What is sound?
Sound is a vibration that travels through air, water or solids as a wave. When the vibrations reach your ears, your brain interprets them as noises, music or speech.
When an object vibrates, such as a guitar string or your vocal cords, it pushes nearby particles together and then pulls them apart. These areas of high and low pressure travel outward as a sound wave. Sound needs a medium to travel through, so it cannot move through the vacuum of space.
The pitch of a sound depends on its frequency, how many vibrations happen each second, measured in hertz. Humans can usually hear from about 20 to 20,000 hertz. Loudness depends on the wave's amplitude and is measured in decibels. A whisper is about 30 decibels, and a rock concert can exceed 110.
How fast sound travels
Sound travels at about 343 meters per second in air at room temperature. It moves more than four times faster in water and even faster in solids like steel. That is why you can sometimes hear a distant train through the rails before you hear it through the air.
Did You Know?
Elephants and whales communicate using infrasound, below the range of human hearing, while bats and dolphins use ultrasound, above it.