Sound and Sound Waves

Posted on February 1st, 2011

Sound is often considered more the stuff of music class but there is a great deal of science involved as well.  From the beautiful notes produced by a violin to the explosive crash of thunder and fireworks, we can use science to make useful predictions about how sound waves will travel, and how they will, well, sound. 

During the Lab Ratz “Sound and Sound Waves” workshops we discuss how sound waves moving through air do not really look like waves in the ocean.  Instead, they are formed by molecules bouncing into one another and transferring energy.  This type of wave is also called a compression wave because the molecules in the air compress, or become closer together as the wave passes by.  The wave is formed by an object vibrating – Such as a guitar string – and causing the molecules of air to begin moving also.  But, sound will also move through a solid and a liquid.  Actually, sound travels more quickly and more efficiently through solids and liquids than through gasses because the molecules of a solid or liquid are packed together more closely than a gas, which makes it easier for them to bump into one another transferring the sound energy. 

One way to test this is to find a long metal railing like the type on a chain link fence.  Have a friend stand at one end of the railing with their ear against it.  Then, at the other end of the railing, use a rock and smack the metal with it.  Your friend should hear two sounds at two different times – One as the sound arrives through the metal and another as it travels through the air – Both occur at different times because the sound moves at different speeds through different materials. 

Because sound travels so well through solids, instruments like guitars and violins use hard, dense wood bodies to amplify the vibrating strings.  The body vibrates with the string and transfers the vibration to the air inside the hollow space of the body.  The hole or holes in the front of the instrument allow the sound waves produced to escape and travel to your ears.  You can build an instrument of your own using a wooden box, such as an empty cigar box.  With some adult assistance, cut a hole in the front of the box and devise a method to anchor some strings – Guitar strings from a music shop work best but wire or even rubber bands will do the trick.  Use a small piece of hard wood for a bridge – the part the holds the strings above the body and transfers the vibrations into the body – and come up with a way to attach the other end of the strings.  You can use bolts or screws for this or you can buy actual guitar tuners at a music shop.  If you want your instrument to be played like a real guitar, you can build a neck out of wood and attach it to the body using glue or bolts.  The tuners should go at the end of the neck farthest from the body.  If you want to get really serious about building this instrument check out www.stewmac.com.  They have everything you need to build guitars, violins, basses, mandolins and dulcimers.  Many professional musicians build their own instruments and some play guitars made out of wooden cigar boxes.  Experiment with the design and materials and see what works best!

And, here is a classic science experiment that uses the same idea – The tin can telephone.  While this is a simple one, it works very well.  All you need are two empty food cans and some string or wire.  Punch a small hole in the bottom of each can and anchor the string inside the can, pass it through the hole and along to the other can.  If you speak into one can the vibrations are carried along the string and to the other can where the can vibrates and creates sound waves in the air inside the can.  Again, experiment with the materials you use to see what works the best. 

The next time you are watching fireworks or you see lightning in the distance try using a little science to calculate the distance between you and the storm or explosions.  You can do this if you know the speed that sound travels through air.  When you see a firework explode or a flash of lightning, start counting the number of seconds it takes for the sound to reach you.  Sound can travel about one mile in six seconds,  so if it takes the sound only three seconds to reach your ears, that means that the storm is about half a mile away.