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Hearing Detonation

DIY knock detection - cheap, easy and very effective!

by Julian Edgar

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At a glance...

  • Cheap, simple and incredibly effective
  • Can also be used to diagnose gearbox and idler issues
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Way back in 1999 we covered the quick and simple construction of an electronic listening device that would allow you to hear detonation while tuning. Well, 15 years later, here is an update. The good news is that the quality has got higher and the cost lower!

The approach

While you might think that we’d be using a purely electronic detonation system that uses a knock sensor and lights a LED when knock occurs, a good audio detection system being monitored by a skilled operator is in most cases more accurate and easier to set up.

But the problem in detecting detonation by simply listening for it is that it's hard to hear the noise clearly from inside the car. The sound deadening that comes from thick firewalls mean that it's often hard to even hear the engine, let alone any specific odd noises being emitted from it.

Click for larger image

So why not make an electronic stethoscope? OK, let's do a bit of parts pricing - mmmm, here's an amplifier module ($7), here's a box ($4), here's a microphone insert ($3), here's a 9 volt battery clip (30 cents), here's a cheap pair of headphones ($6), and here's a headphone jack ($2). Add an on/off switch, volume control and some hook-up wire and it shouldn't cost too much - maybe $25 and a few hours' work.

But hold on - here's one that's almost completely finished - and it’s available for just $7. (You’ll need to add a decent pair of earphones or headphones – but most households will already have those.) The unit is called ‘Listen Up Portable Personal Sound Amplifier’ and you’ll find it on eBay, available from lots of sellers.

Building the knock detector

I am sure that you've read this before: "Building this project shouldn't take longer than 10 or 15 minutes." But in this case, it's the truth!

The first step is to buy Listen Up, four metres of cable (I used shielded two-core microphone cable) and a 10-amp battery clip (you can also cut a clip off any old battery charger – that’s what I did).

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Open the box by first talking off the end caps (one covers the battery – a single AAA cell) to reveal four small screws. With it open, it should look like this.

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Note the microphone (arrowed) – both the fact that it is a large and relatively good quality unit, and that it is attached to the printed circuit board with flying leads.

You can now take one of two approaches. If you are good at soldering and have a fine tip soldering iron, you can unsolder the microphone and solder in place our new long shielded cable. If you are not so good at soldering, cut the leads and join the cable by twisting and using tape to insulate the joins.

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Here are the new wires soldered to the original locations. I also soldered the braid of the cable to the negative connection of the battery (arrowed), to provide better shielding of the signal. Make a suitable hole for the cable to escape and then close up the box.

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Jon the other end of the microphone cable to the microphone. If you have a soldering iron, solder these connections. If you don’t, twist connect the new wires to the old. Keep the polarity the same as original.

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Use hot melt glue or similar to mount the microphone to the inside arm of a metal clip, then cover in heatshrink.

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The earphones supplied with the Listen Up device are pretty crap, so supply your own. High quality, fully enclosed headphones will work best – but any decent quality earphones should also be fine. With fully enclosed headphones on, the sound quality is excellent.

Using It

Using the detonation listener is very simple. You simply clip to the microphone to whatever you are interested in listening to. Noises are transmitted through the metalwork directly to the clip and microphone, making the device extremely sensitive.

To detect detonation, the clip is best placed directly on the block, in the type of place that the factory knock sensors are positioned - no surprise there! Adjust the volume control to give a comfortable loudness level, and sit back and listen. Over the clatter of pistons, valve gear, explosions and gearbox whines, detonation sounds like a sharp "splat!, splat!".

Note that it's best to listen from the passenger seat while someone else drives the car - that way, the driver can still hear emergency vehicles and concentrate on driving, not listening to strange sounds…

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