Resonance: Ocean Sounds
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Fun Experiment to Try at Home!
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Purpose
To determine why sea shells have an ocean sound.
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| Materials |
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two empty 1-liter plastic soda bottles |
Procedure |
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Blow across the mouth of one of the soda bottles to produce a constant sound while holding the second bottle next to your face with its opening near your ear. |
| 2. |
As you make the sound by blowing across the first bottle's mouth, move the second bottle away from your ear and then near your ear again. Note any difference in the sound when the second bottle is near your ear. |
Sum It Up!
All objects have a natural frequency, or rate of vibration, depending on their size and shape. Because the two soda bottles are alike, they have the same natural frequency. Blowing across the bottle causes the air in the bottle to vibrate, which results in vibration of the air around the bottle's mouth. The vibrating air moves outward, causing the air in the second bottle to start vibrating. This action is called resonance, or sympathetic vibration.Resonance can occur when the natural frequencies of two objects are the same. If one taps a tuning fork, for example, and places it near another tuning fork, the second tuning fork will resonate. This is because the vibration of the first tuning fork is the same frequency as the natural frequency of the second tuning fork. The vibrations move through the air between them and cause the second tuning fork to vibrate. The second bottle, like a seashell, vibrates without air being blown across it because the vibration of the air entering it has the same frequency as the bottle's natural vibration. Every seashell has its own sound because every seashell has its own natural rate of vibration. |
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Ocean sounds are not heard in seashells.
The sound that you hear when you hold a seashell to your ear is not the sound of the ocean, as some say. Instead, what you hear is due to sound outside of the shell causing the air outside of the shell to vibrate. This causes the air inside the shell to vibrate, which makes the shell itself vibrate, or resonate. Resonance is a condition in which the natural frequency (number of vibrations per second) of vibrations of an object (such as the shell) is matched in frequency by an outside source of vibration energy (in this case, the air outside the shell). Resonance results in an increase in the size of the vibrations, and the vibrations result in sound. Every seashell has its own distinct sound, because every shell resonates at a different frequency.
For more experiments about the ocean, read "Janice VanCleave's Oceans for Every Kid." (New York: Wiley, 1996)
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