What's the cheapest way to detect vibration with Arduino?
What's the vibration sensor that I need?
I want to detect PING PONG NET vibration and light on led if touched.
Piezo sensors are cheap, reliable and designed for the purpose you suggest - arduino tutorial
In the uk, bitsbox has one for 75p. You can probably find others cheaper, or salvage from electronic toys.
EDIT following poster's clarification of use: Sound is vibration through the air, I see no reason why a piezo sensor cannot accomplish what you suggest, but the form factor may not be ideal for fixing to ping pong nets! I think a flex sensor would be more suitable, sewn into the top of the net.
An optical or magnetic sensor mounted mid net, bottom edge might be a good way to acomplish this. Might also detect vibrations in the table, as might any sensor.