Originally Posted by mega squirt website

Spark knock is the sound of abnormal combustion in an engine. Once combustion in a spark-ignition internal combustion engine is initiated by a spark, the flame front is designed to spread from the spark plug and travel across the combustion chamber rapidly and smoothly. As the flame front propagates across the chamber, the remaining unburnt air-fuel mixture can ignite spontaneously (auto-ignites) before the flame front arrives, due to the increasing pressure and temperature in the combustion chamber. When this occurs, there is a sudden jump in the pressure in the cylinder. This causes in the characteristic knocking or pinging sound. Knocking results from ignition timing that is too advanced for the fuel octane rating and operating conditions at that moment.
The knock sensor is tuned to a specify frequency, like a tuning fork. When this frequency is applied to the sensor (through its connection to the engine), a peizoelectric crystal inside the sensor generates a small voltage (~1 volt), much like a microphone. As an example, some Corvette knock sensors (GM PN 1997562, 1997699, OR Standard Motor Products KS45, KS46, KS49, or KS117) had a design frequency of 5200 hertz, and they would produce a signal between 4800 rpm and 5600 rpm