Post
by MikeC » Fri Jul 17, 2009 10:41 am
OK, to clarify the 2 fans vs 1 fan SPL measurements and perceived noise, I ran some extended tests this morning on a pair of brand new Yate Loon sleeve bearing 120x25mm fans. These are exactly the same as the Nexus 120, except the top speed is slightly higher than that of the Nexus which is 1000~1100. All the tests were conducted in the anechoic chamber, whose ambient was reading ~11 dBA. The two fans were blowing in the same direction, spaced 2" apart on a piece of foam at the edge of our test table. rpm was set using a calibrated strobe light.
either fan at 1000 rpm: 14 dBA/1m
both fans at 1000 rpm: 18 dBA/1m
This makes sense, more or less. Theory says it should be a 3 dBA difference at 1m, but there are some reflections in the room from the table, other gear, etc...
either fan at 700 rpm: 11 dBA/1m -- 12 dBA/0.5m
both fans at 700 rpm: 12 dBA/1m -- 14 dBA/0.5m
The first reading above -- 11 dBA/1m for the single fan at 700rpm (which is roughly the same speed as 7V w/ the Nexus 120) -- is suspect.
Why? It's the ambient level. This means the fan could be anywhere between 0 and 11 dBA/1m and there's no way to tell, because 11 dBA is the lowest we can measure. The second reading for the same single fan at 700 rpm, of 12 dBA at half a meter tells us that the fan would probably measure 4-6 dBA lower in a 0 dBA chamber, or 6~8 dBA/1m.
So when both fans at 700 rpm measure 12 dBA/1m, it makes sense. Each fan by itself was actually only 6~8 dBA/1m. Why then, does it go up only by 2 dBA with both fans? uh.... I don't quite get the why of it but I can hear it.
To sum up...
Larry measured 12 dBA for 1 Nexus fan or 2 in his original test. I measured 1 dBA difference this morn. Why? The levels are so close to the chamber ambient... we could call the difference within the margin of error.
It could also be that the first Nexus fan was a bit noisier than the second so that when the second quieter fan was turned on, no increase registered. The Yate Loon fans I used are brand new, while the Nexus 120s Larry used have been in use around the lab for quite a while (probably at least a year).
The gist of the matter is that any of these fans at 700rpm is really not audible under any normal circumstance -- ie, in an enclosed computer a meter away. Two are still inaudible in almost any ambient. When you get really close to the fans -- less than a foot -- you can hear when one fan is turned off but you'd really be hard pressed to say it's quieter.
Hopefully, that clarifies things a bit.