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RE: Cooling Fans -- Noise levels

Bill Wichers wrote, in part, regarding dbs (or decibels):

> It is a logarithmic scale, with a 3 dB increase representing a
> doubling of
> noise level, and 10 dB representing a 10x increase. 30 dB is a 1000x
> increase.

I would respectfully add this embellishment.  There's a difference
between energy levels and sound (or perceived or audibility) levels.  
The basic fact is that it takes a lot of energy for something to be
perceived as louder by humans.   Actually, while a 3 db increase in
sound level correlates to a doubling of the energy level, 3 db is a
slight increase in perceptable sound level.  Twice the sound level
(twice as loud) requires about 10 times the energy level or a 10 db
increase.  Cases in point:  A 33 watt audio amplifier won't play twice
as loud as a 30 watt amplifier -- it will seem barely louder at all. 
To get twice as loud, you would need an amplifier putting out about 300
watts.  This is why small power ampfliers have limited dynamic range. 
One of the problems of designing good audio equipment is this fact that
you need to produce twice the energy (watts) to produce a mere 3 db
increase in sound.  If the main sound level is about 1 watt (not
uncommon for many audio systems in the average listening room) than
short peaks that are four times as loud require 100 watts.  You want
Ten-to-one dynamic range?  Fuggettaboutit unless you have a monster
amplifier -- and speakers that can handle lots of watts (i.e., heat)..

So what does this mean for aquarium hood fans?  Just this:  two fans
that produce 40 dbs individually will produce only 43 dbs together --
it is only a doubling of the energy and you would need a tenfold
increase or 10 fans to make twice as much noise.  A single fan that
produces 60 dbs will be louder than the two fans that are rated for 40
dbs individually.  Of course, a screech at 40 dbs is less tolerable
than a pleastant hum at 70dbs.

Scott H

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