Speakers won't shut up.

[Kimchi]

When ever I have my speakers plugged into the outlet the subwoofer makes a continuous hum when not in use. Even if they aren't plugged into my computer it still goes off.

[UltimateDave]

Umm, not even plugged into the computer? 2 things, 1. They are plugged into the wall and whatever is causing it is still causing it or 2. you're hearing things

Edit: jesus im stupid you said it was in the outlet. I don't know much about speakers but most stuff does it at some level or another. But if it's quite noticeable then i'd ask someone who knows alot more than i do.

[Sharif]

Try a different power outlet/power strip 

[joncppl]

Few possibilities. Noisy power is unlikely.
Somehow noise is getting to the subs amp, or it is being created in the sub.
If there is a cable from the sub (or setup) floating, it may be picking up noise (like an antenna).
If there is no cable, then the noise is coming from the subs internals, and there is nothing you can do other than replace it.

[DIV1D3]

As @Sharif said, try another power outlet, ideally from another circuit. 

[DIV1D3]

that happened with me recently, and i figured out the cause was mic was set to listening device, just right click on volume button select recording devices, double click on microphone and select listen tab. There you will find the option "Listen to this Device" make sure it is unchecked.

I have no knowledge with this but seeing as it has to do with sound devices and audio managing it could be the problem. 

[Recon97]

Use a duct tape to cover its mouth.

[Guest]

Tell them to shut up.

[KarsonC]

As others mentioned, try another outlet using different circuit. There could be some interference in the circuit, the power is not "clean".

[Tsuki]

that happened with me recently, and i figured out the cause was mic was set to listening device, just right click on volume button select recording devices, double click on microphone and select listen tab. There you will find the option "Listen to this Device" make sure it is unchecked.

normally i'd agree with this, however he mentioned that it happens even when its not plugged into the computer, so it has to be an issue with either the speaker itself or an enviormental issue.

i would try a different outlet, make sure you dont have anything next to it (my old speakers would buzz 10 seconds before i recieved a text on my phone that was 6 inches away).   also check the cable, make sure you dont have loose wires.(another set of speakers that would actually pick up radio frequency somehow cuz the cable was jacked up)

[Mojo-Jojo]

What you're hearing is the 50/60Hz mains hum. Usually it's caused by a badly shielded setup or a setup where the main's earth isn't connected as it should be (ground loop).

The hum will get into the wires and get amplified. You cna hear it especially well on the subwoofer cause it's made for the lower frequencies like mains hum.

 

I'm not sure about your setup, but it's possible that a cable has poor shielding performance due to a loose contact (Or the wires aren't shielded at all, but usally preamp subwoofers should be.)

 

You're not supposed to leave your speakers on when not in use anyway. Usually a preamp sub comes with its own on/off switch. It may stop being grounded once the main amp is powered off but the sub keeps getting power, hence only getting hum after powering everything off.

[phillrulz]

When ever I have my speakers plugged into the outlet the subwoofer makes a continuous hum when not in use. Even if they aren't plugged into my computer it still goes off.

 

Called a ground feedback loop. Not much you can do unless you are good with electronics or an electrician. http://www.native-instruments.com/en/support/knowledge-base/show/250/how-to-resolve-ground-loop-issues-humming-and-buzzing-sounds/