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I opened today my Philips audio system (30W woofer + 20w satellites) because it has a lot of noise. There is something weird with that amplifier: when I knock into the circuit board with my finger nail I hear in the sub-woofer a noise EXACTLY as when you knock with your finger into a microphone. To make it clear, doesn't sound as a component not being well soldered (one that randomly connects/disconnects when you touch it). The noise is very smooth and "high fidelity" (if i can call it like that) exactly as a microphone (connected directly on the mainboard).

I spoke into the circuit board (yes laugh if you want) but it won't pick my voice. However, the slightest touch of the circuit board will reproduce the sound into the woofer. So, it works only for very low frequencies.

I can upload a video (with audio track) if necessary.

I know that loose coils of inductors can create this effect but there is no single coil on that circuit board. What can create this effect?

Update:

It is not on the input. I shorcuted the input of the amplifier and knocked into the circuit board. The sound is still there. But the sound is soooo smooth, I cannot believe it could be a loose component.

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  • \$\begingroup\$ What happens if you use an isolated screwdrive handle? Are you sure you aren't listening to mains hum induced in the circuit by your hand? Do you have pictures / video of what you're doing? \$\endgroup\$
    – jippie
    Commented Jan 31, 2015 at 10:00
  • \$\begingroup\$ @jippie - Hi. I never said that is electric noise that I introduce into the system with my hand. The 'microphone sound' happens when I knock into a non-electric part of the circuit board. It really picks us mechanical vibrations, not electric signals from my hand! \$\endgroup\$
    – IceCold
    Commented Jan 31, 2015 at 10:19
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    \$\begingroup\$ Some ceramic capacitors can act as electret microphones or vibration sensors. \$\endgroup\$
    – user16324
    Commented Jan 31, 2015 at 10:53
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    \$\begingroup\$ @BrianDrummond: Bingo!!! The most susceptible to vibration are the high-capacitance, High K caps with titanate dielectrics. \$\endgroup\$
    – EM Fields
    Commented Jan 31, 2015 at 11:25
  • \$\begingroup\$ Does it change if you ground yourself? (Your body can act like a big antenna.) \$\endgroup\$
    – George Herold
    Commented Jan 31, 2015 at 16:22

2 Answers 2

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You may want to read this article: Stress-induced outbursts: Microphonics in ceramic capacitors

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    \$\begingroup\$ Link-only answers are discouraged on Stackexchange as they tend to go bad over time. It would be good if you added a summary of the page to the answer. \$\endgroup\$
    – DoxyLover
    Commented Jan 31, 2015 at 12:22
  • \$\begingroup\$ Well, short answer or not it answered my question and clarified my voodoo electronics problem. Thanks Ambiorix. I wonder how is the circuit affected at high volume, since it is enclosed into woofer's box... \$\endgroup\$
    – IceCold
    Commented Jan 31, 2015 at 12:31
  • \$\begingroup\$ This could of course be a better answer, but the key fact one needs to know - that ceramic capacitors can be microphonic - is actually contained in the text of the link alone. \$\endgroup\$
    – Chris Stratton
    Commented Oct 20, 2020 at 14:07
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I can confirm the real world occurrence of microphonic effect from ceramic capacitors.
I built an AM radio which had the same sort of effect.

I saw Ambiorix's comment about ceramic capacitors acting as microphones
(as described in the link he provided), and it turned out to be what was happening in my case.

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    \$\begingroup\$ this must be posted as comment not answer. \$\endgroup\$
    – IceCold
    Commented Oct 20, 2020 at 8:34
  • \$\begingroup\$ @Ultralisk It's not the world's most brilliant answer but it is an answer to the extent that it provides additional confirmation of the effect. Ambiorx's useful link is effectively saying the same thing as StocksandStonks - but rather more technically. \$\endgroup\$
    – Russell McMahon ♦
    Commented Oct 20, 2020 at 11:48
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    \$\begingroup\$ Have to agree this is an anecdote not a unique answer. While the other answer would benefit from more detail, the text of the link states the same reality this does, even if the link is not followed. If @StocksAndStonks want to make a real contribution to the site, that would not be done by reporting a personal experience, but rather by writing some actual technical content explaining the issue rather than simply stating that what has already been stated to exist does exist. \$\endgroup\$
    – Chris Stratton
    Commented Oct 20, 2020 at 14:06
  • \$\begingroup\$ @RussellMcMahon - no need for confirmation from a "Random" internet guy. Ambiorix's link to a VERY well established source on information is enough. Therefore StocksAndStonks should post his experience as a comment - UNLESS he has more authority than TexasInstruments and overrides their saying :) \$\endgroup\$
    – IceCold
    Commented Oct 23, 2020 at 12:47
  • \$\begingroup\$ Therefore, I marked "answer" this as "not an answer". \$\endgroup\$
    – IceCold
    Commented Oct 23, 2020 at 12:48

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