Noctua states that cooler will cause the 3950x to run below it's base speed:
"Possible but sustained clock speed will be below base clock."
The NH-L9x65 is a taller, performance-enhanced version of Noctua’s award-winning NH-L9 low-profile coolers. With 65 instead of 37mm height and four instead of two heatpipes, the NH-L9x65 is more powerful than its smaller siblings yet retains their 95x95mm footprint, which guarantees 100% RAM and...
noctua.at
And
"The NH-L9x65 is a highly-compact low-profile quiet cooler designed for use in small form factor cases and HTPC environments. While it provides first rate performance in its class, it is not suitable for overclocking and
should be used with care on CPUs with more than 84W TDP (Thermal Design Power). Please consult our
CPU compatibility list and
TDP guidelines to find out whether the NH-L9x65 is recommended for your CPU."
I'd definitely look for a better cooler that will fit your case.
Read these:
Intel and AMD's finest mainstream chips go toe-to-toe in a seven-round faceoff.
www.tomshardware.com
Turn the dial up to 16
www.tomshardware.com
"Unlike other mainstream Ryzen processors, the Ryzen 9 3950X doesn't come with a bundled cooling solution. AMD recommends using a 280mm AIO cooler to keep the cores chilled, but we found even that to be somewhat inadequate. In our
tests of the 3950X, our Corsair H115i struggled to keep core temperatures low enough to maintain the factory boost frequency. To extract the full potential of a 3950X, you're best with a custom water-cooling loop. "