Dr. Gadget's Science Machine
Vibrations with strings and containers
By Dr. Gadget
Here's a fun look at science working in the world around you, plus a cool gadget or experiment to test it out.
Aloha! In my last column two weeks ago I discussed a few ideas about sound. Sounds are vibrations caused by energy making something move back and forth. Sounds can either be produced by vibrating strings or vibrating air columns. If you stretch a rubber band and pluck it, this is an example of a vibrating string. If you blow across the mouth of an empty soda bottle, you are causing an air column to vibrate.
Sacred Hearts fifth-grader Serra Alo demonstrates a resonance experiment.
There is a Hawaiian pipe instrument called the "singing bamboo." Long pieces of bamboo that are open on both ends and have the nodes opened up inside are tapped on a pad. The up and down motion causes them to "sing" or produce a tone. Why? The tapping causes the walls of the bamboo to be set in motion, which causes the air column inside to vibrate and produce a sound. This is an open pipe.
How can you make sounds louder? At a rock concert it is done electronically with amplification systems. The speakers cause more air to vibrate so more energy reaches your ears. You hear it as a louder sound. Sometimes it's so loud that it is painful.
How does nature do it? The same way! More air is caused to vibrate, but it's done mechanically instead of electronically.
Let's test this idea.
• Find a rubber band, and a container like an empty tennis ball can, a soda can or a yogurt container. If you can find all three, use them!
Stretch the rubber band between the last three fingers of both of your hands and pluck it with your thumb. Notice the degree of loudness. Now put one end of the rubber band over the container you have chosen and hold the container with the mouth toward you. Stretch the rubber band with the last three fingers of your other hand and pluck it with your thumb. How did it sound? Was it louder? Why?
The rubber band caused the container, which is holding air, to vibrate. The vibrating container caused the air inside to also vibrate and connect with air outside of it. The walls of the container cause air outside to vibrate. The sound energy was carried to your ear. This is called resonance, and the container is a resonator.
What if you turn the resonator (the container) around so that the mouth is not facing toward you? How loud do you think it would sound? Try it. Did it sound as loud as before? Probably not, because the major air column that could cause the energy to get to your ear was facing away from you.
• Here's one last resonance test: Get two identical empty bottles. They can be either glass or plastic. Blow over the mouth of one so that you know how to get a sound. Hold the other bottle close to your ear with only two fingers on the neck of it. Now blow into the first bottle. What did you hear? Did the tone seem to come out of the bottle next to your ear? Why?
Since the bottles are identical, they both vibrate with a sound of the same tone. The one you caused to vibrate by blowing over the mouth set the surrounding air into vibration. This caused the walls of the second bottle to resonate. Resonance caused the air inside of it to vibrate, and this energy was directed right into your ear. Until next time . . . a hui hou!
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