Reference information about Knock Sensors

[steveb at newkla.kla.com]

I would appreciate it if one of you in the know can point me to a 
good reference work that covers the function of the knock sensor.
I am interested in knowing if they are tuned for particular freq-
uencies, and such.  They appear to be very much like ceramic ele-
ment microphones ... is there any signal processing circuitry on 
the sensor, or is the processing done in the ECU?  

TIA!
Steve Buchholz
s_buchho at kla.com
San Jose, CA (USA)