AHA! Now i understand the differens between pulses and freq, maybe this lib. is more suiteble for me?
http://interface.khm.de/index.php/lab/experiments/frequency-measurement-library/
But he is writning:
The library returns the measured period length in 1/16 us steps as a long integer. To get the frequency, the clock frequency has to divided by the returned value. The clock frequency is set in this example to 16000400 to compensate inaccuracies of the Arduino board.
What is that meaning for me and my arduino?
This might also work?
http://www.pjrc.com/teensy/td_libs_FreqMeasure.html