You may also hear a hum or buzz from the UPS. This is because, when on battery, there is circuitry to create an AC signal but it does not necessarily have the same wave form as what comes out of your wall. Theoretically, it should be a nice sine wave but it will not be. Power supplies in your PC and peripherals will have filtering circuits that can attenuate small deviations from ideal wave forms. Your UPS first creates a "square wave" and then smoothes it somewhat but it's not perfect. Square waves will cause the buzzing noise. This just means that the PS in your speaker system does not have the extra filtering needed to remove the electrical noise from the UPS and passes it to it's amplification circuits. You hear it as buzzing. UPSs of this variety are not meant to allow you to continue using your PC normally when power is interrupted. They capture short power drops or surges that provide protection from damage or data loss. When the power drops out completely, the UPS should provide enough time for you to finish what you have begun or find a place to shop and sequence down the system normally. Hope that helps.
I was testing to see what happens when I would unplug my battery backup out of the wall what windows would say. After I did that I noticed that my subwoofer of my speakers started making a buzzing sound, so I turned off the woofer and plugged it back into the power outlet and turn on the woofer and the buzzing was gone. Any idea for this buzzing sound? My only guess is that the speakers require a lot of power and the backup battery can't provide enough power. APC Backup UPS ES 725 and Logitech z560 speakers

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