Wednesday, October 22, 2014

A blade of grass makes sound- Sound Sandwich

If you pick a blade of grass or a leaf, stretched it in between your fingers or thumbs, and blow into the gap you will hear a high pitch sound. A lot of our students have had played with this effect. The working of a sound sandwich instrument, which can be made using simple items like craft sticks, a straw, one wide rubber band, two smaller and a narrow rubber band is no different in working as a blade of grass sandwiched between fingers and thumbs.
How to make a sound sandwich?
Stretch a wide rubber band length-wise over one of the craft sticks, and cut two pieces of straw, each about an inch to 1 ½ inches (2.5 to 3.8 cm) in length.
Put one of the small straw pieces under the wide rubber band, about a third of the way up from one end of the stick.
Take the second craft stick and place it on top of the first one, wrap one of the smaller rubber bands around the end of the stick a few times, about ½ inch (1.25 cm) from the end, on the same side where you placed the straw.
Make sure the rubber band pinches the two sticks tightly together. Take the second small piece of straw and place it between the two craft sticks, at the opposite end. This time, though, place the straw on top of the thick rubber band, so it sits just under the top craft stick. Wrap the second small rubber band around the loose end of the stick, about ½ inch (1.25 cm) from the end. When you are done, both ends should be pinched together and there should be a small space between the two craft sticks.
When your sound sandwich is complete, put your mouth in the middle, as if playing a harmonica, and blow. Notice that you can make different sounds by blowing through different areas of the instrument, blowing harder or softer, or by moving the straw closer together or farther apart.
What’s going on?
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 tension leads to higher-pitched resonance.
Reference: Exploratorium Teacher Institute (2009). The Exploratotrium Science Snackbook: cook up over 100 hands on science exhibits from everyday materials (revised edition). Robert J. Semper: United States of America. 

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