Sound and vibrations — Physics, 7–10 years
Sound begins when something vibrates and makes nearby air vibrate too. Those moving vibrations can reach our ears and be heard.
What sound is
Sound starts with a vibration: a quick back-and-forth movement. A plucked string, a drum skin or your vocal cords can vibrate. The vibration travels through air, water or a solid until it reaches an ear.
Why we study vibrations
People wanted to understand voices, music and noises, and to send messages across a distance. Looking for the vibrating part explains why a bell rings and why a guitar string makes a note. It also explains why sound needs something to travel through instead of crossing empty space.
A ruler making sound
Hold 10 centimetres of a plastic ruler over the edge of a table and press the rest down. Pluck the free end: it vibrates and makes a note. Now leave 20 centimetres free and pluck again. The longer part moves more slowly, so the note is lower.
A common mistake
It is tempting to say that loud sound and high sound are the same. They are different: bigger vibrations usually make a louder sound, while faster vibrations make a higher-pitched sound. A small whistle can be high but not loud, and a large drum can be loud but low.
Where it is used
Musical instruments use controlled vibrations to make notes. Microphones change sound vibrations into electrical signals, and speakers change signals back into vibrations. Doctors can also use ultrasound, which has vibrations too fast for human ears, to make images inside the body.
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