Knock Sensors

[Tom Cloud]

>KNock sensors are basically a microphone, and most modern microphones are 
>polarized, since they have two leads, one ground and one is the audio 
>output and combined VCC in.

this is assuming that the mic is an electret mic (is it ???).
If it is just a dynamic or crystal (piezo) and has no active
impedance matching drivers inside it, it should not be polarized.
(reason I question whether it's an electret is that all electrets
I've worked with -- and I've actually purchased sheets of the plastic
used to make them -- use a metallized plastic that has been stressed
so that it's permanently 'charged' so it can simulate a condensor
mic.  anyway, the plastic is easily damaged by heat -- wonder
if it could stand up to engine temps -- maybe there's some
new substance since I worked with it in the mid-80's ??)

 ... from one who knows lots about mics, and n-nnothing about knock
sensors -- but wants to

Tom Cloud <cloud at peaches.ph.utexas.edu>