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Sound travels as a wave through materials called mediums. It can travel through gases (like air), liquids (like water), and solids (like metal and wood). Sound moves faster through solids and liquids than through air because the particles in these materials are packed more closely together. Sound cannot travel through empty space (a vacuum) because there are no particles to carry the vibration.
Have one student tap on one end of a long table while another listens with an ear to the table surface, then compare to hearing the tap through air. Use a string telephone (two cups connected by a tight string) to experience sound traveling through a solid. Discuss why you can hear sounds underwater in a swimming pool.
When you clap your hands, the sound reaches your friend across the room almost instantly. But how does it get there? Sound travels as a wave through a material called a medium. The air between you and your friend is the medium. When your hands clap, they compress the air right next to them. Those compressed air molecules bump into the next ones, which bump into the next, and so on, until the wave reaches your friend's ears.
This type of wave is called a compression wave (or longitudinal wave). Unlike ocean waves that move up and down, sound waves push particles back and forth in the same direction the wave travels. Imagine a long line of standing dominoes — when you push the first one, the push travels through the line. Sound works similarly, except the "dominoes" are air molecules that spring back into place after the wave passes.
Sound does not just travel through air. It can travel through liquids and solids too — and it moves faster in those materials. In air, sound travels at about 343 meters per second (roughly 767 miles per hour). In water, it moves about four times faster. In steel, it moves about 15 times faster than in air. The reason is simple: particles in solids and liquids are packed much more tightly than in gases, so vibrations pass from one particle to the next more quickly.
You can experience this yourself. If you press your ear against a table and someone taps the other end, you hear the tap much louder and faster through the table than through the air. Native Americans historically put their ears to the ground to hear distant horses approaching. Whales sing to each other across hundreds of miles of ocean because water is an excellent medium for sound. The one place sound cannot go is through a vacuum — empty space with no particles at all. That is why, despite what movies show, explosions in space are truly silent.