On Wed, 15 Jul 2009 14:52:33 GMT, Keiron
<pop07@NOSPAMsheffield.ac.uk> wrote:
>Hello all.
>
>I was thinking about taping up the edges of my case in an effort to
>deaden fan and harddisk noise whilst hopefully preventing dust entering
>the case. Is this viable generally speaking?
>
Yes, no, maybe.
It won't deaden fan noise significantly, unless the case is
extremely crude with unspeakably large gaping holes.
It will prevent dust from entering, to some extent, but is
this important? If you have a filtered case intake too,
then it may be useful as it keeps dust on a filter panel you
can periodically clean.
Generally speaking, on a good filtered system the case has
positive pressurization, so you don't need to tape up gaps
because the gaps are where the already-filtered air
exhausts, rather than enters.
However, you didn't mention the case, the specifics of what
you are thinking of doing, nor how bad the dust is in your
environment.
In other words, there is an ideal, there is doing testing
and novel ideas, experimentation as a hobby if the case
isn't right, then there is the amount of time to change a
design versus the amount of time it would take to clean out
a filter every now and then.
In other words, I have modified many cases to be positive
pressure when they weren't originally, and it does keep
parts much cleaner, but ultimately the service interval is
not much lower as the filters (if effective enough to
matter) still have to be cleaned at, at least the same
interval as the whole system would need to.
I have thought about it and started abandoning the idea of
cases built to support filtered intake, instead opting for
cases with ample free space behind the front bezel so I
could add a very large area filter which, due to it's high
surface area, would not only impede airflow less, would also
not need cleaned as often so long as the fans were not
tweaked to be such extremely low RPM that there was not a
fair airflow margin to account for some reduction due to
filters being less than perfectly clean.
>Additionally I was thinking about adding noise damping material on the
>inside of my case but not being prepared to pay the premium of commercial
>products was thinking about completely DIY system. Any recommendation on
>this? I had been thinking corrugated card treated with heatproof
>embrocation of some description, or perhaps covered in duct tape?
It all starts with a good case but we don't know what you
are thinking of using. That will determine how much effort
or cost is worthwhile. We also don't know how loud what you
are using as the example to spur these ideas, versus your
goal.
Generally speaking, if you avoid cases with very thing
sheeting and minimal folds (cheap cases), then the first
step to avoiding noise escape is to not have noise
producing parts within a direct line of holes to the
exterior. For example, some like front intake fans to cool
the HDD rack, and they can do well at that purpose, but if
the case bezel has a front open area next to such a fan, the
fan noise has no mitigation as it is directly exiting
without being reflected off any materials which deaden
sound.
However, there is ideal, there is extreme, then there is
practical. Once you start talking about modifying a case,
there are plenty of extremes to fit every budget. It really
has to be taken one case (literally speaking) at a time,
considering which parts make noise, the budget, and what the
acceptible noise level would be.
>
>Lastly, some questions on fan arrangement. I understand it's best to have
>positive pressure inside the case as to keep heat and dust going in an
>outward direction?
To force the maximum amount of air through filters that are
well sealed as a chamber the intake fans are mounted in,
yes.
You don't have to worry about it for heat reasons though,
negatively pressurized cases deal ok with heat too, the same
factors apply relating to airflow volume and heatsinks used,
because in the grand scheme of things whether it is positive
or negative pressure, the pressure difference still isn't
very big in the grand scheme of things, axial fans simply
can't cause a very large pressure change even it you tape up
the whole case.
>This being the case what is the best configuration for
>exhaust fans?
Depends on the case you use. Generally the best config is
boring but most common, PSU and a rear chassis wall fans
exhaust.
I should pause a minute and mention that not only do we not
know what case and fans you'd use, we don't know the parts
you'd put in them, nor the highest load it would face, nor
the highest ambient temp.
It's an engineering exercise, these things can be
scientifically resolved if only enough time is spent, though
realistically a little extra margin is good too, and
allowing for that margin makes it much easier and quicker to
get the job done.
I should pause a minute and mention something, I have no
idea if it applies to your situation or not...
Many medium or lower end cases have low quality fans running
at excessively high RPM, and/or mostly obstructed fan intake
or exhaust areas. You have to look at all the factors in
cooling, make what is present optimal and only then go to
further extremes... and in most cases more won't be needed,
unless highly overclocking, usign an ancient case which
wasn't designed with modern system heat levels in mind,
using it in a high ambient temp environment, or building a
gaming system with more than one high heat video card.
One or more of these may apply to your use, but we can only
generalize so far, after which point the specifics have to
be considered to optimize noise and dust buildup vs cooling.
It is a science, and an art, and even then the owner of a
system can, by monitoring that system, tweak it even more.
> If the exhaust moves less CFM than the intake fan does this
>positive pressure on the exhaust fan have any negative effect?
Not significantly, except that if you design this way it is
important to make the intake be the opposite end of the case
from the exhaust so the airflow is over the most parts in
the case as possible, particularly the hard drive rack and
chipset heatsink if it is a passive, fanless 'sink.
>Would it
>perhaps be best just to have just an intake and no exhaust fan?
No, axial fans can't maintain much pressure, and directed
exhaust reduces short loops, meaning airflow doesn't
re-enter the case after being heated by parts it cooled, so
soon after mixing with lower temp ambient air external to
the case.
Plus, redundancy is good. If you want to accept front
intake noise for it's benefits, then the rear exhaust noise
you won't necessarily hear at all, is a backup strategy in
case the front fan(s) fail.
Plus, even with a positive pressurized case, having some
rear exhaust fans still increases the total chassis airflow
rate.
>Equally
>if I get a jet of an exhaust fan and an intake fan of less CFM, would the
>exhaust fan be strong enough to create the positive pressure inside the
>case by sucking the air though the case?
I don't understand the question.
You can't just assume anything by a fan's free-air CFM
rating, you have to also look at it's pressure rating when
it's a push-pull fan config, but briefly, if using similar
fan types besides the RPM or size, you should assume that
for positive pressurization, and for low noise, you will
need at a bare minimum, 2 x 92 mm fans.
We are back at the point where we can't generalize much,
have to instead consider the specifics of the particular
case, whichever fans are present and/or not going to be
replaced versus those that might be replaced, whether any
have obstructed intake or exhaust due to fan grills
including half stamped-out metal sheeting holes, or filter
panels (which if they're effective filters, will reduce
airflow quite substantially before being cleaned).
>
>Your views and comments appreciated.
We should step back and look at what problems you have faced
in the past and whether they remain, and how the same (or
different) system might need design changes to counter
them... or might not.
Finally I will give an example. I wanted a fileserver
which would run for years unattended just for the novelty or
margin in it, whatever... As originally designed (for an era
when systems producted less heat). it had only a rear
redundant PSU exhaust (so about 2 x 60mm fan average), plus
a rear 80mm fan on the case wall).
To make it positively pressurized, I cut out holes and
installed 3 x 92mm fans. I could have instead fit at least
one, maybe two 120mm fans but I did not want to do that.
Being a fileserver I wanted the intake airflow to be
entirely through the hard drive racks, so a 92mm fan will be
able to do that while a 120mm fan is too wide, necessarily
most of the airflow will be around the HDD rack unless the
case length is extended a bit (not a typical DIY job unless
designing an entire case from scratch) to put in a ductwork.
Anyway, I took a case and sawed a bit off the front bezel,
cut a window air-conditioning filter panel to the right
size, glued in mounts to position it correctly, and put
adhesive foam insulation tape around the front bezel
perimeter to keep the intake as filtered as possible, to
minimize leaks.
This project was several years ago, I have cleaned the
filter panel once since then. I can't say it was or wasn't
worth the time, I would have to have an otherwise identical
system side-by-side to say for sure, but I am totally
satisfied with the results of the modification I did to that
case.
Unfortunately I didn't take pictures of the whole case, I
suppose I assumed it was just a typical full tower case
beyond the front bezel modifications, and that I cut out the
front steel frame so 3 x 92mm fans would fit. A typical
midtower would at least fit 2 x 92mm fans unless they put
the HDD rack low then you have to be creative with what you
have in front of you.
http://69.36.166.207/usr_1034/filtered/ 
Noise dampening/dust prevention/fans 
Linear Mode
