Microphone in engine bay

[oldjohnno]

It's getting close to winter here, and while I'm not too concerned about heat my old '56 was starting to fog the windscreen. As a quick fix I installed a small heater box under the dash and jury-rigged a fresh air supply that consisted of a 2" hose running from alongside the radiator, through the engine bay and firewall to the heater box.

It fixed the screen fogging but that wasn't the interesting bit. As well as conducting air it also channelled some noise from the engine compartment; not enough to be annoying but noticeable. Now I've had this old car for years and never ever noticed any signs of det. But on the first hill I encountered with the new air duct I could hear it lightly pinging as plain as day - it'd obviously been doing this all along, I just couldn't hear it. I'd imagine that a newer car, with better soundproofing would obscure the knock even more so, and that it'd actually be quite heavy before it was audible from the cab. Some time later the water pump bearings started to go and again this was plainly obvious through the "stethoscope".

Anyhow it occurred to me that a microphone that could be turned on and off in the engine bay might be a useful tool. Thoughts?

[Kevin Johnson]

It would be nice if you could move it around. Maybe a nice protective sheath so you would not have to worry too much about frying it.

[lada ok]

when you coupled that pipe from the engine bay to the cab, did you notice any ' pong '
may be the engine is just having a game

[emmarinc]

I use a tool called "engine ear" with magnetic sensors and ear buds to locate noises on vehicles at the shop. Takes a lot of batteries to run but it works.

[user-23911]

I've built all sorts over the years.
An earpiece out of an old phone works well as a microphone and doesn't need power to it.
I've hooked one of those to a pre amp module (jaycar) and in turn wired the output to a cassette tape adaptor. Pop it into the tape player and you get it coming out of your car speakers with a bit of noise and hiss.
Alternatively use a proper resonant knock sensor and hook that up to an amp, a bit of noise to that too and not much else.

An earpiece for a phone. It's about 50 ohms and works well with twisted pair, is handy for listening to odd noises but the knock sensor is better if you want to hear knock.
Plus if you can hear it with your ears, it's too much.
I've also modified a knocklink to automatically drop the boost to zero if the knock gets over a certain (adjustable) level.

It's what every car needs.

[Belgian1979]

A knock sensor is essentially a microphone tuned to listen to the specific knock frequencies. If tuned right, this can be a standalone unit at a very low price (see knocksense).

It's the tuning part that's difficult as inevitably it will also pickup other mechanical noises. In a more sophisticated system the engine ecu will only verify the knock frequency is present during a combustion stroke and ignore all other episodes.

[user-23911]

There's 2 main sorts of knock sensors, they work in completely different ways.
The older type is the resonant sensor, they work on the same principle as a tuning fork. It picks up a shockwave and it vibrates at it's own tuned frequency, the peak voltage varies with how hard you hit it. Like a tuning fork, the oscillations reduce over time. They're usually tuned 8Khz up to maybe 12KHz depending on the engine.......NOT the bore size, the piston to head distance. The output is a sine wave at one single frequency only.
They work really well and it's very simple to get a reliable output with knock.

The newer ones are like a microphone, have the same frequency response, 20Hz ish to 20KHz ish.
They're very cheap, that's the only advantage, software is also very cheap so it costs pretty much nothing to mass produce them for an OEM type situation.
You're not going to get individual cylinder knock detection with a resonant sensor, not when the output decays over several engine cycles.
But do you need that?
There's so much mis information on the net about knock detection , its a big joke.Like people trying to tune a car with headphones on....lol.
Can you hear 11KHz?.....
Over the top of other noises?
Maybe the 6 million dollar man can with his bionic ears?

[Xnke]

I have no problem hearing engine knock at 6 and 8 khz, and yeah, it's bore diameter...not piston-to-head clearance. Set up a mic with a wideband filter and push an engine of known bore diameter into detonation, look at the output through a spectrum analyzer. Knock pitch correlates strongly to bore diameter, to the point that it is easily calculable as to what frequency you should hear from a given bore. From that point, you can narrow up the filter to only hear knock events, which is how knock sensing used to be done on all analogue and some early digital ECUs.

11khz knock frequency is a pretty small bore...For a 94mm bore (what I'm working on right now) the tone is 5.9khz. I guess you could be looking for the second harmonic at near 12khz if you wanted to, at a reduced amplitude. Computer can hear that easier that You or I could, but it's pretty easy to hear tones up to about 15khz unless you've damaged your ears by not using hearing protection in the shop or at the track.

[user-23911]

Have you put any thought into how much bore there actually is when the piston is at the top?(with mine it's 2 to 3mm at 15deg ATDC)
How many waves are going to bounce across the bore compared with how many waves bounce up and down between head and piston.
One engine with a 91.1mm bore has a 11.6KHz sensor.
A similar one with 93mm bore has a 11.2KHz sensor.

The commonly accepted theory is BS and based on an SAE paper written which states that the minimum frequency you'll pick up is based on bore size.
That statement is true but knock is not a single frequency, it's more than one band and consisting of several frequencies within each band. The very small band of frequencies at low level given by bore size isn't what you're trying to detect.

When you've got really bad knock at low RPM high load on something that doesn't make a lot of power...you hear it clearly with your ears.
It's got a descending pitch.....that's the piston going down the bore.
It's the distance between the head and piston , getting bigger.

[user-23911]

Plus if you're over 40 , you've got about zero chance of ever hearing 15kHz.
As you get older you hear less high frequencies, so if you're tuning cars and relying on ears, you're going to blow more of them up as you get more experienced?

[Kevin Johnson]

Plus if you're over 40 , you've got about zero chance of ever hearing 15kHz.
As you get older you hear less high frequencies, so if you're tuning cars and relying on ears, you're going to blow more of them up as you get more experienced?

https://www.youtube.com/watch?v=RDgUuls73EY

https://www.youtube.com/watch?v=qNf9nzvnd1k

[Rizzle]

https://www.youtube.com/watch?v=qNf9nzvnd1k

Human hearing is greater than 20hz-20khz. However the 20-20 is the range of the common .mp3 format. At least I hope so, cause otherwise it means my memory is starting to make stuff up.

[user-23911]

Human hearing is greater than 20hz-20khz. However the 20-20 is the range of the common .mp3 format. At least I hope so, cause otherwise it means my memory is starting to make stuff up.

If you can hear 20KHz.........you're no more than 3 years old.

Now thinking back to maybe 1988, that's when I started fixing TVs, I had a wall of them in my lounge on soak test. The PAL system has a horizontal scan frequency of 15625Hz (same for NTSC), some TVs would get a bit of a whistle, usually a ferrite core in a choke.
The kids could hear it as could the missus but I could only hear it when I unplugged the aeriel so it lost horizontal sync and ran at a slightly lower frequency, as they do.
One of the reasons the 15625 was chosen is because most people won't hear it if there's a fault.

Digital audio formats are a bit different, even though you can't hear the higher frequency components, they're there and combine with other high frequency components that you also don't hear so that you do hear them.
Otherwise you'd just have telephone quality which has an 8KHz sampling rate with 4KHz (or less) limit to transmission.

[turbobaldur]

You guys are completely overengineering this. A proven solution is what we call det cans, a pair of ear muffs with 8mm fuel hose running to them, attached to a piece of copper tube that's bolted to the engine. Can hear knock very clearly and immune to electrical noise.

[Kevin Johnson]

I suggest a skinner box to train a small cat that will roam about the engine compartment. Operant conditioning will develop a marked twitch of the tail at specific frequencies. This is a green solution as it will provide a home for an otherwise stray cat (be sure to be a responsible owner and spay or neuter your free-ranging shop cat).

I would suggest a small tethered bat but this would be too distracting for the driver to try to distinguish specific movements.