Feel the energy from banging a drum
By Joe Lazlo
A fun look at science working in the world around you, plus a cool gadget or experiment to test it out
Sacred hearts fifth-grader Kelsey Wakuzawa plays with a balloon drum made from a water bottle.
The first drums were probably hollow tree trunks that were hit with sticks. Later an animal skin was stretched over one end of the hollow "body." Hawaiians had pahu drums made of the hollowed-out wood from the trunk of the coconut tree and shark skin for the drum head, the part you beat on.
What is a drum? Technically speaking, it is a "closed pipe," something that causes an air column inside to become set into motion. Let's make a drum.
Find an empty 16-ounce plastic water bottle or an empty 20-ounce plastic soda, juice or sport-drink bottle.
For the 16-ounce bottle, cut the bottom off about ¥ inch below the first groove, or 1 inch above the bottom. Or, if you're using a 20-ounce bottle, cut the bottom off about 1¥ inches above the bottom.
Cut the neck out of a balloon and stretch the remaining balloon rubber over the opening. Use a rubber band and masking tape to secure the balloon onto the bottle. The balloon will be the drum head.
Tap the drum head. Does it make a sound? It's not very loud, is it? Why not? The balloon rubber is not the best material for a drum head. The drum is a vibrating pipe that sets the air inside into motion. That air must connect with the air in the room to carry the energy to your ears. The balloon is too flexible and will not cause a large amount of air to be set into vibration by tapping. Also, the mouth of the bottle is small.
Next, try this: Pinch some of that balloon rubber between your thumb and first finger. Pull it out and release it. How did the drum head sound? Aim the bottle toward your face and repeat this. Did you feel something? What was it? It was the kinetic energy of the vibration that you put into the drum head by stretching it.
How far do you think the energy will travel? Try this: Get a plastic shopping bag and stand it up. Stand close and aim your drum at the bag. Pull the drum head and release it. Did you see any indication that the energy hit the bag? Did the bag rustle?
Here's another kind of drum: Get a plastic tennis-ball container and cut the bottom off of it about 1 1/2 inches above the bottom. Replace the lid. Hold the container near the lid and tap the lid with a pencil. Notice the volume of the sound. Place the drum on a table top and tap as hard as before. Notice the difference.
Why is the sound muffled? Because you prevented the air inside the pipe from escaping. That's why much of the energy didn't get to your ears. If you can find some other containers of different sizes that have plastic lids, you can create a set of "tuned drums," much like the kettle drums of an orchestra. Until next time ... a hui hou!
Do you have a question or suggestion for Dr. Gadget? Write: Dr. Gadget, c/o The Honolulu Advertiser, P.O. Box 3110, Honolulu, HI 96802, or fax 535-8170. You may send e-mail to email@example.com only if you're 13 or older. "Dr. Gadget's Science Machine" is written by Joe Laszlo, a retired science teacher and winner of a Presidential Award for Excellence in Science Teaching. His column alternates in this spot with "Hawai'i Nature Squad."