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I have a Clevo M57RU laptop here. Whenever it goes to stand-by, sleep or is shut down I hear a VLF sound (presumably close to the ~20 kHz maximum of the human ear) that's even better to hear if the battery led blinks because the sound is intermittent then. When I disconnect power and such the LEDs go off, the sound goes off, as well. When it is operating the sound is probably there, as well, but I can't hear it since the fans are terribly noisy.

I tried to use this Sound Analyzer App on my tablet to give you more information but I can't even calibrate it since:

Failure: Unstable sound source, ΔL = [5.5–10.5 at different tries]dB

What's causing this sound and is there a possibility to get rid of it?

(I'm absolutely able to master advanced tools like screwdrivers, soldering guns, etc. I had it open dozens of times for fan cleaning and thermal compound renewing.)

UPDATE

I read the suggested Buzzing power supply. The instructions how to possibly get rid of it, however, are a bit too vague for me (I'm a high-power electrical not an electronics engineer):

add a load at the output (a resistor)

Of which size?

put some insulating viscoelastic caulk on the noisy components

What about heat if I put that around a DC/DC converter? 20 V (external power supply) converted to a voltage suitable for LEDs can produce quite an amount of heat, IIRC (crafted 12 V car to 5 V CMOS once), can't it?

Gerold Broser
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  • same phenomenon: under low load, the switch-mode supplies inside your laptop make noise – Marcus Müller Apr 19 '17 at 18:21
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    While RF signals in the 3kHz-30kHz range are considered VLF, for audio (sound) purposes, that's a very high-pitched sound, and "audio VLF" would most likely be considered to be something around 2Hz-20Hz...probably best not to use VLF in this question as it tends to be a bit confusing with regards to a high-pitched sound. -- With that said, @MarcusMüller is correct that what you're hearing is SMPS/LED Driver "whine." – Robherc KV5ROB Apr 19 '17 at 18:26
  • @RobhercKV5ROB / Marcus Müller Thank you for the info. It's not the (external) power supply in my case, however, but it's at the front of my laptop. The LEDs, where the sound is best to hear are ~12" away from the power socket. Does this mean there's going 20V a rather long way through the casing and then there's the LED's power electronics that makes this noise? – Gerold Broser Apr 19 '17 at 18:33
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    It's coming from an *internal* DC/DC converter (or LED driver circuit) that runs off of the laptop's battery... *not* from the external power supply. In truth, it is *possible* for the LED to be making the sound itself (pulsating ohmic thermal expansion), but the PWM-driven SMPS/LED driver is where the *frequency* for the sound is being generated...the whatever component has the highest losses (often the magnetic core of an inductor, though sometimes a resistive element such as a wire or LED) broadcasts the frequency as sound waves. – Robherc KV5ROB Apr 19 '17 at 18:38
  • @RobhercKV5ROB Thank you for this detailed explanation. However, it runs off of the external power supply here – always, since my battery pack reached its end of life a while ago and I'm just leaving it in because otherwise the laptop misses one "foot bump". – Gerold Broser Apr 19 '17 at 18:46
  • OK, then the internal circuit is running "parallel to the laptop's battery"...but the rest of the explanation is still valid, and the offending circuit was *intended* to run "off the laptop's battery," lol. – Robherc KV5ROB Apr 19 '17 at 18:50
  • @RobhercKV5ROB I think it's most likely that it's simply the first stage of SMPSs that power the laptop from the external supply which drift into audible oscillations in low-load situations. – Marcus Müller Apr 19 '17 at 18:59
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    @GeroldBroser -- The viscoelastic (silicone) caulk *would* have a negative effect on themral transmission, and should be used very much with care. As to the value of the parallel resistor, that would have to be determined by experimentation. Without far more information than you have to give us (full schematics & specs on offending circuit, precision scope readings on the control PWM signal, etc.), no real calculation is possible. As a first attempt, you could start with 2x the resistance of the LED+series diode (to add 50% to the power draw), then reduce resistance from there until if needed. – Robherc KV5ROB Apr 19 '17 at 21:56
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    Caveat: The resistor method I just mentioned is assuming the LED driver is either the source of the offending frequency, or that it can handle enough power output to sufficiently load the SMPS that is. If that's not the case, then using too low a resistance could kill the LED driver & leave you still hearing the noise. – Robherc KV5ROB Apr 19 '17 at 21:58
  • @RobhercKV5ROB It's a pity that the question is flagged now (you can't post an answer due to that, right?). If you had put all the valuable information you gave in your comments in an answer I'd accepted it. – Gerold Broser Apr 19 '17 at 22:05
  • Correct, questions that have been "put on hold" can't have new answers posted...only commenting & voting is allowed. However, if you could edit your question to comply with the [ask] guidelines, *and* be notably distinct from the listed duplicate question, it can be reopened. – Robherc KV5ROB Apr 19 '17 at 22:08
  • Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/57370/discussion-between-robherc-kv5rob-and-gerold-broser). – Robherc KV5ROB Apr 19 '17 at 22:12
  • @RobhercKV5ROB Spontaneous idea: are you able to edit alex.forencich's answer? Perhaps you can hijack it. ;) – Gerold Broser Apr 19 '17 at 22:14

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This is coming from an internal DC to DC converter. I have a Dell XPS laptop that produces similar high pitched whine and/or hiss under certain conditions. These converters can operate in several different modes. One of these is constant on time, where the converter pulses on for a constant period at a variable frequency. It's possible that the LED provides just the right amount of load to get the converter switching in the audible range - LED off results in switching so infrequently you don't notice and computer on results in switching well outside the audible range.

alex.forencich
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  • Thank you for your answer. If you add how to exactly get rid of it (see the UPDATE to my answer) I'm going to accept it. – Gerold Broser Apr 19 '17 at 20:11