Which kind of sensor for "knock" detecting

Other Hardware Sensors
1

Hey folks,

imagine following scenario:
you have a metal bar, 30cm long and cross area is 2x2cm. On the one end there is a light knock, mostly the same, like cracking your knuckles or bones.
On the other end of this bar you want to detect this knocking.
Which sensor might be the best to indicate this?

There may be some disturbing sounds as well, but the knock is unique.

I Thought about a piezoelectric sensor, but a first try yesterday, where i just put the plate on a table and knocked on another point at the table, didnt gave me satisfying results (i checked with an oscilloscope directly connected to the Piezo). Only when you knock right ON the piezo you see the signal..

any ideas on other sensors?

2

Using a microphone/guitar pickup or even accelerometer?

3

I assume you are trying to detect small mechanical vibrations so perhaps a phonograph needle w/ pre-amp.

4

How is this bar supported? Does it lie somewhere?

5

An automobile knock sensor from the junkyard?