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Total Fan Noise - how to calculate ??

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Anonymous
a b B Homebuilt system
April 21, 2005 12:51:17 PM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

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).

What I am wondering though, is if there is a very general ballpark way
to estimate how much noise a system will create. TIA
Anonymous
a b B Homebuilt system
April 21, 2005 12:51:18 PM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

Al Dykes wrote:
> In article <l68f61tf70ec0l84iu99efir0kc82tp9bc@4ax.com>, Ken Roberts
<> wrote:
> >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

I wouldn't even consider it. db's doesn't even indicate how the sound
will be percieved either. High pitch noises may not sound as 'noisy'
as low pitch ones which are just as noisy. There are also loads of
other things to take into consideration, such as how much sound will be
absorbed before it leaves the case etc.

Your dBA for some of those fans seems very low. CPU fan 15dBA, and
video card 12dBA.
Anonymous
a b B Homebuilt system
April 21, 2005 1:03:04 PM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

In article <l68f61tf70ec0l84iu99efir0kc82tp9bc@4ax.com>, Ken Roberts <> wrote:
>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).
>
>What I am wondering though, is if there is a very general ballpark way
>to estimate how much noise a system will create. TIA


Google for "how to add dba" came up with this...

http://www.overclockers.com/articles607/

To simplify, if you have two 28dBA fans, combined they are 31dBA. Do
that again (2x2 fans) and you're up to 34dBA, so your 5 case fans are
about 35dBA. The rest of the fans, combined bring you up to maybe
37dBA.

It'll generate high numbers (ie lower perceived sound) because, for
example, given identical fans, one moving air into the case and one
exhaustiong air out, the exhaust fan will be a bit louder.

Nobody needs 5 case fans for anything like a standard desktop PC. It
the're not set up right they are fighting each other. One on and one
out should do it.
--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.
Related resources
Anonymous
a b B Homebuilt system
April 21, 2005 1:43:44 PM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

Ken Roberts wrote:
> 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.
Anonymous
a b B Homebuilt system
April 21, 2005 2:14:58 PM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

petermcmillan_uk@yahoo.com wrote:

> Al Dykes wrote:
>
>>In article <l68f61tf70ec0l84iu99efir0kc82tp9bc@4ax.com>, Ken Roberts
>
> <> wrote:
>
>>>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
>
>
> I wouldn't even consider it. db's doesn't even indicate how the sound
> will be percieved either. High pitch noises may not sound as 'noisy'
> as low pitch ones which are just as noisy.

You have the basic idea right but the scale backwards. High pitched sounds
seem 'louder', and more objectionable.

Which, btw, is why people perceive that cheap stereos can put out a 'lot of
sound'. They can't really handle the power needed for 'loud' and, so, clip
the hell out of the audio, which generates all kinds of harmonics up the
frequency scale that the human ear interprets as 'loud', and 'painfully so'
(so it must be *really* 'loud'). It's the high frequency distortions that
give the perceived effect.

The reverse is true for bass. It takes a *lot* of power to get 'volume',
which is why people can comfortably (that's a joke folks) listen to bass
that's shaking the house off it's foundation.

Sensitivity also changes with the volume, which is what the 'loudness'
control on stereos is for: to boost them when the volume level is turned down.

The frequencies we hear the easiest are between 3 and 4 KHz.

When you get above 9KHz, or so, human hearing drops off again, like on the
bass end, but the 'objectionable' nature of it remains.

> There are also loads of
> other things to take into consideration, such as how much sound will be
> absorbed before it leaves the case etc.
>
> Your dBA for some of those fans seems very low. CPU fan 15dBA, and
> video card 12dBA.
>
Anonymous
a b B Homebuilt system
April 21, 2005 6:23:42 PM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

> Total = 190 dBA

Ah, noooooo :-)

For 2 fans of A1 dB(A) + A2 dB(A)...
o Combined = 10 * LOG (10 ^ (A1 / 10) + 10 ^ (A2 / 10))
So for 2 fans of 21dB(A) + 21dB(A)...
o Combined = 10 * LOG (10 ^ (21 / 10) + 10 ^ (21 / 10)) = 24dB(A)

For n fans just keep adding onto the end...
o Combined = 10 * LOG (10 ^ (A1 / 10) + 10 ^ (A2 / 10) + 10 ^ (A2 / 10))

Thus for 5 fans of 21dB(A) the combined total is 28dB(A).
o Multiple exhaust fans are additive in cfm
---- eg, 5x exhaust fans of 25cfm each = 125cfm
---- subject to case resistance & c/sectional area of the case intake
o Adding intake fans is not additive in cfm, but assistive in pressure
---- so allowing exhaust fans to get closer to their Free Air airflow

If you end up at 90dB(A), worry as that's the limit for machine rooms :-)
--
Dorothy Bradbury
www.dorothybradbury.co.uk for quiet NMB & Panaflo fans
Anonymous
a b B Homebuilt system
April 22, 2005 3:27:17 AM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

Matt wrote:

> David Maynard wrote:
>
>> dB = 10 * log 10 (power in W/m 2 ) + 120, to get the dB(A) sound level
>
>
> W/(m^2) is a unit of intensity, not power.

Quite right.

The audio industry is lax about that terminology because the m^2 term
cancels in the ratio.
Anonymous
a b B Homebuilt system
April 22, 2005 10:21:57 PM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

> Sensitivity also changes with the volume, which is what the 'loudness'
> control on stereos is for: to boost them when the volume level is turned
> down.

To boost what? Does the loudness control do more than increase/decrease
the volume?
Anonymous
a b B Homebuilt system
April 23, 2005 1:06:30 AM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

>> Sensitivity also changes with the volume, which is what the 'loudness' control on stereos is for: to boost them when the volume level is turned down.
>
>
> To boost what? Does the loudness control do more than increase/decrease the volume?

It exaggerates the base and treble, fooling your ear into thinking
that you have the volume higher than it actual is.
Anonymous
a b B Homebuilt system
April 23, 2005 2:13:48 AM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

Matt wrote:

> sorry, having a little trouble with my news server ...

Are you with HOTMAIL? They had a major burp at 1421 ET.
Anonymous
a b B Homebuilt system
April 23, 2005 9:18:17 AM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

Matt wrote:

> David Maynard wrote:
>
>> Sensitivity also changes with the volume, which is what the 'loudness'
>> control on stereos is for: to boost them when the volume level is
>> turned down.
>
>
> To boost what? Does the loudness control do more than increase/decrease
> the volume?
>

Yes, the 'loudness' control boosts the low and high frequencies, at low
volume, because the ear's sensitivity to those frequencies drops off,
relative to the middle frequencies, as the volume is turned down.
Anonymous
a b B Homebuilt system
May 4, 2005 11:36:35 AM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

Matt <matt@themattfella.zzzz.com> wrote:

(for the third time)

> To boost what? Does the loudness control do more than
> increase/decrease the volume?

Besides double and triple replies to the same post, your posts
have been showing up in duplicate and triplicate for weeks. Once
is plenty.

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Anonymous
a b B Homebuilt system
May 4, 2005 10:33:21 PM

Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

John Doe wrote:
> Matt <matt@themattfella.zzzz.com> wrote:
>
> (for the third time)
>
>> To boost what? Does the loudness control do more than
>> increase/decrease the volume?
>
> Besides double and triple replies to the same post, your posts
> have been showing up in duplicate and triplicate for weeks. Once
> is plenty.
>

God you are a bitchy little thing
!