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Re: Cooling Fans

Gamera wrote:

> I will be building a new light canopy for my new 30-gallon aquarium. 
> I want
> to replace the AC fan I am using in my current fan because it
> produces a
> relatively loud “whirring/whining” sound that can be very annoying at
> times.
> I have been doing some research on AC fans and have found several I
> am
> considering.  I’ve read up on their CFM and dBa ratings, but don’t
> really
> understand what they mean.  To be more precise, I KNOW what they
> mean, but I
> cannot correlate the ratings to anything that means anything to me. 
> I know
> the CFM rating of my current fan (32 CFM), and it seems to do an
> acceptable
> job of cooling my light canopy.  However, I don’t know the noise
> (dBa) level
> of the fan.  Without knowing this, how can I possibly understand how
> a new
> “quieter” fan would compare to my current fan?  Unfortunately, I
> cannot find
> any information on the noise level of my fan.  I was hoping someone
> here
> could give me a hand (***Applause*** Ha! Very funny!).  I am
> currently using
> a Radio Shack 3” 120VAC cooling fan catalog #273-242.  Does anyone
> know the
> dBa noise level of this fan?  If so, it would really help me in
> choosing a
> quieter fan for my new hood.  TIA!
The stuff Radio Shack sells can be manufactured by any of a number of
overseas companies, and this surely changes from time to time.  So the
only way to get a reliable value for the noise level is to ge a decibel
meter (Radio Shack sells them but doesn't actually manufacturer them
itself) and measure the fans with all other sound makers off (if
possible.  Radio Shack actually sells a cheap decent db meter -- decent
enough for these purposes -- even passable for designing a loudspeaker.
 But it could cost as much as buying a fan.

The overall noise level is not all that matters though.  If most of the
sound energy is focused around a particular strident frequency, then 32
dbs can sound like hell while 60 dbs from another more pleasant source
can be much less bothersome.  Audio afficianados refer to this factor
as listening fatigue -- you just get tired of hearing it -- some would
say that it makes pink noise instead of white noise.  If your fan
produces a sound that you would describe as a whine, then how loud it
is probably isn't important -- a whine is a bad sound at any level.

If a 32 cfm is your target for air flow (you say that works for you),
then look for the quietest fan you can find and afford that moves 32
cfm -- it might be AC or DC and if it's DC it might be worth the cost
of an AC-DC adapter.  My own rule of thumb is that, if the noise level
isn't published, or at least avaialable from the maker, then it's
probably not worth bragging about.

Larger tubeaxial fans will tend to be quieter than smaller ones because
all of the oomph is near the blade edges and, for example, the working
area near the edge of a 4" fan is about twice that of a 3" fan --
roughly speaking.  Other things being equal, a 3" fan will have to spin
twice as fast as a 4" fan to move the same amount of air -- and its
noise, therefore, will tend to be at a higher frequency.

I would expect a good quality 3" fan, AC or DC, that produces 32 cfm to
make about 35 to 40 dbs of noise (when new and when unobstructed). 
larger fans that move 32 cfm would be quieter than 35-40 db.

Scott H.

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