Mar. 04, 2011

Science Dad on Sound Waves

by Vince Harriman

Click to enlarge images
The long tradition of tin can phones

Beckett and I have tried to make a paper cup phone before, just for fun, but this week, Beckett asked if we could do it for a science project. Our first challenge was finding the right materials. Cups and string are usually the kind of thing we have handy, but we finally had to resort to using soda cans and string we found in the shed.

We measured out enough line to stretch from one side of the house to the other. Beckett tied his line inside the can and stretched it all the way to the other side of the house. We tried whispering, but that didn't work so well, so we talked into the cans with our normal voices -- and they worked!

Now was the fun part. Beckett and I have talked before about different types of waves, starting with a discussion of ocean waves at the beach. In this case, the bottom of the soda can turns the sound waves from our voices into mechanical waves. The mechanical waves then travel down the tight string until they reach the bottom of the second soda can, which transforms them back into sound waves.

The great thing about this experiment is that so much of our audio electronics still use this principle to function. Even the coolest smart phone has something in it that resembles the bottom of a soda can connected to a string -- only now the phone is connected to a sensor that turns the sound wave into an electrical wave that can be transmitted to a speaker. In the speaker, as in the bottom of the second can, the electrical impulse is turned into mechanical energy, which vibrates the diaphragm and creates sound.

If you want to make your own tin can or paper cup phone, here are some hints. Plastic and paper cups work well -- probably better than metal cans since they react more to the sound vibrations. Use a string that doesn't stretch much -- the more the string stretches, the more it will dissipate and dampen the 'signal'. Make sure you keep the string taut as you use your 'phone' -- any slack will destroy your signal.

There are plenty of other ways to 'see' sound waves. If you want a visual representation, place a large bowl of water next to a speaker (or tie the receiving end of your tin can phone to the bowl) and shine a strong light on the surface -- the reflection of the light will move with sound vibrations. If you feel ambitious, you can experiment with aluminum foil -- position a piece of straight smooth foil next to a sound source and watch what happens.

We are planning an upcoming post on digitizing information: the process by which one type of information (sound, images, math problems) is turned into digital data and manipulated. A digital signal is made of discrete packets that approximate the original data. In the case of the tin can phone, and most early phones until just a couple of years ago, the signal was analog -- a continuous representation of the original data in a different form.

The neighborhood foxes

Finally, an update on our Urban Wildlife post. Late one night last week I heard a horrible noise in the backyard and went to investigate. The fox was back, barking and yelping. I had never heard a fox bark -- it is a very distinctive and unusual sound. I found the fox next door in the grass, just hanging out. It hopped the fence into our yard, then hopped another fence and disappeared. When I mentioned it to our neighbor a couple of days later, she sent this picture. Apparently the foxes are quite comfortable.

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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