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I recently bought a ten year old 10-bit color fidelity computer monitor (NEC PA301W) that has brilliant display quality but to my sensitive ears has an audible/annoying low pitched humm noise along with a medium to high pitched buzzing noise, which my sensitive hearing can hear and my body can almost feel while working in the quiet evening hours. My other EIZO monitor has an almost inaudable amount of the same kinds of noises even to my sensitive hearing. On the old NEC, the spectrum of low, medium and high pitched noises all together makes concentrated working on this monitor in quiet evening hours uncomfortable.

The brightness can be set from 0 (40 cd/m²) all the way to 100%. The noise is the lowest at brightness setting from 55 cd/m² to 70 cd/m². The noise goes away at a brightness of 160 cd/m² or higher, which is too bright for me. The lowest point where the monitor is the quietest is at brightness 7% (62 cd/m²).

  1. Which component(s) can be causing this noise? (See picture with alphabetic letter designations on the elkectrical components).
  2. Why is humm/buzz coming from the monitor louder at the lowest brightness setting?
  3. Why does the monitor make the the loudest noises inbetween 70 and 160 cd/m²?
  4. Why is the monitor quiter between 55 and 70 cd/m²?
  5. Why is the monitor the quitest beyond 160 cd/m²?

I have opened up my monitor to see whats inside. Its very difficult to hear exactly where the sounds are coming from. Since touching the board with your ears can be lethal, I decided to not risk a fried ear (as an audiophile) and instead resort to knowledge and information online about these components to guide me in the process of isolating the sources of the noises. 6. Could I replace any of these components in order to silence the noise? 7. Which components could I safely glue on (with a glue gun, hot molten plastic) to the PCB motherboard, without any harm whatsoever? My guess is that if the component(s) making the sound are solidified, weightened and encapsulated with soft plastic, it will reduce noise and make the monitor more silenced.

Any and all ideas are welcome, thank you!

General photos of the power board and the main board.

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Identification and labeling of suspicious components.

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Some more photos from close by.

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    \$\begingroup\$ You've labeled almost all components suspicious. And based on the fact that buzzing changes with the brightness setting, which indicates it's the backlight driver PWM duty change that alters the sound. The backlight driver might not even be in these PCB pictures you see here as it's usually at the panel. \$\endgroup\$
    – Justme
    Nov 18, 2021 at 12:17
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    \$\begingroup\$ (a) The components marked with a capital "I" are electrolytic capacitors. They don't whine, but they are sometimes attached to a PCB with flexible adhesive for physical security. They can, however, degrade after several years, and that is often evidenced by the silver end bulging out. (b) Rather than putting an ear near a component, you could touch the component with a long, dry plastic stick and listen for a change in the sound. \$\endgroup\$ Nov 18, 2021 at 15:45
  • \$\begingroup\$ Excellent suggestion @AndrewMorton thanks! \$\endgroup\$
    – Sam
    Nov 18, 2021 at 15:56
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    \$\begingroup\$ You can try the chopstick test. While it's running, poke everything with a wooden stick. See if you can get the noise to stop. \$\endgroup\$
    – Kyle B
    Nov 18, 2021 at 16:12
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    \$\begingroup\$ get a mechanics stethoscope from harbor fright to isolate the audible source. \$\endgroup\$
    – dandavis
    Nov 18, 2021 at 18:37

4 Answers 4

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What component(s) can be causing this noise?

Vibrating components cause hum. The amplitude and the frequency of the vibration are the key points. Ceramic capacitors tend to vibrate and buzz more due to their construction. Inductors and transformers can also cause excessive humming and buzzing (See Magnetostriction).

Why is humm/buzz coming from the monitor louder at the lowest brightness setting?

Why does the monitor make the the loudest noises inbetween 70 and 160 cd/m²?

Why is the monitor quiter between 55 and 70 cd/m²?

Why is the monitor the quitest beyond 160 cd/m²?

Basically, all have the same answer:

I'm not 100% sure but I can say that the power supply is highly likely an off-line flyback converter. Most of these converters are designed to operate in discontinuous conduction mode (DCM) at light loads (e.g. 20% and lower). They switch to continuous conduction mode (CCM) at higher loads (i.e. higher than DCM-CCM threshold to full load).

At DCM, the AC rms currents are high and this creates the worst case for EMI and audible noise. Since the switching currents have large amplitudes at DCM, their harmonics have higher amplitudes. For a switching frequency of 100kHz, it's quite possible to hear the one twentieth harmonic (5kHz) at DCM.

The audible noise is lower at higher loads (i.e. higher brightness) because the converter switches to CCM and thus the RMS currents decrease.

A similar thing occurs on my graphics card: As the number of white pixels increase, the GPU starts to whine. It tops when the screen is fully white. Because the power need is higher to generate white pixels (all three sub-pixels are driven 100%).

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Point 1 question : it is called Coil Whine and due to one or more audible frequencies in coils or duty cycles. Point 2 question : because frequencies changes to adapt it I think and drops to an audible one.

and At my knowledge level answer to the other 3,4,5th question is just the same, frequencies vary on one or more of the coils and it buzz/humm is more or less audible depending on frequency.

You can try to limit that by adding some resin on coils

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You can identify which components are making noise by making a home-made noise detector.

Buy or borrow a small, clip-on piezo pickup (for guitar / violin / trumpet / etc) with attached cable and plug. Cheap ones are available on Amazon for less than $10. Plug this into any device that accepts a microphone input, such as a voice recorder, and plug headphones into the device. Best to use full ear coverage headphones or ear buds so you only hear amplified sound, not sound coming through the air.

Now take a nonconductive rod such as a wood dowel, a chop stick or a plastic knitting needle, and clip the pickup onto it. Any vibration in the rod will be amplified into your headphones. Best to hold the rod with a tool so your hand does not damp the vibrations.

Now touch (with the rod) each component that you suspect of making noise. You may hear the noise from all components due to the conduction of vibrations all over the circuit board, but the device making noise should be louder than all other components.

Use caution when setting the volume level for your headphones so you do not blast your ears.

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I don't know how driven you are about finding this noise, but you can narrow it down by temporarily replacing each board with a used one from eBay, etc. (Look at the cost as a rental fee, before reselling them when you're done.)

I've heard of using tiny microphones to isolate noises, but have never tried that myself, outside of using a funnel or screwdriver handle to find mechanical noise in an automotive setting.

Know that more than one part may be contributing to the overall sound, and that some components, such as the coil-based ones, may be unavailable to you for off-the-shelf purchase.

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