When a subwoofer isn't set up properly, it can create a high-pitched sound known as ground loop. It commonly occurs when there is a difference between ground points on electrical devices. The exact source of the problem can vary, and you may have to disconnect and reattach every wire on your audio unit to find out where it's coming from.
Ground loop is one of the most common causes of subwoofer hum. When ground loop occurs, you will hear either a low or high-pitched humming noise whenever the unit is plugged in. The problem can stem from any electrical component. To solve ground loop, disconnect the component cables and then reattach them one by one until the hum stops. Install a coaxial isolation transformer on the cable that's causing the problem. If this doesn't stop the humming, install a line-level ground loop isolator onto the live feed. If this still doesn't solve the problem, you probably have a faulty subwoofer.
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Setting up the wiring incorrectly can also cause ground loop. Before you tamper with any of the wiring, make sure the noise is actually coming from your audio unit and not your power socket. To do this, simply plug it into a different socket. If it still occurs, the problem is almost certainly coming from your audio unit. Turn off your power outlet and unplug your audio system. Check the setup instructions provided in the manufacturer's instruction manual. Make sure all of the connections are in the right place. For example, if you have your subwoofer connected to a 5.1 surround sound system, make sure the cables leading into the front and back speakers match up with the cable labels properly.
Loose connections can result in either low or high-pitched humming sounds in your subwoofer. Check all of the connection points on your subwoofer, such as the plug screws and wire connectors. If any of them are loose, either replace them or solder them back together. If your subwoofer is near another speaker, the problem could stem from that. In this case, check the positive and negative terminals of your speaker setup. The colored wires should be connected to the corresponding terminal on the audio unit.
The low frequencies of a subwoofer can make objects in your room reverberate when you are playing audio. While this shaking may be subtle and barely noticeable, it can cause high-pitched noises. If you have anything balanced on your subwoofer, remove it. Objects should never be placed on top of audio equipment. If the noise continues, try moving your subwoofer to another area. The unit could be causing the surface it's resting on to reverberate.