So I did some testing and research and thought I'd share.
In case you didn't know the Low Noise Adapters are just inline resistors. Resistors reduce Voltage and Amperage/Current. I measured the resistance of the adapters I have with my NF-F12 and NF-A14. I'm using math to calculate the Low + Ultra specs and Voltage but it could be slightly off.
Also I wasn't sure what the minimum rpm of the NF-A14 was so I didn't know if it could go to 750rpm but I can confirm it does.
1200 rpm - no adapter 68CFM SP 1.51mm, 19.2 dbA
1050 rpm - Low Noise Adapter NA R10 50ohm (-1.5v) 60CFM SP 1.18mm, 16.4 dbA
900 rpm - Ultra Low Noise Adapter NA R11 100ohm (-3v) 52CFM SP .89mm, 13.8 dbA
750 rpm - Low + Ultra Low Noise Adapter 150ohm (-4.5v) 44CFM~ SP .60mm~ 11.4 dbA~
1500 rpm - no adapter 55CFM SP 2.61mm 22.4dbA
1200 rpm - Low Noise Adapter NA RC6 80ohm (-2.4v) 43.7CFM SP 1.83mm 18.6dbA