Tuesday, October 14, 2014

PET Bottle Membranophone

The PET (Polyethylene terephthalate, the plastic found in most platic bottles) bottle Membranophone is fun to make and even more fun to play. Using simple materials like a clean empty plastic bottle, a balloon, a rubber band, a straw, and A4 size paper we can make and play with one. This instrument produces sound from a vibrating stretched membrane or balloon.
 How to make:
Get a PET bottle and measure down about 3 inches (7.5 cm) from the top of the bottle. Using scissors, cut along the measured line. Make sure you cut evenly along the edge. Trim off any bumpy spots and recycle the bottom of the bottle. You will need the top half of the bottle to work with. Take out a punching machine and punch a hole near the cut edge of the top half bottle as far as you can get it. Put the straw through the hole to test it for size. It should be a tight fit. If the hole isn’t large enough for the diameter of the straw, re-punch in nearly the same spot to widen the hole a bit. Cut the neck off the balloon to form a sheet of elastic material- a membrane. Stretch the membrane over the hand cut opening of the bottle, making sure that the hole you punched in the side does not get hidden by excess material. Attach the membrane to the bottle with a rubber band. Wrap the rubber band around the bottle several times, making sure that the membrane is taut. Twist the cap off the bottle and set it aside. Roll a piece of A4 size paper into a tube, making it as tight and straight as possible. Put the rolled up tube into the neck of the bottle, where the cap had been. Let go of the tube when it barely touches the bottom of the membrane. It should fit securely in the hole. Tape it to the neck of the bottle so it stays in place. Insert the straw into the punched hole on the side of the bottle, and you’re ready to play.
Now that your instrument is ready, simply blow into the straw on the side of the bottle and your pet bottle Membranophone should play.
video

What’s going on?
As you blow into the straw, you create pressure in the space between the outer wall of the paper tube and the inner wall of the water bottle. That pressure forces the membrane to rise, allowing air to flow into the top of the tube and escape out the bottom.
As the air escapes, the membrane returns to its position. But as you continue blowing air into the instrument, you force the membrane to rapidly rise and fall, over and over again. If you place your finger over the top of the membrane, you can feel it vibrate. These vibrations produce sound.  


video
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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