Why does sound travel faster in helium that is lighter than air but also travels faster in water that is more dense than air?
In: Physics
Sound is a physical pressure wave of particles. They act like little balls – one knocks the one beside it, which hits the one beside it, on and on, until one hits your ear drum. It’s like ripples in a pond. The “speed of sound” is how fast these ripples move along. It has to do with how heavy the particles are, and how close together they are.
**Case 1: helium vs air** – As long as they’re at the same pressure and temp, all gases have the same density of particles. If you have a balloon of helium and a balloon of air, the He atoms in one are the same distance apart as the “air atoms” (N2 and O2) in the other. All things are equal in this comparison *except the mass of each single particle*. He is MUCH lighter than O2 and N2, so when you give them the same push they’re able to move along knocking into each other more quickly.
**Case 2: Air vs water** – Now the biggest difference is the distance between particles. In liquids (and solids), the particles are packed MUCH closer together than they are in gases. The speed of sound is much higher in liquids and solids because of this. Each ball has a very very short distance to go before hitting the one next to it so the wave moves quickly. Imagine pool balls lined up touching each other. Hit one end and one flies off the other end of the line almost immediately. Also single water molecules are somewhat lighter than air molecules, so it has a double-advantage.
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