Re: Pump noise -- such nostalgia

["S. Hieber" <shieber at yahoo_com>]

Jean -- JOlson8590 at aol_com posed some hypotheses on why permanent
magnet AC induction motors sometimes get noisy.  Shorts in the stator
coil is a good guess.  Decaying potting is another -- if the potting
dries out or shrinks, it can let the stator coils vibrate enough to
hum.  And the strength of the rotor magnet can weaken over time.

But Jean recalled the venerable Silent Giant air pumps.  They ran for
years.  Many of the pumps, I think might have run longer than the
company.

A very simple way to quiet an air pump is to put it in a box stuffed
with polyester batting --  let the air pump be suspended in the
batting.  Or hang the air pump by it's cord -- in a nice way, I mean --
inside a larger thick-walled enclosure.

Jean gave a more detailed design that would surely be even more
effective.  More detailed even than the Silent Giants that I remember
-- the one with metal housings.  The motor (pump) sat inside the
housing, which was filled with ordinary gravel.  The housing, which was
really nothing more than specialized coffee can, sat on foam feet and
there was a length of cotton string inside the the tube from the motor
to the outlet nipple -- the string evened out the pulses of the air
flow, and thereby reduced the peak noise.

Before powerheads and power filters, before those first
green-perforated-tube undergravel filters, Silent Giants drove the back
filters of an awful lot of aquaria.

At the lfs back then, if you wanted to buy an aquatic plant, you
usually could get your choice of three:  anacharis - egeria - elodea. 
:-)

Scott H.

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