General Knowledge
Why does a struck tuning fork get louder when you touch it to a table?
- The table surface moves much more air
- The table adds energy
- It rings for longer
The answer is The table surface moves much more air. More air is moved, so it is louder and dies away faster.
Because the fork itself is terrible at moving air. Its prongs are thin and slice through it, so almost all the vibration energy stays in the metal. Touch it to a table and the whole tabletop is driven into vibration - a large surface pushing a large volume of air - and you suddenly hear it.
Nothing is added. The energy is the same; it is being converted to sound far more efficiently, so it is louder and dies away much faster. A fork left ringing in the air can sound for half a minute; pressed to a table it is gone in a couple of seconds.
Every acoustic instrument is built around this. A violin string alone is nearly inaudible - the bridge transmits its vibration to the soundboard, which moves the air, and the body of the instrument is the loudspeaker. The same reason explains why an electric guitar unplugged is quiet and why a music box on a wooden shelf is much louder than one held in the hand.