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Why are sound waves invisible?
Category: Physics
Published: February 11, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Sound waves are not always invisible. Sound is simply the ordered vibration of matter (as opposed to heat, which is the disordered vibration of matter). Any time you see a vibration, you are seeing sound. For instance, hit a bell and watch it shake. That shaking motion of the bell is the sound waves traveling through the bell. When a jet flies close by over your house, you hear a rumble and see the silverware dance around on the table. That dancing motion is the sound waves from the jet traveling through your table and silverware.
Talk quietly to your friend and you indeed cannot see the sound leaving your mouth and entering her ear. This is not because sound is invisible, but is because air is invisible. When you talk, you are sending sound waves into the air and the air itself is what is doing the vibrating. Since air is invisible to begin with, there's no way for you to see the air once it starts vibrating. (Strictly speaking, air is not perfectly invisible. But air is so close to invisible that you have to look through a lot of air, or through air with extreme temperature/pressure differences, in order to see the air.) Also, most sound waves consist of material vibrating too quickly for our eyes to pick out. For instance, take your hand and wave it back and forth very rapidly in front of your face. What do you see? You see a blur because the motion of your hand is faster than the speed at which your eyes are able to process images. The vibrating motion of most sound waves is far faster than your waving hand, and is therefore just a blur to human eyes. The sound waves traveling down a plucked guitar string are not invisible. They are just moving so fast that the plucked guitar string looks like a blur to our human eyes. A high speed camera has no problem capturing detailed, non-blurry images of the sound waves traveling down a guitar string.