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Re: Low Noise Fans
- To: aquatic-plants at actwin_com
- Subject: Re: Low Noise Fans
- From: Bill Wichers <billw at waveform_net>
- Date: Wed, 30 Jul 2003 22:51:07 -0400
What dB level, in your opinion, qualifies as a "low noise fan?" Also, I am
soon going to be trying my hand at a marine setup with soft corals (and
eventually stony corals). I am building a hood to house anywhere from 440
to 660 watts (depending on how much I can fit in it) of VHO lighting.
Eventually, I will add some MH bulbs, probably 2 x 250 watts. What size,
number, and flow rate of fans would you recommend for this endeavor? It is
for a 4 foot long x 18 inch wide tank.
The noise level you find acceptable is going to be something you'll need to
find out yourself. I typically find 30-35 dB fans fairly quiet, with the
<30 dB fans nearly silent. The 20-25dB fans are very difficult to hear
running. Anything over about 40 dB is going to sound loud. FYI, 90 dB is
considered the "pain threshold" for humans, although I doubt you'll find
any normal fans that even approach that.
The more tricky part is what kind of sound you will find to be annoying.
Many fans have the same noise level in dB (usually dBA actually), but will
have a lower or higher pitch sound to them. I personally can tolerate a
louder fan that is lower in pitch much more than a fan of the same noise
level in dB that generates a higher-pitched whine. Typically the faster the
fan spins in RPM, the higher the pitch of the fan noise will be. Also, a
higher speed small fan will blow the same amount of air as a larger fan
running a slower speed (over simplified a bit, but the general idea is the
same). For low noise you are much better off using a LARGER, SLOWER SPEED
fan over a SMALLER, HIGHER SPEED fan.
Just for example, from the Digi-Key catalog, page 1034 (ebm brand fans), an
80mm (common computer size fan) moving 33 CFM of air will produce 35 dBA,
while a 92mm fan moving 36 CFM of air produces only 27 dBA. 10dB is a 10x
reduction is sound energy, 3 dB is a 2x reduction. The actual results you
will "hear" will sound smaller than that, and I can't remember at the
moment exactly what the conversion is. Unfortunately they do not list
rotational speeds so I can't make the comparison there. The larger fan
should generate noise of a lower pitch, and will sound *significantly*
quieter than the smaller fan, even though it is moving slightly *more* air.
The larger fan also uses about 1/2 watt *less* power to move *more* air.
Unless you need the higher air pressure that a higher speed fan can
generally generate, for common air-circulation-for-cooling applications a
larger, slower-speed fan will generally be a better choice for low noise.
The slower-speed fans will also generally last longer before their bearings
For your hood, you could take the rated temperature rise of your ballasts,
the exposed surface area of the ballasts, and the ambient air temperature,
and then calculate the minimum airflow required to keep the temperature of
the ballast below it's rated maximum. It's far easier to just put enough
fannage to turn the air volume of your hood over a few times an hour and
position the fans near the heat sources in your hood. And remember that
cooler parts will only last longer, so a few extra fans won't hurt anything.
BTW, you can further reduce the sound levels the fans will generate by
isolating their vibration from the enclosure. The easiest way to do this is
to mount the fans on rubber washers and adhesive weather stripping. Also,
the less obstructions to the fan's airflow there are, the lower the noise
due to air turbulence will be.
One last thing -- if you are planning on moving to MH lighting in the
future, you should probably consider starting with MH instead of VHO. It
sounds like you are going to be building your hood, and MH parts aren't
terribly expensive when purchased through electrical supply houses instead
of fish places. You then won't have the expense of redoing everything
later, and I think you'll find that the MH bulbs will be cheaper in terms
of parts replacement over time than the VHO bulbs will be. The newer PCF
lights are also longer-lasting than the older VHO lights and will be
cheaper to run over time. Something to think about...
UNIX Systems Administrator