I installed a new fan, a low speed inexpensive Aerocool 80mm:
A little work with the voltage tester reveals that the motherboard is delivering ~8.25 volts to this fan even when the fan isn't being ramped. I left the case cover off, went into the PC Health Status of the bios and set the control to smart fan. With the cpu at 45C and the smart fan set to 60C the fan is being delivered ~8.25 volts, up from the 5 volts that the stock fan was delivered. Both fans are running at the same ~1800 RPM at their respective voltages. Leaving the voltage tester hooked up I lowered the ramp temp until I reached 44C where the fan began to ramp, then lowered the ramp temp in 1C steps and observed the voltage at each step. The fan was delivered ~.5 more volts at each step.
I only have one sample other than the stock fan to work with but it appears at this point that the on-board smart fan controller initially sets the fan not by voltage, but by rpm (~1800). From there, at every degree above the ramp temp setting, the fan is delivered ~.5 more volts until 12 volts is reached.
But there at first appeared to be an anomoly. Using the manual fan settings (ultra low, low, mid, & full) the respective voltages and rpms between the two fans at first didn't seem follow any rhyme or reason. Stock is delivered 5v, 6v, 7.5v, and 12v @ ~1800, ~2200, ~2800, & 3800, respectively. The Aero is delivered 8v, 10v, 11v, & 12V @ ~1800, ~2000, ~2200, ~2300. They didn't seem to follow any rhyme or reason until I looked at the percentage of increase for the second and third steps (low and mid). If the first step (ultra low) ramps the voltage until ~1800 rpm is reached it would account for the differing voltage but matched rpm of the respective fans at that setting. The voltage increases from ultra low to low and low to mid are ~1.2 times the previous, with the exception of the 10V to 11V (low to mid - aero) jump which is only ~1.1. The rpm increases are the same, varying from ~1.1 to 1.2. The last setting (full) just delivers the full 12v. It would seem that the second and third steps are a ratio of the preceeding steps with the initial step (ultra low) being whatever voltage is necessary to achieve ~1800 rpm.
So what does this mean? It means bad news. If the bios is looking for rpms then it's going to ramp the voltage being fed to the fan until it sees 1800 rpms. If we install a fan that runs at 1500 rpm at 12v (the nexus I have ordered and expect delivered monday, for instance) the fan is going to get 12v. The problem here is that while the fan may be running slower, and therefore quieter, the fan controller will already be maxed at 12 volts. If for some reason the cpu temps rise the controller has no more juice to give and the cpu fries. Alternatively, placing a voltage reducer between the fan and motherboard (like the fanmate I ordered and expect delivered wednesday) will have the same affect as running a fan that maxes out below 1800 rpms. If the board doesn't see 1800 rpms it's going to send 12v. The fanmate may cut those volts back to 5 and reduce noise, but as the board will be delivering all it can (12v) there is no protection from an overheat.
I will confirm my speculations when both the Nexus and Fanmate arrive. If true, then quieting the Zen will be a matter of going the opposite direction. Reducing fan size rather than increasing it. The test will be to see if a solitary 60mm, or even 40mm fan running at 1800 rpms will cool the beast. The question is, is this a deadend? Is it unreasonable to expect a 60mm or 40mm fan to run any quieter than an 80mm fan at the same 1800 rpm?