This question came up again in the context of a long discussion about case airflow techniiques, strangely entitled, question on an oddity ive noticed here
. I decided to do some empirical testing to answer the question with 2 Nexus fans versus one. The following, slightly edited from the orignal, was posted deep in the above thread where it will never be found.
NOTE: A basic rule of acoustic addition states that 2 identical sound sources will result in a 3 dB increase in overall noise. Four identical sound sources will result in +6 dB.
2 Nexus 120 fans, powered by a fanless PSU, in "the quiet room", in which the B&K SLM was seeing an ambient reading of 15~16 dBA (at dawn). Very hard to stop the meter from moving -- even a car 2 blocks away can have an impact when it's this quiet.
EDIT: A Nexus 120 @ 12V blows 37~40cfm; at 6V, it blows around 18~20 cfm.
from 1 meter:
1 fan at 6V (about the setting in my lab PC): 16~17 dBA.
2 fans at 6V (just like in my lab PC): 16~17 dBA.
I could not hear the change from one meter. But from a foot, I could hear it... so I put the meter closer.
from 1/2 meter:
1 fan at 6V (about the setting in my lab PC): ~17 dBA.
2 fans at 6V (just like in my lab PC): ~18 dBA.
OK, so that shatters the idea that 2 identical noise sources means a 3 dB increase. Obviously not a universal rule! I learned something new here!
This also confirms my impression that the lab PC, which has a very slow Nexus 120 for exhaust and a Seasonic Super Tornado with Yate Loon 120 fan, is ~18 dBA/1m. I should really measure it.... so I did.... and got 18~19 dBA.
Time to try the Nexus fans at 12V.
from 1 meter:
1 fan at 12V: 21 dBA.
2 fans at 12V: 22 dBA.
This is actually kind of amazing. It is not what I expected. So maybe the +3dBA rule only holds for louder noises? Or maybe it's the low freq balance and absence of midband of the noise that makes this rule not apply? Perhaps the rule is only valid without A weighting.
What did I hear at the 12V drive level? At 1m, the difference was very marginal, but I could hear it. From less than ~1/2m, the increased noise was more obvious but not big.
I tried the meter w/o any weighting. Levels jumped into the mid-30s and higher, and the needle was jumping up/down ~10 dB! I stood there and listend for a while to see if I could hear what the meter was picking up. 2 blocks away is Main St, by now starting to get busy. There was some distant rumbing that was audble but at a very low level. I am guessing that the meter was picking up on the low freq traffic rumbling <30Hz that was not so audible to me. In any case it was not usable w/o the low filter of the A weighting.
Time to try something more drastic: A much louder 120mm fan, which shall remain nameless for now.
from 1 meter:
1 fan at 12V: 30 dBA.
2 fans at 12V: 33 dBA.
Yup, +3dB "rule" confirmed at this level -- with this fan. Tried the test both ways -- either of the fans read 30 dBA by itself. Adding the other always brought the reading up to 33 dBA. And there was no question of the audibility at 1 meter. Totally obvious. This fan also had a whole lot more noise in the midband.
My preliminary conclusions:
1) The +3dB rule is valid but only when not weighted by any compensation curve. (Like the "A" weighting we use.)
2) The +3dB rule does NOT always reflect human hearing perception.
3) The exceptions are when the noise is mostly in the low freq, or probably when it is very low in level, say under 20 dB in the midband.
The issue of multiple vs single fans is clear with the Nexus 120. Two of them at 6V move as much air as a single one at 12V, and make 2-3 dBA LESS NOISE!!
This jibes with my experience of Panaflo 80Ls... which I found puzzling for a while. My wife's PC has 3 80mm Panaflos -- one in the PSU, one on the HS and another at the front to keep the suspended B-IV drive cool. The fans all run at 5V or less. It measures 17~18 dBA/1m and sounds quieter than any PC I've built -- except for experiments with notebooks drives. I remember taking out the front fan; heard no difference so stuck it back in to keep the drive cool.
Again, I think it's a question of freq balance. When you slow any decent fan down enough, the sound shifts to a lower freq balance. This is true of every size of fan, I can vouch from years of experimentation. Both the whistling caused by blade edges and midband whooshing at high rpm drops dramatically at low speed. So the whole sound moves down in freq to where our hearing is less sensitive.
I think it is safe to say that when you combine low speed fans, that +3dB rule gets moot because...
1) your hearing is not sensitive to that noise because it's mostly low freqs. Certainly you don't hear volume changes as linearly as in the midband. So you just can't hear the increased acoustic energy / SPL even though it's there.
2) the audibility may be borderline because it's at or near ambient level, so again, you can't hear it or not well.
BTW, AFAIK, low freq sensitivity in human hearing does not change much with age -- except maybe for those who really elderly or with abnormal hearing loss/damage.
In the original thread, NeilBlanchard
followed up with this post:
Here is a link to an explaination of the Fletcher Munson
curves. Huh? What does this have to do with this discussion?
Just about everything...
EDIT: A note about the above curves: These are the inverse of the frequence response of human hearing at different loudness levels. The top curve tell us that when subjected to a broadband sound of 110 dB SPL, we hear the frequencies between 1000 and 7000 HZ most acoutely, with the rest being fairly linear. When the sound is 50 dB, the 30Hz portion of the sound is heard as ~30 dB quieter than the signal at 1000 Hz. Ditto the extremely high frequency sounds beyond 10KHz. When we get down to the 20 dB level, the 30Hz portion of the signal is perceived to be ~50 dB quieter than the midband portion.
This phenomenon of rapidly decreased ability to hear low frequency sounds as loudness level drops is precisely what is reflected in my experiments here. The shrewd PC silencer will want to exploit this aspect of human hearing to maximize cooling airflow while still keeping audibility to a minimum.
After which I added:
I see the +3dB rule as an interesting physical phenomenon that can have a bearing on what we hear. By itself, +3dB rule does not dictate what we hear; it is simply a relationship between physical phenomenon and the meaurement standards/tools we have. The acoustic energy in the space is much less important than what you hear
-- the 2 are not the same at all, far from it.
The A weighed dB scale does not try to read the acoustic energy (SPL is not reallly an energy measurement anyway...); rather, A weighting is an attempt to simulate
the human hearing frequency response documented by Fletcher Munson (thank you NeilBlanchard
)and others. It's a bit rough, but much better than unweighted SPL readings, which have almost no correlation to what we hear when levels / freqs are low.
So when I measured 15~16 dBA ambient, then got 16~17 dBA with one or both fans turned on a meter away, this is very very close to what I heard. IE, I could not tell whether one fan was on or two fans from a meter away. I had to get within ~a foot to hear that difference clearly. The dBA readings did a very good job of simulating what I heard, and this is what really matters: the measurements jibe with what I hear.
If the measurement don't reflect with what I hear, then, for our purposes, the measuring tool/system has to be made better -- NOT the other way around.