[quote="Gxcad"]I must jump in. Bigger fans are LOUDER at the same RPM. For example, the famous black label 60mm delta (38cfm) was about 6800rpm and 48.5db. The 80mm 68cfm I believe is 4700rpm and around 48.5db as well (approximate off the top of my head guesses). As you can see, the rated noise is the same yet the bigger fan rotates slower. Do not confuse bigger fans = more cfm for the same noise (true) with bigger fans = quieter because they spin slower. Bigger fans are louder per spin, but create more cfm per db. So in actuality, bigger fans are louder per rpm, and therefore harder to silence as much as smaller fans.
Actually there are several factors that contribute to the noise of a fan. What makes fan noise unacceptable is a combination of pitch and volume. Much of the noise is created by the air hitting obstructions in the flow path, by imperfections in the blades, and by the tips of the blades themselves. Larger fans create sound that is lower in pitch by virture of the longer path the air takes through the blades. Larger fans have a smaller % of restriction caused by the fan case and the supports which means lower noise per rpm from that cause. Your general statement that "bigger fans are louder per spin" is not necessarily true. They are much easier to silence than smaller fans to product the same work (CFM) with more pressure through the system. If you would like to learn more about the general subject, read http://www.amdmb.com/print-article.php?ArticleID=132