zeus2kx:
Oscillation of a steel tube which is oscillated by some mechanism.
I can't calculate it as it would change by changing certain parameters (which I want to test).
Precision - I can't be sure but at this stage let's say 1% error.
If the tube is creating audio frequency sounds, which your nominal 300 Hz frequency suggests, this recent thread should be relevant: http://forum.arduino.cc/index.php?topic=540969.0;topicseen
The sensor used in this case is an FC-109 amplified electret microphone module or similar.