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Any powered external speakers I connect to my laptop make a buzzing noise when my laptop is plugged in. I also have battery-powered noise-canceling headphones that make the buzzing if noise canceling is enabled. Quick summary:

Laptop       Speakers                                      Buzzing noise?
Battery      (any)                                         No
Plugged in   Internal                                      No
Plugged in   External, plugged into wall outlet            Yes
Plugged in   Headphones, active noise canceling enabled    Yes
Plugged in   Headphones, active noise canceling disabled   No
Plugged in   Earbuds, passive                              No

The buzzing is louder if my display brightness is low, and especially loud if the display is turned off.

The buzzing happens with any power outlet: I've tested it in several buildings in two different cities. I have also tried plugging the speakers into a separate outlet from the laptop (one connected to a different circuit breaker), but the buzzing persists. This leads me to believe that the issue isn't due to some sort of ground loop (and this is supported by the fact that the buzzing happens with the battery-powered noise-canceling headphones).

The volume of the buzzing does not change if I adjust the volume on either my computer or my speakers.

The buzzing stops if my CPU is at 100%. For example, if I run two instances of yes (one for each CPU core), I don't hear anything out of the speakers.

I recorded a segment (you'll probably have to turn your volume up) of the buzzing noise with my phone and performed a frequency analysis:

frequency analysis of buzzing noise

The peak just to the right of the 100 Hz line is centered at 120 Hz, which is double the frequency of the AC power here.

Why does this happen and how can I fix it?

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I'm betting your PC is sending extra power to them when it can spare it, and it translates as buzzing. I don,t think you can fix it... Doesn't sound like something you can adjust. – Ariane Jan 13 '13 at 21:14
+1 for extensive testing and very detailed question – That Brazilian Guy Jan 13 '13 at 21:23
You mention display brightness. Do you mean an external display that is plugged into the AC outlet? If so, that's quite significant. In any event, it sounds like a "hum loop". What type of laptop do you have, and does it have a 2-pin AC plug or a 3-pin AC plug? – Daniel R Hicks Jan 13 '13 at 21:36
How are your noise-cancelling headphones powered? – Daniel R Hicks Jan 13 '13 at 21:38
@DanielRHicks: The laptop's display, not an external one. It's a ThinkPad T500 and it has a 2-pin AC plug. – Snowball Jan 13 '13 at 21:38
up vote 9 down vote accepted

Sounds like a power supply issue. In a computer's power supply, a constant voltage is maintained by a component that switches the power to a storage capacitor on and off very rapidly.

I'm willing to bet that what you're hearing is the voltage regulator switching on and off. When you do anything that requires additional power (i.e. turning up the brightness up, or running the CPU at 100%) the regulator is kept on for a longer period during the cycle. When you use less power, the regulator "on" time is shortened. This switching is generating electrical noise which is being amplified by your speakers.

Power supply manufacturers go to great lengths to minimize the electrical noise generated. It's possible that your power supply may be faulty. Do you know anyone else with a Thinkpad? If so, see if you can reproduce the issue when your laptop is plugged into their power supply. If it's still happening, it could be an issue with the motherboard. If the ferrite core has become detached from the end of your laptop's power cable, that could also be the cause.

There's a chance that a driver update could resolve the issue. It's a small chance, but worth a try. Assuming you're running Windows, download and run Lenovo's ThinkVantage System Update utility and install any available audio driver updates.

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Yeah, odds are it's "hash" from the power supply. Not entirely clear why it's present in some modes and not others. – Daniel R Hicks Jan 13 '13 at 22:39
I'll try out out a different power supply in a couple days. I think the newer ones have 3-pin plugs. – Snowball Jan 14 '13 at 5:28
The odd thing about the power supply hash explanation is that it's heard only with amplified devices. Apparently some sort of signal is being generated that gets through to the amp. It may be that the signal is very high frequency and the above chart is just showing the "envelope". Or it may be that the signal is "common mode" and the amps are more sensitive to this. An interesting experiment would be to put a ferrite choke on the cable to the speakers. – Daniel R Hicks Jan 14 '13 at 13:01
Good news! As you suggested, I tried a different power supply and it doesn't make the buzzing. The one that doesn't cause the buzzing is the 65 W supply shipped with a ThinkPad T520i. The one that causes the buzzing is the 90 W supply shipped with the ThinkPad T500. – Snowball Jan 15 '13 at 3:31
@DanielRHicks: Perhaps I'm misunderstanding the point of a ferrite choke, but wouldn't the envelope (and therefore the noise) still remain even if the high frequencies are filtered out? – Snowball Jan 15 '13 at 3:36

I had the same issue. I use two power bars for all my electrical in my office. My laptop and exterior speakers were plugged into the same powerbar. I unplugged the power for the speakers and plugged it into the other 2nd powerbar. Now no humming or hissing.... Hope that helps.

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I've had it in the past, (Dell and Sony laptops), where I had to use an adapter that prevents ground from connecting on the laptop power supply cable, and the noise stopped.

Not saying that this is the solution to your problem, but it has completely eliminated the issue for me a few years back.

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I am not sure but most likely reason for buzzing is that your electrical outlet(s) is not grounded. The noise is coming from the laptop not from the headset/speakers so you would need to connect the laptop not the sound playing device to socket with ground wire (best thing you could do, I guess, is to connect both to the outlet with the same ground source).

I have no idea if that is the case here as you seems to have strange results with different outputs.

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I had a similar problem with an external speaker being plugged into the USB port on my laptop, with the aux cable jack directly next to it. After reading the reply above about powering the speaker from a different power supply, I used a different USB power supply than the laptop, and the buzzing stopped completely.

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