The minimum % duty cycle for nearly all Asus motherboards that have the BIOS Silent Profile feature has always been 20%. This was changed recently by Asus for some of their more recent Republic Of Gamers (ROG) boards such as the Asus Maximus VI Gene. On these ROG motherboards it is set in the BIOS to something around 35-40%.
On silent preset is 30%, here some screenshots that should put it around 28% but i waited until i gotten the lower rpm on the bios, it will usually go +20rpm, so that should set it very close to 30%.
But is important to note that this is an exceptional situation and not typical of Asus motherboard BIOS control as a whole. That said, the improvement to BIOS fan control being made by Asrock on their latest Haswell boards are to be welcomed. Perhaps at some point in the future Asus might be able to incorporate all or most of the functionality of Fan Xpert 2 in the BIOS rather than as a separate piece of software, and give all OS users the benefit of it.
Didnt know it was still 20% on none ROG boards, on my ROG GENE IV was 20%, but on my ROG GENE VI is 40% as the minimum on the manual preset.
That said, the improvement to BIOS fan control being made by Asrock on their latest Haswell boards are to be welcomed. Perhaps at some point in the future Asus might be able to incorporate all or most of the functionality of Fan Xpert 2 in the BIOS rather than as a separate piece of software, and give all OS users the benefit of it.
If nothing changes ill probably build on Asrock on Skyalke, i also liked a lot the control that Asrock is giving to their users on pure bios.
The other point that should be made is that PWM fan profiles are also part of the equation. Even if you have a motherboard that restricts BIOS PWM fan control to a minimum of 35% duty cycle it is still possible to achieve sub-500 rpm idle speeds with the right choice of fan. An example is the Scythe 140mm GlideStream PWM fan as fitted to the Ashura CPU cooler. This fan essentially has a built-in Silent Profile and holds the fan speed to a more or less a constant level all the way from 0 to 40%. The result is a quiet fan at idle and low system stress levels. However the complaint that has been levelled at this fan is that is does not spin up and fails to cool, when all that Scythe are doing is implementing an ancient silencing technique - sacrificing CPU and system temperatures to a degree in order to benefit from lower fan speeds and lower fan noise. It is possible to see this clearly now that SPCR is producing Fan Xpert 2 PWM fan profiles as part of both CPU cooler and case fan reviews. This is the profile for the 140mm GlideSteam PWM as published as part of the SPCR Ashura review:
Personally i would prefer a fan can be controlled from 0-100%, that can drop to almost 0rpm on 0%, this way with something like the Asrock motherboard you can set it the way you want it. If Asus dont remove their restriction on future generations, i'll not consider them, given that Asus FanXpert2 is the easiest way to tune/quiet fans, almost nothing is needed from the user, it comes pack with AI Suite that its far from a finish product, and specially bundle with so many things i don't need, i would prefer a stand alone FanXpert3. One point that i do dislike about AI Suite III is the way the measure the CPU temp, its werid combination of sensors that they say gives a more accurate reading, given that its not the same as any software out there (this doesn't bother me to much), but it ramps differently also, i could live with +C like an overlay, but if it scales differently then i cant adapt to it to design my desired temp graphs. My MSI Z87-GD65 works great with HWmonitor/HWinfo/CoreTemp, and the temps that i set on bios for the fan control go directly to the cores temps, really what i wished.
Another thing worth mentioning that is bad from Asus is they only offer 1 true PWM fan header, you do get two with CPU_OPT but its not controllable, its just a mirror of the CPU_FAN header, this puts it worst than MSI and AsRock that offers 2 fully controllable, and to end this, the CPU_OPT is not readable by a lot of software like HWmonitor, only with AI suite (this could be lack of support from 3rd party, but regardless its not something that will know if it happens), on MSI i can read both CPU_FAN1 (CPUFANIN) and CPU_FAN2 (SYSFANIN) on HWmonitor, this is very convenient as i have different fans on case fans and cpu.
I honestly prefer to invest more in choosing the fans and a motherboard that can control it in pure bios, and investing a little more time setting up the bios fan control, like Asrock is atm or MSI to some point.