Tuesday 17 September 2013

How Do Sound Waves Travel?

This will throw you back to 7th grade science class. Remember when your teacher was talking about molecules and atoms, those building blocks of the universe that were so tiny you can't see them? Well, if you missed that class, this will be a refresher because sound waves travel through air (among other things that are made of molecules). Sound is vibration, or energy, that gets all the molecules around the source of that vibration all excited, so they vibrate too and start bumping into each other and pass along the vibrations. It's kind of a molecular mosh pit. The molecules lose some of their excitement the further away they are from the source of the vibration so the noise gets quieter and quieter until the molecules run out of vibrations and there the sound ends.

9th Grade Science, a Little More Advanced

    Sound waves are essentially variations of pressure that exists, in this example, in the air. (Air is made up of molecules). To help visualize the nature of sound waves, think of yourself in a tub of water with a plastic toy boat floating in front of you. If you create waves by pushing your hand along the top of the water, the waves push against the boat and move it from one end of the tub to the other. Now think of the waves and the boat as sound. As the waves move along, so does the boat (which is also the sound). The bigger the wave, the more the boat moves up and down and the farther it can travel because the sound waves, like vibrating molecules, dissipate over distance. Now if you were in a large pool, you'd need much bigger waves to push the boat (sound) all the way to the other end.

Time to Play Dominoes

    When a vibration causes the molecules to vibrate, it isn't the first molecule that just gets so excited that it rushes to your ear, bypassing all the other molecules. When it gets excited, and jumps all around, it starts bounding off all the other molecules around it that bound around to the ones next to them and so on. Now set up your dominoes on their side in a line, close enough together so that when the first one falls it will fall into the second and then the second onto the third...you get the idea. Now push over the first one and it will cause the entire line to fall. Now instead of a straight line make a Y pattern using a few of the dominoes. At the top of each Y, start two more branches, and at the top of those branches do the same until you run out of dominoes. Again, knock over the first and watch how they all fall around the Y tree you built. That more accurately shows how sound waves don't move in just a straight line but in a spherical or conical way.

Time to Put the Toys Away

    For sound to travel, it needs a medium that contains molecules. It can be water, steel, concrete, anything that has molecules close enough to one another that they can vibrate and transmit the sound. In fact, the closer the molecules are bunched, the faster the sound travels. That's why when you put your ear to a railroad track you can hear it coming long before you hear it's noise in the air. Steel molecules are packed much more tightly than air molecules. That is also why there is no sound in space. The molecules in space are so far apart that when one of them near a vibration starts dancing the noise jig, it has no other molecules to pass the vibrations to. In essence, there is no medium through which sound waves can travel.

On to College


    As sound waves travel from a source, they are known as compression waves. Because of the excited state of the molecules, they compress while vibrating. But once the vibration has been passed along the molecules left behind slow their vibration because they have essentially transferred the energy along. The ones left behind experience rarefaction. This is a more technical explanation of what scientists mean by sound wave. It looks like this and can be charted as a two-dimensional graph.

Why We Hear Things

    When those vibrating molecules reach the outer ear, they are funneled into the inner ear, where a thin stretched membrane "catches" the vibrations, which are fluctuations in air pressure. The body then translates these fluctuations into electrical signals that are transmitted to the brain, which further deciphers them. Pretty complicated process but those are the basics.

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