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Tag Archives: sound waves

In A Vacuum: Silence

A vacuum is space that contains no matter. Outer space is a vacuum, more or less. We can create artificial vacuums here on earth. The strange thing about a vacuum is that sound cannot travel through it. So, if something big out in space explodes, in a supernova, you can see it, but you will never hear it. If you’re in space, and a bomb goes off near you, you may feel the heat wash over you, but you won’t hear it.

The reason that sound can’t travel in a vacuum is that there aren’t any particles to carry the wave. Sound waves are longitudinal waves that must have a medium to travel through. They cannot transfer energy from particle to particle because they are spaced too far apart, and the energy cannot jump from particle to particle.

Whenever you see a movie that takes place in space, listen to see if they’re following this law of physics. Much of the time, if there’s an explosion, they add sound effects, when in reality it’d be silent. Take the newer Star Trek movie, for example. When people are pulled out into space after part of the ship was destroyed, everything was silent. That is how it should be. Next time you’re watching a space movie, listen intently to your sub and speakers, and see if the filmmakers got it right.

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Rubens’ Tube: Writ in Fire

Probably the coolest way to see sound waves is with fire. In 1905, German physicist Heinrich Rubens invented the Rubens’ tube, or the standing wave flame tube. The Rubens’ tube demonstrates acoustic standing waves through fire.

The Ruben’s tube is a length of pipe that has evenly-spaced holes along the top and is sealed at both ends. One seal is attached to a speaker or frequency generator while the other seal is attached to a propane tank. The tube is filled with gas, and the gas that comes out of the holes is lit on fire. You can now see the sound waves, etched into the air by way of fire.

If a constant frequency is used, then the inside of the tube will have a standing wave, and will create points with oscillating pressure and points with constant pressure. The flames will be lower at the points of oscillating pressure because less gas will escape through the holes in the tube. Naturally, the flames will be higher at the points of constant pressure.

Having a standing wave is cool, but not nearly as much fun as playing music into the Rubens’ tube. Take a look, it’s really quite enjoyable:

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