DC fans versus AC fans (System Saver IIGS)

[supertimer@aol.com (Supertimer)]

Picked up a System Saver for my backup IIGS and the
first thing I noticed was how loud the fan was.  When
I opened the case, I found a huge AC fan.  Are AC fans
just more noisy than DC fans?

Since the tower unit of my flagship IIGS has two fans, I
took one and installed in in the System Saver in place
of the AC fan.  For 12V DC power, I found the power
supply board from an old junk hard disk case I had and
mounted it inside the System Saver.  From the System
Saver's 120V AC leads for the AC fan, I connected the
power leads to the power supply board from the junk HD
case.  Then from the fan leads of that power supply board
(12V DC), I connected the leads to the 12V DC fan from
the tower case.

The DC fan seems to move the same volume of air but is
much quieter.  Why is that?  Are DC fans inherently
quieter?  The AC fan seems to have a huge core and
stubby fan blades.  The DC fan has wide fan blades and
a slender core.  Could this be the cause?

With the AC fan gone, the System Saver is much lighter
too.  Why didn't Kensington use DC fans in the first place?