I initially tried 6KHz and wasnt getting reliable knock detection, so I done a frequency analysis and you can see why...  Frequency is the Y axis on this spectrograph, notice at 6KHz you can hardly tell the difference between the area with no knock Vs the heavy knock.  The best signal to noise ratio looks to be around 8-8.5KHz, you can see quite a clear difference in sound energy between no knock and heavy knock.