General Knowledge
Why does a struck bell keep ringing after you stop touching it?
- The metal goes on vibrating
- Sound bounces off walls
- Your ears keep hearing
The answer is The metal goes on vibrating. The bell itself is still moving, thousands of times a second.
Because the bell itself is still moving. Striking it deforms the metal, which springs back, overshoots, and goes on oscillating hundreds or thousands of times a second - and each oscillation pushes the surrounding air. The sound stops when the energy has all leaked away into the air and into heat inside the metal.
A good bell rings for a long time because bronze loses very little energy internally, which is why bells are not made of lead. A cracked bell sounds dead because the crack rubs against itself and turns the vibration into heat within a second or two.
A bell is also unusual in that its overtones are not in a neat mathematical series like a string's, which is why a bell has a distinctive clangour and why tuning one is skilled work - the founder shaves metal from the inside to bring the partials into a chosen relationship. Big Ben, the bell in the Elizabeth Tower, cracked shortly after installation in 1859 and was left cracked, turned to strike a different spot with a lighter hammer, which is why it sounds slightly wrong to this day.