After New Year’s Day, there’s sure to be a heap of empty soft drink cans lying about. Find a couple of clean ones and wake up the neighbourhood with this simple activity!
Warning: The noise made in this activity is quite loud and high pitched. It’s a good idea to not do this inside. Use tissues in your ears to protect them. Never make the noise close to anybody’s head.
You will need
2 empty aluminium soft drink cans (preferably with no dents in them)
2 tissues
What to do
Screw the end of the tissue up and place gently into your ears.
Loosely hold each can with your thumb on the edge of the bottom and your index finger on top.
Position the cans close to one another so they are very nearly touching. The easiest way to do this is to have them touch and then separate them slightly.
Hold them in this position in front of your face, with your lips close to the gap about half way down the height of the cans.
Give a good, hard blow.
If you don’t get it the first time, practice.
What’s happening?
To understand what made the noise, it helps to think of air particles as people. Since air is a gas, these ‘people’ would be running around like crazy, bouncing off the walls of whatever room they might be in. These bounces are what we refer to as air pressure – the more they hit the walls or the harder they bounce, the higher the pressure will be.
What if there were people also running around in the next room? They’d also be smacking into the wall. If there were more people in there, or they were bouncing harder, they might push the wall down. Yet if everybody in each room is bouncing around the same, the wall will stay in one place.
The cans and the surrounding air are like the two rooms. Air pressure is equal, so they don’t move when you hold them. When you blow through the gap, you’re forcing air to rush through at a much higher speed than when it left your mouth. As the particles are moving quickly along the sides of the can, they’re not bouncing into the can’s walls as hard as they were previously, reducing the pressure inside the gap.
The surrounding air pressure then pushes the cans together (you may feel them tugging slightly), squeezing the tiny bit of air and increasing the pressure slightly. This rapid change in pressure makes the sides of the cans vibrate quickly, creating high-pitched sound waves that are amplified (made louder) by the empty space inside the can.
Applications
Palaeontologists have long known about a family of duck-billed dinosaurs called hadrosaurs, many of which happened to have strange crests on their heads. Corythosaurs, Lambeosaurs and Parasaurolophus are three examples, each with very different structures above their eyes. Nobody was certain what they might be used for, as they were too fragile to be used for protection or fighting.
While there are still several hypotheses, one favoured by many biologists is they were used to create and amplify their calls. The empty chambers inside them would change the sound waves the animals made inside their nasal cavity and increase their volume, much like the insides of the drink cans made the vibrating walls sound much louder. The different shapes would create distinctive noises, allowing herds of the dinosaurs to communicate with one another over long distances.
