I loved my new main computer - my Air - from the very beginning (yes, m-a-i-n computer, I know I'm crazy ;) ). Unfortunately, my enthusiasm faded after 1 hour of working with the default OS X installation. A Macbook Air is a beautiful machine, but I also expected it to be beautifully silent during "normal use". Well, much to my regret, it was not! Whenever the CPU load increased moderately (e.g. due to watching a YouTube video) and lasted for longer periods of time, the Air increased the fan speed to the factory maximum of 6200 rpm. At 6200, the buzz of the Air's fan can be incredibly annoying, particularly if you're sensitive to fan noise. At 2500 rpm, the fan is practically unnoticeable - the Air's pretty aggressive about cooling though, so, these low levels are rare in the non-idle-case.
My research lead me to believe that this NOT an issue affecting my Air in particular, it's behavior by design. Lots of Air owners seem to be annoyed by the fan noise, none of them have found satisfying workarounds and some even returned their Airs or got them replaced in hope for a "working" unit.
Rather than returning the Air or requesting a replacement to fix the issue, I investigated further and tried various ways to bring silence to the otherwise flawless and fascinating Air experience. I experimented with different energy saving configurations, software for controlling the fan rpm minimum as well as CPU voltage and frequency and also changed OS X kernel module configurations to alter the fan behavior. While all of these had noticeable effects on the situation, none of them prevented the fan controller from increasing revolutions to the maximum of 6200.
The fan is controlled by the so-called System Management Controller (SMC). Software allows you to modify the rpm minimum, that however could not solve the issue. Apple now provides an SMC firmware update that affects fan control. According to the release notes, this update could potentially worsen the situation because it apparently tries to lower CPU temperature further to avoid hangs and unexpected shutdown (of a single core and/or the whole system). I tried the update, it didn't have any noticeable effect in either direction.
Despite the 6200 rpm targeted by SMC, CPU temperature levels remain in a perfectly safe region (60 - 70 °C) at all times. So, knowing that the rpm increase is unjustified in respect to temperature levels, I changed the rpm minimum to 4500 and 6200. In respect to CPU temperature at 100% CPU utilization, the temperature diff between 4500 (maximum acceptable level that can be considered comfortable) and 6200 rpm is pretty much insignificant. Knowing that, my goal was to redefine the rpm maximum to fully address the issue at stake. I was unable to find any way to do that during the first round of research. Most utilities, including SMC Fan Control, merely allow you to change the minimum. As it turns out though, that software package provides a console application smc that can be used to changed all writable SMC parameters, not just the rpm minimum. With the help of the source of a Linux kernel driver controlling the fan via SMC, I identified the key representing the SMC parameter controlling the rpm maximum and gave the new maximum a try by executing smc as follows:
smc -k F0Mx -w $(python -c 'print hex(4500 << 2)[2:]')
Additionally, I reset the minimum to 1500 to conserve the fan at CPU idle levels:
smc -k F0Mn -w $(python -c 'print hex(1500 << 2)[2:]')
These commands only change minimum and maximum, revolutions still remain in control of SMC. The results have been fantastic, the Air has been quiet and cool ever since enforcing the new limits. Execute these commands in a startup item to make changes permanent (for the sleep / wakeup case, use SleepWatcher).
Now, I can finally say that it is a real pleasure to work with this elegant and capable notebook, I enjoy every minute of doing so. I strongly encourage you to give this workaround a try before throwing the Air out (at your own risk), you definitely miss something!