Thermal testing was conducted on the Phenom II X4 840 using HWMonitor 1.19 to monitor idle and load temperatures. Idle conditions were established by having the system sit at the Windows 7 desktop with nothing else running, while the load conditions were established by running 3DMark Vantage back-to-back three times before recording the maximum temperature. For comparison purposes, the NH-L12 was tested with the 120mm fan connected directly to the motherboard's fan header, as well as with the LNA (low noise adapter) installed. The AMD stock cooler as well as Noctua's NH-U12P were also tested in this system.
The first chart shows that at idle there isn't much separating the four coolers, although you can give a tie for first place to the NH-L12.
With the system all warmed up, I am now annoyed that I left the stock AMD cooler installed for so long seeing that it can let the processor get close to 80 degrees Celsius. The NH-U12P wound up being the top performer, but the NH-L12 did very well, too. Considering the design differences, I expected the NH-U12P to actually have a wider margin of victory. For a low profile cooler, you definitely get great performance out of the NH-L12.
Whether the cooler was configured to use the low noise adapter or not, the noise level produced was minimal when inspecting the case with the side panel open. Now that the only moving parts in the case are two Noctua 120 fans and the PSU's 120mm fan, the system is silent when all closed up and observed from more than about two feet away. The multimedia experience provided by this system just got a big upgrade since the only sounds we will hear are from the content being enjoyed.