When I look at fan or the fan/heatsink combo, lI ook at the CFM or Cubic Feet per Minute. That is how much air can travel through per minute. a fan with a high cfm, say 60 cfm, is better than one with 30 cfm, even though the 30 cfm fan may have higher rpm. and since you are concerned with sound, look at the dBA, lower means quieter.
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