file as the result of an e-mail discussion with Michele and her 6 year old daughter, and it grew from there. Besides, it has been far too long since I had a noisy experiment, with the potential for irritating older brothers and sisters in the name of science. You will need:

a drinking straw
a tall glass of water or some other liquid Put the straw into the glass of water. Hold the glass in one hand and the straw in your other hand. Holding the straw flat against the side of the glass, bring it up to your lips, just as if you were going to drink. Instead of putting the straw inside your lips, blow across the top of it. You don't want to blow into the straw. Instead, your breath should go straight across the top. As you blow, move the straw back and forth, up and down slightly. When you get the angle just right, you will hear a whistling sound. This may be very faint, but with practice, you can get a very loud, clear whistle.

Once you get the straw to whistle, the fun starts. As you are blowing, keep the straw still, but move the glass of water slowly downwards. As the glass moves down, the tone of the whistle changes. Move the glass upwards and the tone changes again. What is going on?

When you blow across the top of the straw, you cause the air inside the straw to vibrate. This vibration causes the sound that you hear. By changing the length of the column of vibrating air, you change the sound. The longer the column of air, the lower the pitch. The shorter the column of air, the higher the pitch.

Why would moving the glass up and down change the length of the column of air? The water inside the straw will move to the same level as the water outside the straw. As you move the glass downwards, more of the straw is above the water level, so the length of straw that contains air is longer. When you move the straw downwards, more water moves into the straw and the length of the straw that is filled with air is shorter.

This experiment began with two straws, one fitting inside the other. Holding your finger over the bottom, you moved the straws together and apart to make the column of air longer or shorter. After playing for a while (Playing is good! You would be amazed at how many scientific discoveries have been made while playing.), I found the water produced a much better tone.

Can you think of other instruments that change a column of air to produce different tones? What about making a set of Pan Pipes, by cutting straws to different lengths and taping them side by side. Instead of changing the length of one straw, you would blow across straws of different lengths to get different tones. Don't forget to seal the bottom of each one. You might also try straws that are the same length, but different thicknesses, to see what impact that has. What about vibrating other things, such as guitar strings, rubber bands, or different sized metal pots? You can learn a lot about sound and have lots of fun, not to mention making lots of potentially irritating sounds along the way.

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