Author Topic: "singing" board (audible noise), how to pinpoint?  (Read 2368 times)

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Offline rolandTopic starter

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"singing" board (audible noise), how to pinpoint?
« on: February 21, 2020, 05:43:52 pm »
We have a small board (roughly 50mm * 150mm) with medium to complex design (quad core ARM Cortex A17, display, some LEDs, loudspeaker, microphones, networking/WiFi) etc. which is working nicely but there is one big issue: the board produces a clearly noticable, high-pitched whining tone (about 5kHz). After EMC testing of the product, we needed to fit an EMC shield over the CPU and RAM, and this has now unfortunately amplified this noise...

I know the usual suspects, we have checked all inductors and MLCCs near the power supply, DC-DC converters (power companion chip) etc., however we didn't find anything yet that might correspond even remotely to the 5kHz we're seeing (or better, hearing).

Does anyone here have a clue about a method how to pinpoint the issue? Is there any kind of measurement we could do (some special directed microphone or something) that might help to find the root cause of this issue? At the moment we're more or less cluelessly probing around with the scope or running over the board with a mobile phone's microphone (and FFT app). However, since the root case element seems to let the whole board vibrate, it is very difficult to find the element that produces the waveform in the first place. I guess if you had a guitar and didn't know that the string actually produced the sound, you could probe around the body quite some time because the sound seems to come out of the resonant body.

Any idea is appreciated that might help.
 

Offline Twoflower

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #1 on: February 21, 2020, 05:47:49 pm »
Use a small tube as stethoscope and scan the PCB with it.
 

Offline ogden

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #2 on: February 21, 2020, 06:06:17 pm »
Use a small tube as stethoscope and scan the PCB with it.
Use PC with microphone. Hint: old phones came with (wired) headset+mic, you may still have such in scrapbox. Otherwise just buy it for buck or two:

 

Offline Siwastaja

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #3 on: February 21, 2020, 06:44:55 pm »
Or, use a toothpick or similar to touch/press components one by one, likely a small amount of compression makes the sound stop, helping you pinpoint the suspect.
 

Offline Bud

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #4 on: February 21, 2020, 07:48:05 pm »
Try using a 1uF ceramic capacitor on a toothpick or on a cotton swab stick as a vibration sensor  , wired to the oscilloscope input to monitor the amplitude.
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Offline Miti

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #5 on: February 21, 2020, 07:58:04 pm »
Drop distilled water selectively from the tip of a toothpick or from a needle.
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Offline FenTiger

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #6 on: February 21, 2020, 08:32:14 pm »
Clamp the board somehow so it doesn't vibrate, and your microphone might do a better job of locating the source.

Alternatively, look more closely at the waveform, in both time and frequency domains - maybe this'll give you a better clue than the 5kHz frequency alone.

Are there many clocks or other fixed frequencies on your board? Put the microphone signal on one channel of a scope and look at the clocks etc on another channel. Is the noise synchronous with one of them? If so, you've narrowed down its source somewhat.
 

Offline KMoffett

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #7 on: February 21, 2020, 08:44:14 pm »
Get a cheap stethoscope. Remove the head (bell and diaphragm). Just use the tube to snoop out noises. Very handy.
https://www.amazon.com/Ever-Ready-First-Aid-Stethoscope/dp/B00HW8EEOG/ref=lp_3777201_1_1?s=industrial&ie=UTF8&qid=1582317693&sr=1-1
 
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Offline DaJMasta

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #8 on: February 21, 2020, 10:01:51 pm »
Clamp the board somehow so it doesn't vibrate, and your microphone might do a better job of locating the source.

Clamp and even damp if required, some sort of vibration reducing material (foam, rubber, etc.) pressed against the bottom should keep the board from amplifying so it's easier to find the source.  You don't even need a directional mic for the test if you're sensitivity is high enough that you can pinpoint it with intensity alone, so most small mics should do the job.
 

Offline jonpaul

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #9 on: February 23, 2020, 11:45:45 am »
As I was a power electronics EE, for decades I may have a clew.

Singing SMPS check the on-board regulators for sub cycle oscillation, the SMPS is skipping cycles.

Inductor singing is the symptom not the cause


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Offline Ice-Tea

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #10 on: February 23, 2020, 11:51:23 am »
Or, use a toothpick or similar to touch/press components one by one, likely a small amount of compression makes the sound stop, helping you pinpoint the suspect.

^^This.
 

Offline Psi

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #11 on: February 23, 2020, 11:51:26 am »
Could also try a ~5khz disc piezo with a toothpick or wire glued to it.
Hold the thing with some foam to let it vibrate freely.
Then touch the toothpick to various parts while watching Oscope.

if that doesn't work, you could try generating a beat frequency.
Pick a source frequency that, when mixed with the 5hz noise, will generate a unique beat frequency
Feed that frequency into the piezo then go around touching parts and listening for the new sound.
« Last Edit: February 23, 2020, 11:59:28 am by Psi »
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Offline Psi

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #12 on: February 23, 2020, 12:17:20 pm »
Could also try doing an overclock/underclock on the CPU and other peripheral clocks
See what, if any, affects the sound frequency.
Might tell you something.
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Online SiliconWizard

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #13 on: February 23, 2020, 03:27:42 pm »
I know the usual suspects, we have checked all inductors and MLCCs near the power supply, DC-DC converters (power companion chip) etc., however we didn't find anything yet that might correspond even remotely to the 5kHz we're seeing (or better, hearing).

How did you proceed then? Because yes those definitely look like the usual suspects.

Ceramic capacitors when this happens will transmit vibrations to the PCB and it's sometimes hard to locate the source precisely (until of course you shut it down).
 

Offline RoGeorge

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #14 on: February 23, 2020, 04:16:54 pm »
Take a wooden or a plastic rod, and use it like a soundwaves guide.

Touch one head of the rod to the ear cartilage, and the other head of the rod to various parts on the PCB.  You'll be surprised how effective this could be.  If too loud, use the head bone from outside of the back of the ear.

First suspects will be the coils with magnetics (e.g. SMPS coils).  MLCC capacitors might sing, too.

What tune is it singing, tho'?   ;D

Online Conrad Hoffman

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #15 on: February 23, 2020, 10:03:13 pm »
Do you have any much faster clocks that are about 5 kHz apart?
 

Offline richnormand

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #16 on: February 23, 2020, 11:41:50 pm »
Years ago I had a similar issue on a large board  with some high voltage on it too (3-4kV). I could hear it fine but the high pitch made it hard to locate. It was at a higher frequency but with several lower harmonics that were angle sensitive. I used an electret microphone and spectrum analyser and got a good signal. Due to the high frequency there was a complex pattern in intensity distribution in the far field due to interferences. The interference pattern was pretty sharp however either indicating a near point source with diffraction pattern or multiple sources in phase with each other.

Solution: a 6 foot length of 1/4" tygon flexible tubing. One end in my ear and the other one taped to a small stick scanning the pcb at a few mm height of components while using the length of the tube to stand away from the assembly.

Turned out to be a ceramic capacitor that was signing.

People suggesting using a microphone could consider coupling a long flexible tube to it to probe the nearfield and isolate individual components.

Best of luck with it!
« Last Edit: February 23, 2020, 11:45:44 pm by richnormand »
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Offline rolandTopic starter

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #17 on: February 24, 2020, 07:47:43 am »
Guys, thank you all very much for your numerous great suggestions. I have ordered some stuff (microphone, tubing, stethoscope) and will let you know when I've found the culprit. Thanks!
 

Offline RoGeorge

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #18 on: February 24, 2020, 10:39:21 am »
The rod I was telling can be a simple thin, dry and stiff stick from outside, the tubing can be a simple sheet of paper rolled, it will act as a rod and as a tube in the same time.

Offline NiHaoMike

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Re: "singing" board (audible noise), how to pinpoint?
« Reply #19 on: February 24, 2020, 01:43:49 pm »
Get a cheap stethoscope. Remove the head (bell and diaphragm). Just use the tube to snoop out noises. Very handy.
Or 3D print one.
https://hackaday.com/2018/03/17/3d-printed-stethoscope-makes-the-grade/
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