What happens when there is a change in the material through which the sound travels?
The speed of sound is not always the same. Remember that sound is a vibration of kinetic energy passed from molecule to molecule. The closer the molecules are to each other, the less energy it takes for them to pass the sound to each other and the faster sound can travel. It is easier for sound waves to go through solids than through liquids because the molecules are closer together in solids. Similarly, it is harder for sound to pass through gases than through liquids, because gaseous molecules are farther apart. The speed of sound is faster in solid materials and slower in liquids or gases.
The speed of sound is also different for different types of solids, liquids, and gases. Some materials, like nickel or iron, are more elastic than others. Elasticity describes how quickly the molecules return to their original positions. Molecules that return to their original shape quickly are ready to move again more quickly, thus they can vibrate at higher speeds. Sound can travel faster through mediums that vibrate faster. Sound travels faster through elastic solids like nickel or iron than through solids like lead, which is less elastic.
The density of a medium also affects the speed of sound. Density describes the mass of a substance per volume. A substance that is denser per volume has more mass per volume. Usually, larger molecules have more mass. If a material is denser because its molecules are larger, it will transmit sound slower. Sound waves are made up of kinetic energy. It takes more energy to make large molecules vibrate than it does to make smaller molecules vibrate. Thus, sound will travel at a slower rate in the denser object. If sound waves of the same energy were passed through a block of wood and a block of steel, which is more dense than the wood, the molecules of the steel would vibrate at a slower rate. Thus, sound passes more quickly through the wood, which is less dense.
Suppose, however, that two substances are made of different molecules which weigh the same amount. They have the same volume, but one substance is more dense. We know the denser substance must have more mass per volume. Since both substances have molecules of similar weight, this extra mass means the denser substance has more molecules per volume. More molecules are squeezed into the same volume, therefore the molecules are closer together. Since sound is more easily transmitted between close molecules, it travels faster in the denser substance. Sound moves faster through denser air because the molecules are closer together in dense air and sound can be more easily passed on.
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