Perhaps this entry should be longer or more formal, such as a full review, but I am a little short of time.
There has been a great deal of interest expressed in the Silenx fan, but not much objective information is available for it yet, as best I can tell.
Let me say at the outset that one of the biggest problems we have in this forum is that the quest for very low noise levels involves evaluation of phenomena that are at the limits of human sense perception. Measurement of these phenomena is very difficult, requiring very sophisticated and extremely expensive equipment, as Mike's work with the University of Vancouver has shown.
I have recently acquired a device for measuring airflow, the AirFlow 6000VT (http://www.airflow.co.uk/scripted/detai ... r=71839701
). This is not an ideal instrument for measuring low airflow states, but seems useful. The results, which I have gotten with it, have been consistent.
As a first step, I measured airflow for three 80mm low-noise fans at 12.3 volts: Vantec Stealth SF8025L (0.10 amps, airflow=210 ft/min), Panaflo FBA08A12L (0.10 amps, 190 ft/min), and Silenx 825-15-20 (by Ahanix, 0.84 amps, 125 ft/min). As pointed out many times before (most thoroughly in gmJamez' wonderful post, "Choosing Fans, A Scientific Approach", http://xover.mud.at/jamez/fanspec2/fanspec2.xls
), there is a tremendous variation in airflow at 12 volts from different fans on the market. To some extent, quiet fans are just low airflow fans. The results, which I obtained with the Vantec, Panaflo and Silenx at 12.3 volts, support that idea.
Next, I adjusted voltage to the Panaflo and the Silenx, using FanMate controllers, until airflow for each fan was 110 ft/min. The choice of this value was arbitrary, but was based on the likelihood that reliable airflow measurements could be made, and this level of fan function is likely to be employed in everyday use.
At isoflow=110 ft/min, voltages were 6.73v for the Panaflo and 9.0v for the Silenx.
Extended, careful listening to the fans revealed the following: The Panaflo has a faint clicking, a hum and a tiny bit of whine. The Silenx has almost no click, but there is a slight buzz/hum which is probably due mainly to air turbulence. Both fans are extremely quiet, but the Silenx is quieter. This is mostly due to the non-complex nature of the sound it makes, and the sound appears to be at relatively low, poorly penetrating frequencies.
A major caveat: These were only 2 fans, one of each. There is likely to be significant variation between different samples in a batch of fans, which are ostensibly identical due to subtle variations in manufacturing. The results, which I obtained, could have just been due to sampling error.
So, if these results are generalizable, is the obvious conclusion that everyone should buy Silenx fans? Not necessarily. The Silenx is an intrinsically low-flow fan. The Silenx' top end of performance is only 66% of the Panaflo's. In the event that a heavy heat load is imposed on the system, the Silenx may lack the ability to ramp up to meet demand. The Silenx is not widely available (yet), and the cost and difficulty in obtaining the fan may not be worth it.
Another intrinsically low-flow 80mm fan, the NMB 3110KL-04W-B19 (0.06 amps), shows airflow at 12.3v of 124 ft/min, and at the isoflow=110 ft/min level uses 10.68 volts. This fan is slightly but definitely noisier than the Silenx and the Panaflo, but it is still relatively quiet. (I am using 2 of them at 12v in current systems, with no complaints at all.) The NMB has a more pronounced buzzing sound up close. SVC is basically giving these fans away at $2.50 each ($1.85 each in batches of 10), and it is a 3-wire fan, which allows for fan speed monitoring. (The Panaflo and Silenx are 2-wire fans). Although none of these fans are costly, the NMB is easily cheapest.
In summary, options for low noise fans are quite good. Among the low-flow, super-quiet fans, Silenx, Panaflo and NMB are all reasonable considerations. Choice will depend on system need, availability and (to a small degree) budget.