1. Recording engine sounds

• Connect THE KNOCK BOX to the engine in a normal fashion
• Connect signals from Pin 13 and Pin 6 (Left and right headset) into the Blue input of your sound card via a shielded cable and 3.5mm jack. (This is so we can record these sounds using your soundcard on your PC or LapTop
• Connect the shield of the this cable to pin 12 (common ground)
• Select and set the volume control slider on the 'line in' to about half way on the Volume control button
  
  
  
  
• Run the Sound Recorder function
  
  
  
  
• Got to the File menu and select 'File > New'
  
  
  
  
• Record the sound of the engine without knocking at around 4000 R.P.M., say for about 10-20 seconds.
• Make sure the signal is not so loud as to cause clipping (flattening) of the wave form profile. You can tell this is happening is the scrolling green line reaches the top and bottom of the small screen on the sound recorder as in this example below.
• Reduce the sensor input signals if this is happening until the signal peaks appear about half way to the top of the small screen.
  
  
  

• Save the file by selecting 'File > Save As'
  
  
  

• Save the file to a convenient location.
• Increase the timing until you get a moderate amount of audible knock through the headphones. Repeat as above and save this to a different filename. (ie 'modknock4000.wav')
• You should now have two different recordings; one as you would hear without any knock, and one as you would hear with a moderate but not engine threatening amounts of knock. What we will now do is to use these to evaluate what frequency is noticeable when your engine produces knock. We will do this by using the 'Virtins instrument' spectrum analyser function.

2. Open the Virtins Instrument program

• Open the application from your start menu
  

• Open a new wave file without knock
  
  
  
  
• The analyser will show you something like the following screen:
  

3. Discussion

• What this shows in the upper window (Spectrum Analyser) is that there are no particular frequencies that are much bigger than the background spread of frequencies between 2Khz and about 10Khz. And these sounds are all below 7 mV. It also shows in the lower window, (Oscilloscope) that the signal is not clipping and is about 300 millivolts peak, which is near perfect for our purposes. It also shows that we recorded for about 14.5 seconds.

4. Compare a file with knock

• Next load a file that does have knock content. You will now see a stronger signal with an extra frequency component, in this case centred on about 5kHz. (For our example) Your signal may have a different frequency from this.
  
  

5. Discussion

• What we see here is a louder signal of about 1 volt peak. It contains many frequencies as before, but with one exception, a large and obvious signal peak at 5Khz. What this implies strongly is that the frequency of knock for this engine is centred at 5Khz. We will use this new knowledge to 'tune' THE KNOCK BOX logging function to this frequency, much as a radio tunes to a particular station. This will help to discriminate knock events more cleanly and give us a way to detect the occurrence of knock by removing or filtering most of the background noise of the engine.

6. Tuning THE KNOCK BOX

• Connect your green 3.5mm jack cable into the green jack of the sound card on your PC or Laptop.
• Now open the Virtins Signal generator.
  

• Set both channels A&B to 'Sine' at about 200 mV output and set the frequency to your frequency peak as shown by the analyser (5000 Hz if that is where your frequency peak is). Do this either by dropdown box selection, or with the scroll bars on both channel as follows.
  

• Next press the green arrow button. This will create a constant signal into your knock box and you should be able to hear a whistle from your headphones. For preference I would not wear the headphones at this point.
• Connect the precision rectifier pin (pin10) to a multimeter and set it to read about 2 volts full scale DC.
• With the signal going into THE KNOCK BOX from your signal generator, rotate the frequency adjustment pot with a small screwdriver until you see a maximum value minus voltage from Pin 10. As you rotate the pot, the voltage will be small and as you rotate the pot the voltage will go to a higher reading and then drop to a lower reading. Tune the pot until you see the maximum value minus voltage.
• You have now tuned the internal filter to the frequency of knock on your engine and it is ready to test or monitor your engine.