This is a fun science experiment to try the next time you go swimming. It is a very dramatic demonstration of the difference between how sound travels in liquids and gases.
To try this, you will need:
- a swimming pool, lake, or some other large body of water
- two hard objects, such as rocks, spoons, etc.
With my background in geology, I decided to use two rocks for this one, but you can use any two objects that will make a nice click when you tap them together. Hold them out at arms length in front of you and tap them gently together. You want the sound to be just loud enough for you to hear. Pay close attention to how much force you are using to click the rocks together.
Then get into the water. Once again, hold the two rocks out at arms length, but this time hold them under the surface of the water. Try to tap them together with the same amount of force that you used the first time. What do you hear? Depending on how quiet the pool is, you may hear the sound a tiny bit louder.
Next put your ear into the water and try it again. This time, the sound is MUCH louder. Ask a friend to take the two rocks to the other side of the pool and tap them together, first out of the water, and then under the water. With both your ear and the rocks out of the water, you probably can't hear the sound at all. With both your ear and the rocks under the water, you should be able to hear it very clearly, even at quite a distance. Why?
Sound is vibration. For us to hear a sound, something has to vibrate back and forth, and those vibrations have to reach our inner ear, where they jostle nerves, to tell our brains that we heard a sound. The substances that the vibrations pass through to get to your inner ear play a big role in how well you hear the sound.
Two main factors determine how well sound will travel through a substance: elasticity and density. Elasticity is how a substance reacts to movement of nearby atoms. Highly elastic substances, such as steel, quickly move to realign themselves, where substances with low elasticity, such as putty and clay, do not. Substances with a high elasticity are better at conduction sound waves, passing them along faster. That is why solids tend to transmit sound better than liquids, and why liquids tend to transmit sound better than gases, which have a very low elasticity.
The other factor is density, and this is where many people, including myself, can get confused. I had always heard that sound travels faster through denser substances. When you first think about it, solids are denser than gases, and they conduct sound better.
Actually, the opposite is true. Solids tend to carry sound better because of their higher elasticity. If you have two substances of similar elasticity, such as helium gas and carbon dioxide gas, you find that sound waves travel much faster (965 meters/second) through the low density helium and much slower (259 meters/second) through the much denser carbon dioxide. Why?
For that, we have to think about inertia. Imagine that you have a large rock and a large block of Styrofoam. They are both the same size, but the rock is much heavier. If you push both with the same amount of force, which one will move more easily? The Styrofoam, right? It has less mass, so it takes less energy to overcome its inertia. The same is true for sound waves. Sound waves travel more slowly through denser substances.
Between the two properties, elasticity and density, elasticity has a much bigger effect. When you compare water and air, the water is much more elastic. Although the water is denser, its stronger elasticity conducts the sound waves much better, so you hear the sound much better in the water than in the air.
Sitting here, I suddenly realized that the door to my freezer must be very elastic, because I can hear the ice cream calling to me.