The dBA noise rating on a case fan's box is measured at maximum speed, which is exactly the speed you'll probably never run it at. If you've bought a fan rated "quiet" and found it anything but, the box didn't lie to you, it just told you about a speed setting you're not using.
Every Fan Is Loud at Full Speed
Vendors publish airflow, static pressure and noise as one matched trio, and they measure all three at the fan's maximum RPM. A fan rated 39.7 dBA at 3000 RPM sounds nothing like the same fan running your actual PWM curve, which on a typical case fan header sits well below maximum most of the time. The problem is that headline number is the only one most listings give you, so two fans that behave completely differently at the speeds you'll actually use can carry the same on-paper noise rating.
Some manufacturers do publish a second, quieter figure. Noctua states 22.5 dBA at 1800 RPM for its NF-A12x25 G2, and a further reduced figure with its low-noise adaptor in place. That second number, at a stated lower speed, is the one that actually predicts what you'll hear in a quiet room, and it's routinely left off retail listings entirely in favor of the single maximum-speed spec.
Fans With Physical Speed Switches Make This Worse
Some fans ship with a physical switch offering multiple performance profiles, each with its own genuinely different airflow, static pressure and noise figures. A fan like this can be whisper quiet in its lowest setting and genuinely loud in its highest, and a single published noise rating for that fan is almost meaningless without knowing which switch position it refers to. If a listing gives you one number for a multi-mode fan, you have no way to know which mode it's describing.
What to Actually Check
Look for a noise figure tied to a specific, stated RPM, ideally one closer to what your case's fan curve will actually run at (often somewhere in the 800-1200 RPM range for a quiet build) rather than the fan's rated maximum. If a listing only gives you a maximum-speed figure, treat it as the fan's ceiling, not a prediction of daily noise. And if you're comparing two fans for a quiet build, weight the quieter, lower-speed figure heavily, since that's closer to your real-world experience than the number on the front of the box.
Winner
Don't judge a fan's real-world noise by its maximum-speed rating alone. Look for a quoted figure at a lower, stated RPM, and treat the headline dBA number as a ceiling rather than a prediction of what you'll actually hear running a normal fan curve.
Pros and Cons
Maximum-speed noise ratings: Easy to find and directly comparable between fans, but tell you almost nothing about how loud a fan will be running a typical quiet fan curve.
Quiet-speed noise ratings: Far more useful for predicting real-world noise, but published by only a handful of manufacturers and often missing from retail listings entirely.
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