(4) Noise arising from these causes in or between other components upstream of the subwoofer. (3) Ground loop noise resulting from different ground potential at the receiver and the subwoofer and (2) Induced noise in the audio signal path, most likely around cables (1) Electrical defects in the powered subwoofer The four principal likely causes of hum are: To get rid of hum without having to throw out some of the desired audio at the same time, we need to start by understanding what the various possible causes are. Some parts of a low-frequency audio signal are themselves around 60 Hertz, and a filter doesn't know whether a particular wave is part of the intended sound or is noise - it just strips it out. The "brute force" method for getting rid of 60-cycle hum is to filter it out, but that's not a particularly desirable solution. There's nothing that can more effectively dampen one's enthusiasm for a nice powered subwoofer than a persistent 60-cycle hum - and since a subwoofer is intended specifically to do a good job of amplifying low-frequency signals, when a sub hums, it can hum very, very loudly. In a perfect world, power hum wouldn't ever get into the audio signal path, but in this respect, our world is far from perfect.
Unfortunately, there's another low-frequency signal present in every home, which isn't quite so lovely to listen to: the 60-cycle hum of the AC power lines that power everything in the house.
Getting good reproduction of the lower end of the audio spectrum gives sound a more full and realistic quality, and at the lowest audible frequencies and below, a subwoofer adds a tactile quality to home theater - some things are not so much heard as felt. Everyone can appreciate the value of a good subwoofer in a home theater system.