Sound comes from vibrations — Physics, 11–13 years
Sound begins with a vibrating object and travels through a material to reach a listener. Physics, 11–13 years.
Idea
A sound starts when something vibrates back and forth, such as a guitar string, a drum skin or your vocal cords. The vibration pushes and pulls nearby particles in air, water or a solid, passing the disturbance along. Your ear detects these changing movements and your brain experiences them as sound.
Why
We need this idea to explain why a silent-looking object can make a noise and why sound cannot cross empty space. The vibrating source alone is not enough: a material must carry the disturbance from it to an ear or a detector. This separates sound from light, which can travel through empty space from the Sun to Earth.
Worked example
Stretch a rubber band around an empty box and pluck it. Step 1: the band vibrates and pushes the nearby air. Step 2: the box also begins to vibrate, moving more air than the thin band alone. Step 3: those air vibrations reach your ear, so you hear a louder note. If you touch the band gently, you stop its vibration and the sound fades.
Common trap
A common mistake is to think that air itself travels all the way from a speaker to your ear. It is a reasonable picture because you feel air move from a fan or a shout. In sound, nearby air particles mainly jiggle back and forth and pass the disturbance on; the particles do not make one long journey across the room.
In the world
Musical instruments are built so a vibrating string, air column or surface produces a useful sound. Microphones turn air vibrations into electrical signals, and loudspeakers turn electrical signals back into vibrations. Ultrasound scanners use vibrations above human hearing to make images inside the body.
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