Last week we went through some of the difficulties in sensing detonation - the fact that the detonation frequency of sound varies depending on engine bore diameter and construction, and the difficulty of using a purely electronic system compared with a good audio detection system being monitored by a skilled operator.
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 - and wind noise that results if the windows are lowered - mean that it's often hard to even hear the engine, let alone any specific odd noises being emitted from it.
So why not use an electronic stethoscope? This commercially available tool consists of an amplifier, a pair of headphones and a microphone input. In fact - as mentioned last issue - I've seen one such instrument in action, being used on a car being put through its paces on a chassis dynamometer. The instrument had then proved spectacularly successful at detecting detonation, the 'tink, tink' being audible to the headphone-equipped operator long before bystanders or a specific electronic detector could hear its occurrence.
So why not make an electronic stethoscope? Let's do a bit of electronic store catalogue pricing - mmmm, here's an amplifier module (A$7), here's a box (A$4), here's a microphone insert (A$3), here's a 9 volt battery clip (30 cents), here's a cheap pair of headphones (A$6), and here's a headphone jack (A$2). Add an on/off switch, volume control and some hook-up wire and it shouldn't cost too much - maybe A$25 and a few hours' work.
But hold on - here's one that's almost completely finished for just A$10.50!
The Dick Smith Electronics Whisper 2000 Personal Sound Amplifier (cat no Y-2000) consists of a microphone, amplifier, neat box, 9 volt battery clip and mono headphones. It's meant to be worn by the hard of hearing - basically, it's a hearing aid disguised as a 'Walkman' style radio. But would it be suitable for use as an electronic stethoscope? As you can see, the answer to that is 'yes'.
Building the Stethoscope
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 the Whisper 2000, four metres of single core microphone cable (cat no W-2030) and a 20-amp battery clip (cat no P-6418). The total bill is A$14.70.
Carefully pry the front grille off and revealed should be the microphone.
Separate the two halves of the Whisper 2000 box by unscrewing from the back the three self-tapping screws that hold it together.
The microphone is further held into place with some hot-melt glue - carefully remove it from its recess.
Taking note of the polarity of the microphone connections, unsolder the two wires connecting to it. Remove one of the push-on plastic sleeves from the battery clip and slide it onto the new microphone cable. Solder the braid of the cable to the microphone terminal formerly connected to the black wire and then solder the cable's single conductor to the other microphone terminal.
Slip a piece of small diameter rubber hose over the cable at the amplifier end, and then unsolder the original microphone wires and replace them with the wires from the new cable, maintaining the same polarity (ie braid to black wire terminal, core to red wire terminal). Temporarily insert a 9-volt battery and check that the Whisper 2000 unit works as it should with the long microphone cable in place.
File two adjoining crescent-shaped cut-outs in the two halves of the box so that a round hole is created when the box is re-assembled. Position the piece of rubber hose that you slipped on the cable so that it will be clamped into place by the two halves of the box (or instead you could wrap the microphone cable in electrical tape) and then reassemble the box. Using hot melt or another type of adhesive, glue the microphone into place face-down inside one arm of the clip. Slide the plastic sleeve back over the microphone and arm of the clip, hiding the microphone from view.
Then sit back and admire the device that all this hard work has resulted in!
Using the electronic stethoscope 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 stethoscope 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!".
Testing was carried out on an RB20DET-powered C210 Skyline running 15 psi boost and water/air intercooling. Even in cool ambient temps, holding full boost in third gear resulted in some low-level detonation, knocking that was simply impossible to hear from inside the car without the electronic helper. Depending on the location of the microphone clip, gearbox noises, injector clicking, suspension clunks, turbo whistling - the whole lot could be individually identified.
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.
DIY Detonation Detection - Part 1