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The Sound of Science - 'Resonance'

What do kids on a swing, musical instruments, and microwave ovens have in common? They all use an underlying scientific concept called resonance as well as being common parts of our daily lives.

With me are two tuning forks, each tuned to 256 Hz. I’m going to strike one and only one tuning fork. After a second or two I’ll put my hand on that tuning fork to stop it. I’m not going to touch the second fork at all. Listen closely.

Did you notice anything odd? I didn’t use any trickery, but I was able to continue the note using the second fork without hitting it. This is what resonance sounds like! But what does that have to do with playgrounds and kitchens?

Imagine a girl on a swing and her dad pushing her. The dad doesn’t just push once, he pushes her until she jumps off or it’s time to go. Every time the swing comes back to the top of arch, the dad gives a hard push. The dad’s pushes and the daughter’s swing are in resonance as his pushes are timed to add energy on each pass. In the case of our tuning forks, the air pressure from the first tuning fork is pushing and pulling the second fork at just the right time and just the right place. The second fork is now vibrating just like the first fork, thus creating the same sound. This would not work if the tuning forks were even a few hertz off!

Okay, so what about the microwave ovens? They use electromagnetic waves – not sound waves – to heat your food. However, like the tuning forks and the swinging girl, the waves are designed to resonate. The microwaves are tuned to oscillate at the same frequency as molecules of water which causes them to shake and heat up. Most foods contain water and the water molecules heated by the microwave then heat up the rest of your food, leaving you with a tasty treat.

Although they can be more complex, musical instruments rely on the same concept of resonance. Whatever chamber the air is vibrating in, such as the body of a guitar or the pipe of an organ, is damping or “weeding out” frequencies that are not designed to be heard, while using resonance to amplify the desired frequencies.

I’m Sam Watt. This is the Sound of Science on WNIJ, where you learn something new every day.  

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