(adsbygoogle = window.adsbygoogle || []).push({}); The German magazine ComputerBild is currently running an analysis with a PS4 debug unit. They have measured the unit's noise level while navigating the system menu and compared it to the original PS3.

After 10 minutes of operation
PS3: 0.83 sone
PS4: 0.815 sone

According to the magazine, a "slight noise" is audible from 40cm (= 15.75 inches) distance.

After 60 minutes of operation
PS3: 2.1 sone
PS4: 0.83 sone

A sone is a unit of perceived loudness: You must login or register to view this content. (with examples).

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This is just navigating the UI and not running a game on ps3 and ps4 but least it's quieter than the ps3 in that respect