Microphone recommendation for finding sources of noise in electronic equipment

[matthuszagh]

I sometimes come across a piece of test equipment that produces annoying high-pitch audible noise. It's often difficult to locate the offending component without additional tools. So, I'm thinking of purchasing a microphone that I can use with an audio-frequency spectrum analyzer (probably an HP 3585A) to help locate these noise sources. Any recommendations?

Some preferred requirements:

1. Small, so it can reach into tight spaces.
2. Electrically insulating, so I don't accidentally short something out. If needed, I can place heat shrink over it (if I do this over the microphone part, I'll have to see how much this degrades the performance).
3. An articulating arm would be nice. But at the very least something that has some rigidity.
4. It should cover at least close to the full range of audio frequencies. I don't know the maximum frequency I can hear, but I'm relatively young so it's probably near the higher end.

I have almost no experience with audio electronics, so if I need a preamp for it too, recommendations for that are also appreciated. Without a preamp, I was planning to plug it into the high-impedance input of my 3585A due to the fairly high microphone source impedance.

It doesn't need to be very high performance since this is for simple troubleshooting.

I'm happy to construct or modify something myself if that's the best bet. For example, maybe this would involve purchasing a microphone and articulating arm separately and constructing a complete unit from that.

Any other thoughts such as other ways to locate these noise sources also welcome. And, if anyone else has something they use for this purpose I'd be interested to hear it.

From a quick look, it seems like pulling the microphone from a headset could be a good bet, but it would be good to have information about the frequency range the headset's microphone supports.

[nctnico]

What you need is a stethoscope for (car) mechanics. Works like a charm.

[matthuszagh]

What you need is a stethoscope for (car) mechanics. Works like a charm.

That does sound ideal - thanks. Do you know the frequency range of the sensor? I expect most medical stethoscopes operate with low audio frequencies in mind, but I would imagine mechanic stethoscopes would be designed to support higher frequencies. I can't find much information on that though.

Also, the sensors look like metal rods. Any idea how their function would be affected if I insulated them (e.g., heat shrink or a rubber coating)?

[Sorama]

We have an acoustic camera for analysis of building acoustics ald finding air leaks in walls or windows and wonder if it would suit for this also.

[matthuszagh]

We have an acoustic camera for analysis of building acoustics ald finding air leaks in walls or windows and wonder if it would suit for this also.

Super cool! Didn't know those existed. But they seem well out of my price range, unfortunately.

[Sorama]

There are indeed expensive. Ours is about 15k.

[TheoB]

A simple phone app can act as an all in one audio spectrum analyzer. Try for example Spectroid. An external small microphone can also be used like the one present in a headset.

[Coordonnée_chromatique]

How can we localize a sound with one microphone ?

[matthuszagh]

How can we localize a sound with one microphone ?

The signal gets stronger when the microphone is closer to the source of the noise and weaker when it's further away.

[Coordonnée_chromatique]

How can we localize a sound with one microphone ?

The signal gets stronger when the microphone is closer to the source of the noise and weaker when it's further away.

Here we should face a sensitivity problem IMHO

[Gyro]

The simplest way, and probably cheapest, is to attach a length of thin plastic tube to the front of a small microphone (probably electret) in an airtight manner. Sound will pass down a considerable length of tube with reasonable frequency response, think in terms of the long tube acoustic headphones used in MRI scanners.

If you need more steerability, tape an insulated length of thin garden wire to the outside of the tube.

[Coordonnée_chromatique]

The simplest way, and probably cheapest, is to attach a length of thin plastic tube to the front of a small microphone (probably electret) in an airtight manner. Sound will pass down a considerable length of tube with reasonable frequency response, think in terms of the long tube acoustic headphones used in MRI scanners.

If you need more steerability, tape an insulated length of thin garden wire to the outside of the tube.

Have you an idea of the frequency response and the sensitivity of this soluition ?

[Gyro]

Sensitivity is as good as the gain on your microphone. Frequency response isn't going to he Hi-Fi, but should be good for reasonably high frequencies. It's an old car mechanic's trick to use a thin length of hose, one end in the ear, to listen for the high pitched hiss of small air leaks etc.

[richnormand]

If you can hear it: I use a tygon hose. One end on my ear the other to probe around the circuit.
Provides electric isolation and pinpoints the specific component.

Otherwise the electrec microphone and a scope is a good idea but I would still use a length of hose to isolate electrically and pinpoint the component.

Many of the car stethoscopes require contact to the surface. Not a good idea in PSU and such.

[themadhippy]

If the idea of shoving a raw bit of tube in your ear dont appeal theirs plenty of ear pieces with a tube attached , 1 random example https://www.amazon.co.uk/sourcing-map-Acoustic-Earpiece-Replacement/dp/B0BG87C4XD

[RoGeorge]

Try bone conduction.  For example a wood chopstick in touch with the ear cartilage (kept with the hand), while with the other end touch it to the presumably singing coil.

[Coordonnée_chromatique]

IMO you should investigate a little before choosing any technical solution, the sound propagation in the air is not the same that in solids and it is presumptuous to correlate them so early, moreover in the audible range the short waves are very easy to diffract and refract especially on a very populated PCB with a lot of thermal gradients.

Could you describe how is your noisefloor ? level and distribution ?
Could you describe how is your noise ? have you already tried to record any of this noises when you've succed to find one ? 

[RoGeorge]

If it has to be a microphone (and not just a passive stick used as a bone conduction hearing aid), then I would use an electret microphone.  They are very small, cheap and easy to use.  Can be recovered from defective headsets, or from 2/3G mobile phones, fix phones, TVs/boomboxes, etc., electret mics are almost everywhere.  Some are as small as 2-3mm in diameter.

Many may work as high as 100kHz or more.  If you like more fun with that, try an audio heterodine, and you'll can hear ultrasounds and bat ecolocation barks, too.  Here I did something else, used the comparator of an ATtiny13 instead of properly amplifying the analog sound, then a counter to divide the frequency, like this:


The image is from this other topic https://www.eevblog.com/forum/projects/want-to-hear-the-bat-flying-in-my-front-yard/msg5582713/#msg5582713 where you can find, too, the code for the microcontroller.  It is for a MEMS (still analogue) microphone, but should work as well with an electret microphone.

Same as before, use a thin wood/plastic stick (anchor the microphone at the tip of a baguette), to make an audio probe.

If you order instead of scraping old microphones, beware that most recent models may be digital, but there are plenty of analog electret microphones still in production.

[al777]

My vote also goes for an electret microphone, followed by a simple amplifier. Attached are photos of my solution, I've built it exactly for the reason of weeding out a whiny ceramic capacitor on an LCD panel "scaler" board. I gave it local battery power and housed it in a metal enclosure to avoid any chance of EMI hum. The Plastic tube is a body of a BIC ballpoint pen, guts removed. The microphone was from an old wired mobile headset. Be careful when soldering it, they are easily ruined by overheating.

In my experience, having the acoustic probe for scope helps with remediating the problem, not only identifying an offending component. Sometime the component is not faulty, while the design/manufacturing decisions are :-). Hence it's very useful to actually see what is the signal that you are hearing, such as some DC/DC conversion frequency will show clearly in the acoustic domain, and you can use second scope channel to see the electrical domain at the same time - that way you know what is going on and can try to mitigate the root cause of the problem. In some cases I've seen it was not a faulty capacitor, but simply an underspec-ed one... or several... economy is driven by MBAs :-). So, replacing it with the same value would not get much effect (although one could try to use more expensive caps with interposer substrates, like some of muRata's series), but bumping the capacity back to what the datasheet has suggested may help :-).

Here is a link to muRata's related article: https://www.murata.com/en-us/products/capacitor/ceramiccapacitor/library/apps/notepc

Happy holidays to all!

[Coordonnée_chromatique]

Is there any ready made product of this probe ?

[al777]

Is there any ready made product of this probe ?

If you are asking about my probe (photos above) - then sadly, no - it is a one-off "custom tool" I've made to extend my line up of probes when a need arose... it joined a pair of other probes in the same form-factor, optical probe and Hall-effect current probe... I'm not aware of a ready-made version for sale anywhere, but I wasn't searching :-)... I just made it, it was faster that way :-)...

[5U4GB]

I've used a Dodotronic UltraMic, no direct link but they're available from all sorts of sources although the price seems to have gone up a bit since I got mine many years ago.  Has a rubber cap so you won't short anything out by poking around with it, and a USB interface for connecting to more or less anything.

[Coordonnée_chromatique]

I've used a Dodotronic UltraMic, no direct link but they're available from all sorts of sources although the price seems to have gone up a bit since I got mine many years ago.  Has a rubber cap so you won't short anything out by poking around with it, and a USB interface for connecting to more or less anything.

There is not "tube" on this microphone, could you tell what is the purpose of a tube, it is a kind of helmholtz amplifier for an already known frequency range ?

[Georgy.Moshkin]

IMO sound pressure may be kind of stronger at some points that are farther from the source. In a sense that close-range measurements are too sensitive to the microphone orientation and position. What about a toothpick coupled to some piezo or microphone, makimg it a contact measurement: couple the vibration from some chip capacitor directly to the sensor.

[nctnico]

Many of the car stethoscopes require contact to the surface. Not a good idea in PSU and such.

You actually need the contact measurement to pinpoint the actual source. A piece of plastic pipe does the job for insulation if necessary or find a plastic stethoscope. In many cases it is the board itself which propagates the sound, you need to find the part which causes the board to vibrate.

[tooki]

I’m amazed that, other than the talk of putting microphones in hoses, nobody is talking about the directional characteristics of different microphones! Because what OP needs here is a highly directional microphone, which is not what most microphones are.

When it comes to microphone capsules (the actual microphone elements), they’re fundamentally omnidirectional or unidirectional, and this is determined by the mechanical construction of the microphone. (Dave did a fascinating interview with a microphone engineer some years back that goes into how this works.)

When it comes to “finished” microphones (the thing you’d give a user), the combination of capsule type and enclosure determine the ultimate directional characteristics. Those are shown in “polar patterns” that show sensitivity in a circle around the microphone.

Common polar patterns are omnidirectional (equally sensitive in all directions), cardioid (sensitive mostly to the front), and figure-8 (equally sensitive front and back, but insensitive left and right). Cardioid mikes vary in their directionality, with “supercardioid” and “hypercardioid” ones being progressively narrower (but also picking up a bit more from directly behind).

One of the most common type of highly directional microphones are frequently called “shotgun” mikes because of their long bodies.

[Coordonnée_chromatique]

“shotgun” mikes because of their long bodies.

Interesting, i'm amazed that today designs are still totally empirical and that some engeeners works profesionnaly with an "inacurate shotguin microphone"

[tooki]

“shotgun” mikes because of their long bodies.

Interesting, i'm amazed that today designs are still totally empirical and that some engeeners works profesionnaly with an "inacurate shotguin microphone"

Huh? I don't understand what you mean.

[Coordonnée_chromatique]

“shotgun” mikes because of their long bodies.

Interesting, i'm amazed that today designs are still totally empirical and that some engeeners works profesionnaly with an "inacurate shotguin microphone"

Huh? I don't understand what you mean.

The recommended microphone with a body of a BIC ballpoint pen 

[tooki]

“shotgun” mikes because of their long bodies.

Interesting, i'm amazed that today designs are still totally empirical and that some engeeners works profesionnaly with an "inacurate shotguin microphone"

Huh? I don't understand what you mean.

The recommended microphone with a body of a BIC ballpoint pen 

I don’t understand what you’re trying to say with any part of “ Interesting, i'm amazed that today designs are still totally empirical and that some engeeners works profesionnaly with an "inacurate shotguin microphone" ”.

[Georgy.Moshkin]

I’m amazed that, other than the talk of putting microphones in hoses, nobody is talking about the directional characteristics of different microphones! Because what OP needs here is a highly directional microphone, which is not what most microphones are.

wow, I've totally missed that. I guess it makes it very difficult to make an usable device. Even a sound camera has some limitations. Therefore, I thought about much simpler solution. Forget the microphone! How about using our own ears? Put some soft synthetic material on a chopstick. For example, a tiny EVA ball. Simply poke and listen if there are any audible changes. I have actually done that once, long before covid, while searching for a high pitched noise source coming from the LED driver PCB.

[G0HZU]

A while back I bought one of these SMD microphones to make a bat detector as bats fly in my garden every summer.

https://www.mouser.co.uk/datasheet/2/218/Knowles_SPU0410LR5H_QB-2935249.pdf

The frequency range is good to 80 kHz and these things are tiny at just 2.5mm x 3.5 mm!

Part number Knowles SPU0410LR5H-QB and they are really cheap at about 60p each. Note that these are active devices so you would need to provide about 1.5V to 2.5V to power it. A watch battery (button cell) would be one option here.

 I never got around to making the bat detector, but because these things are so tiny, you will need to make a specialist PCB to mount it on as the SMD pads are tiny. I was going to mill a tiny PCB for my microphone.

You could mount it inside a short tube to make it directional. Even then the whole thing could be tiny. The alternative is a classic electret microphone element but this will be much larger.

[Stringwinder]

A closed microphone capsule will be sensing absolute pressure no matter what
the direction to the source is. It will be omnidirectional even if you put a
tube on it.

A microphone sensitive to the pressure difference between front and back of the
diaphragm will be directional and it can't be closed on the rear side.
Bidirectional in its simplest form, sounds from the sides will be canceled.

Using a pair of pressure gradient capsules you can make many fancy variants of
directional patterns.

[SteveThackery]

I'm not entirely convinced that the microphone needs to be highly directional. Provided it's small enough to be pushed between the components and get up close, I think it could still work. I recommend experimenting with some MEMS microphones, as shown in the picture.