Originally Posted by carnageX
Backup battery power supplies not only usually filter out noise before it gets to the computer's PSU,
Yes, it does filter. And then view manufacturer's spec numbers. That filter is near zero. Just enough above zero so that the many who recommend it subjectively can claim 100% filtering. Little difference between a subjective claim and a bogus one.
How well a UPS filters is further made irrelevant by other factors. A 230 volt power supply converts 'cleanest' or 'dirtiest' power into well over 300 volt radio frequency spikes. Far dirtier than anything found on the mains. Then superior filtering inside a supply convert that much dirtier power into rock solid, low voltage DC.
Anything done by a UPS is first undone. And then done better inside each electronic power supply.
View specifications for a cited: APC Back-UPS ES BE350G UPS - Newegg.com
UPS. How much filtering does it do? Manufacturer specifications say virtually none. When not in battery backup mode, it connects a computer directly to AC mains - and to noise from that fan.
A UPS is expensive. So it must do cleaning. Hearsay says so. Even the manufacturer does not make that claim.
Where is noise best solved? At its source. A most effective and the least expensive solution.