I believe most knock sensors are essentially piezo microphones. They're designed to produce voltage in response to certain frequencies. The ECU has a table that determines what the response is to the amount of voltage at a given RPM. By doing this the engineers that set up the car's engine management can configure less or more timing to be pulled depending on how much voltage the mics produce at what RPM. They know where false knock is more likely to be detected. I know this is how it works on several Japanese and American cars, no idea about German cars. I assume most German cars use Bosch Motronic engine management.