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By making simple adjustments to this noisemaker, you can raise or lower its pitch and make different kinds of sounds.
When your Sound Sandwich is complete, just put your mouth in the middle, as if you were playing a harmonica, and blow! (Remember to blow through the sticks, not the straws.) Notice that you can make different sounds by blowing through different areas of the instrument, blowing harder or softer, or by moving the straws closer together or farther apart. Experiment to find out how many different sounds the Sound Sandwich will make.
When you blow into the Sound Sandwich, you make the large rubber band vibrate, and that vibration produces sound. Long, massive objects vibrate slowly and produce low-pitched sounds; shorter, less-massive objects vibrate quickly and produce high-pitched sounds. The tension of a rubber band also will change its pitch: Higher tensions lead to higher-pitched resonances. When you move the straws closer together, you shorten the part of the rubber band that can vibrate, so the pitch gets higher than the original sound. You may also have played with this effect if you’ve ever stretched a blade of grass between your fingers and blown on it to make the grass vibrate and buzz.
Like the rubber band in the Sound Sandwich, your vocal cords also vibrate when you speak or sing. The more tension they’re under, the faster they vibrate and the higher the sound they make.
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Attribution: Exploratorium Teacher Institute