Physics

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Video

Materials

• 1 drinking glass
• 1 drinking straw
• 1 pair of scissors
• Water

Explanation

In air, sound consists of waves of denser air, moving from the source of the sound. Here the lower half of the straw works like any wind instrument. Inside is a column of air that can be made into waves. The length of this column of air can be changed and thus different musical notes can be obtained.

The air inside the lower half of the straw is made into waves by the air you blow past its upper opening. Just as water waves start if you hit a water surface, sound waves start when the moving air hits the air "surface" in the opening of the lower straw half. The sound waves also spread out through the open end of the straw and reach your ears.

The air column in the straw, and anything else that can vibrate, can only do so at one or a couple of speeds and can thus only be heard as one or a couple of notes. Exactly how fast something can vibrate depends on a number of factors, but the factor that can change here is the length of the air column. A short straw gives a fast vibration and short sound waves, which we humans perceive as a high note.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you make the cut 5 cm (2 in) from one end of the straw?
• What happens if you use a wider straw?
• What happens if you use a straw from another manufacturer?

Variation

Blowing over the opening of a glass bottle partially filled with water works according to the same principles as the water whistle. If you have many glass bottles, you can fill them all with different amounts of water and thus get a whole set of "keys" that play different notes. A tip is to tap the bottles with a pencil, which can be an easier way to set the air columns in motion. That's what the Glass bottle xylophone is about.

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