Short Version
The relative noise level produced by the reference cooler.
Long Version
We rate the reference coolers according to personal experience and comments and reviews from other sites. There are 5 sound levels: Silent, Relatively Quiet, Moderate, Loud and Notoriously Loud.
Silent means the card has a passive cooler with no fan. Relatively Quiet means the card has either a very small fan, or a cooler that's known to be very quiet. Moderate means the card is about as loud as your average mid to high range card (i.e. it will be audible above other fans in your case if you have a quiet case). Loud means several sources have confirmed that the cooler is loud to the point of irritation and should definitely not be considered by those looking for silent solutions.
The Notoriously Loud rating is reserved for a very special group of cards that are not only loud, but have acheived a bad reputation for the noise they generate.
Retail cards don't necessarily use the reference cooler and therefore may be louder/quieter than the reference card rating suggests.
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