Everything is vibrating. Absolutely everything is wiggling and jiggling, and most of those things are doing it really fast! Now, I can hear you saying “Hey…maybe you need to check your eyesight or lay off the coffee because in my house, I’m not seeing everything jiggling.”
You can easily make a humming (or screaming!) balloon by inserting a small hexnut into a balloon and inflating. You can also try pennies, washers, and anything else you have that is small and semi-round. We have scads of these things at birthday time, hiding small change in some and nuts in the others so the kids pop them to get their treasures. Some kids will figure out a way to test which balloons are which without popping… which is what we’re going to do right now.
This is one of my absolute favorites, because it’s so unexpected and unusual… the setup looks quite harmless, but it makes a sound worse than scratching your nails on a chalkboard. If you can’t find the weird ingredient, just use water and you’ll get nearly the same result (it just takes more practice to get it right). Ready?
NOTE: DO NOT place these anywhere near your ear… keep them straight out in front of you.
Sound is everywhere. It can travel through solids, liquids, and gases, but it does so at different speeds. It can rustle through trees at 770 MPH (miles per hour), echo through the ocean at 3,270 MPH, and resonate through solid rock at 8,600 MPH.
Sound is made by things vibrating back and forth, whether it’s a guitar string, drum head, or clarinet. The back and forth motion of an object (like the drum head) creates a sound wave in the air that looks a lot like a ripple in a pond after you throw a rock in. It radiates outward, vibrating it’s neighboring air molecules until they are moving around, too. This chain reaction keeps happening until it reaches your ears, where your “sound detectors” pick up the vibration and works with your brain to turn it into sound.
You can illustrate this principle using a guitar string – when you pluck the string, your ears pick up a sound. If you have extra rubber bands, wrap them around an open shoebox to make a shoebox guitar. You can also cut a hole in the lid (image left) and use wooden pencils to lift the rubber band off the surface of the shoebox.
When a guitarist plucks a string to start the vibration, it not only vibrates the string, but it also vibrates the entire box of the guitar. This is called a forced vibration, which means that the motion of the original source vibration is also causing another object to vibrate (the box of the guitar). Since the box is larger than the string, it amplifies the vibration and makes it louder.
An electrical signal (like music) zings through the coil (which is also allowed to move and attached to your speaker cone), which is attracted or repulsed by the permanent magnet. The coil vibrates, taking the cone with it. The cone vibrates the air around it and sends sounds waves to reach your ear. Here’s how speakers work and also how to make your own out of cardboard (it really works!):
We’ve already looked at standing wave patterns that are created when a reflected wave interferes with an incoming wave. It looks like the wave is fluctuating in place, when really it’s just an optical illusion of two waves interfering with each other. The point is, this effect are created at specific frequencies called harmonics, and now it’s time to learn about vibrational modes using a really cool experiment by Ernst Chladni.
A vibrational mode is the standing wave pattern that give the highest amplitude vibrations with the least amount of energy input. If you vibrate an object at it’s natural frequency, you’ll get the highest amplitude during the vibration. Sometimes the amplitude (which is related to the energy of the vibration) that the object vibrates at is so high that the object will actually will tear itself apart. Here’s a video where the wind was blowing the bridge, which started a natural vibration in the bridge which tore itself apart.
Ella Fitzgerald was famous for breaking the wineglass with her voice at the end of the Memorex commercial:
Let’s see this in slow motion using lab equipment (video courtesy of MIT):
