Ken said:
For example, if I have the following:
5 case fans - 28 dBA
1 CPU fan - 15 dBA
1 PSU fan - 23 dBA
1 Video Card fan - 12 dBA
------------------------------------------
Total = 190 dBA
Now, I am certain that you cannot calculate the amount of noise you
will hear by adding all the fan's dBA rating together. I am also sure
it would be an extremely complex formuyla to calculate, because there
are so many variables (placement of fans, distance from user's ears,
vibration, etc).
Yes, and more. In particular, humans find higher pitched sounds more
objectionable than lower tones so even if the 'noise level' is the same the
'objection level' is different. (which is not the same thing as dbA
weighting. dbA weighting takes into account that the human ear is more
sensitive to certain sound frequencies than others but not that they're
emotionally 'objectionable')
A 92mm fan putting out a measured 26 db of 'noise' will 'sound better' than
a 60mm fan with the exact same measurement because the 92mm job does it at
a lower RPM, which translates (a significant amount, anyway) to a lower
frequency.
What I am wondering though, is if there is a very general ballpark way
to estimate how much noise a system will create. TIA
Sorta, but not really.
At any rate, the basic decibel (actual base unit being the Bel and deci the
base ten prefix for .1 ) equation is logarithmic and is measuring one level
against a reference point (ratio), which is essentially arbitrary (but
standardized for particular db scales).
http://www.analogrules.com/dbwatts.html
For sound power levels the equation is
dB = 10 * log 10 (power in W/m 2 ) + 120, to get the dB(A) sound level
(the equation presumes the standard reference level and is why you do not
see a ratio in the power section but a + 120 to the log)
To add db you need to convert back to power level and add the power levels
together, then reconvert to db.
There is a shortcut and that's when the db levels are the same. I.E. two is
a doubling, 4 is a doubling of the doubling, etc. And a doubling of the
power adds 3db. So two 21 db fans would add to 24 db, four 21 db fans would
add to 27 db, 8 to 30, ... etc.
However, what may seem counter intuitive is that when the db levels are
greatly disparate, as like one is 28 and the other is 12, then the increase
in db over the larger one, I.E. 28 in this example, is rather minuscule
because the louder one is *so* much louder that the quieter one adds little
to the scale which, if you remember, is logarithmic.
I.E. it looks sorta like
s .. to equal a small db change up here.
o ..
u ..
n ..
d ..
..
p ..
o ..
w ..
e ..
r ..
db -------------------------------->
takes a lot down here
So, the point is, in your case you can significantly discount the 12 and 15
db jobs since the 28 and 23 db fans will dominate.
Now, it gets even more confusing because the human ear does not interpret a
doubling of sound power as a doubling of 'the sound'. I.E. even though a 26
db fan is putting out twice as much noise power as a 23 db fan humans do
not perceive that as 'twice as loud'. Rather, blind studies show that
humans tend to indicate something is 'twice as loud' when the db level
changes by 10. (this is a source of great confusion with people arguing
about what a 'doubling of sound' is and it depends on what one is looking
at. I.E. if you're looking at noise levels to determine ear damage and
hearing loss, like standing next to a 747 jet engine, then the actual power
level is the item of interest and 3 db is a doubling of it. If your talking
about how one perceives noise, or the local opera singer, then 10 db is a
doubling of the *perceived* sound)
To make matters even worse, db levels are based on coherent sound and
'noise' is non coherent, As such it tends to add together less (non
coherent cancellation). So what would be a 3 db increase for coherent sound
adds to less, say between 1.5 and 2.5 db, depending on the nature of the noise.
Which is why I said "Sorta, but not really."
And all that is before we even get to case dampening, how far away one is,
direction, conducted vibrations and secondary radiation from things (such
as case walls, etc.) and the fact that the fan rating is it sitting all by
its little lonesome in an acoustic chamber and not whistling air through a
stamped case grill or fan blade tone beating it across heatsink fins.
