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I have a MacBook Pro and last time I observed a strange behavior. When the fridge is turning on (to start cooling down again) I get a loud noise through the headphone jack on the external sound system. This also happens if the fridge is itself turning off (but not so loud as before).

I never experienced such behavior before on any of my devices (including PC). But I noticed this noise doesn't appear if I listen to music on the internal speakers of the laptop.

Has somebody an idea why this is happening and what I can do against this?

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    Does this only happen when you've got the notebook plugged into AC, or does it also do this when running (only) off battery? What exactly is your "External sound system"? Does it have an amplifier that's powered by plugging it into the wall?
    – Ƭᴇcʜιᴇ007
    Jan 22, 2016 at 20:44
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    It also happens if I'm running on battery. I'm using a Logitech Z2300 and it has an own amplifier powered by plugging it into the wall. It is probably the same circuit the fridge is running on.
    – testing
    Jan 22, 2016 at 20:46
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    Yeah, that's generally a bad idea. High-powered devices really should be on a different circuit than computing equipment. Even with low-pass filters, the power adapters of most computing equipment let through dirty voltage in such cases which can drastically reduce their life.
    – Mekki MacAulay
    Jan 22, 2016 at 20:48
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    It's most likely the amplifier in your Logitech speakers amplifying the power fluctuations in the AC line when the fridge turns on and off. Plug it into a different AC circuit and see if it behaves the same way.
    – Ƭᴇcʜιᴇ007
    Jan 22, 2016 at 20:49
  • What do you mean with drastically reduce their life? The life from the computing equipment?
    – testing
    Jan 22, 2016 at 20:53

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Magnetic fields do horrible things to speakers. Fridges draw a lot of power so send out a lot of electromagnetic radiation, especially when starting up (and then a lack thereof when stopping).

A similar thing will likely happen if you put your cellphone beside your laptop speakers and call it from another phone.

It means that the grounding of the setups wherever you hear it is sub-optimal. Usually it's laptops that are worse, but sometimes the battery can do odd things.

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    What do you mean with battery can do odd things?
    – testing
    Jan 22, 2016 at 20:51
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    I wasn't very clear. By "odd things", I mean that how laptops switch between AC power and battery power is usually not visible or controllable by the user at the level at which it affects grounding/electromagnetic interference. Some laptops will use the battery as a power filter by passing AC current through it, resulting in cleaner power fed to the mainboard. Other laptops will have a bypass circuit when AC power is connected so the battery isn't part of the equation. Unfortunately, the former is less common, hence the "odd" effects from the user perspective.
    – Mekki MacAulay
    Jan 22, 2016 at 20:54
  • One more question: I tested it with another laptop. Here it works without problems. Is the sound card of the MacBook faulty? Or is the difference in the power filter the reason?
    – testing
    Jan 23, 2016 at 10:19
  • It's not likely a fault in the sound card. It's mostly likely that they way the MacBook handles grounding is different. If you really want to get creative, you can try to improve the grounding of the MacBook to see if it changes it. It's a bit difficult with laptops. In PCs you can just add a wire somewhere to ensure the power supply and case are grounded properly. For a laptop, you'd have to find some sort of exposed metal that is likely in contact with the sound setup and try to connect that to a ground sink somehow... Likely not worth the trouble.
    – Mekki MacAulay
    Jan 23, 2016 at 15:56
  • But, regardless of what you're hearing, I'd strongly encourage you to not connect your electronic devices to the same circuit as high powered devices like fridges, freezers, microwave, etc. Even if you don't hear anything, minute, continual damage to the devices is probable. This thread explains the electrical theory behind the effect: [ superuser.com/questions/182002/what-is-dirty-power ]
    – Mekki MacAulay
    Jan 23, 2016 at 15:57

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