Which case fan setup is quieter. Enter the fans of two configurations and the airflow the case needs. Each fan is set to the rpm that delivers it; the result is the noise level of each setup.
Add fans to both setups.
Delivered airflow per fan is CFMmax · derate · rpm/rpmmax.
The derate is the share of catalog airflow that survives the mounting:
| Mounting | Derate | Pressure-optimized |
|---|---|---|
| Open grille | 0.88 | 0.88 |
| Dust filter | 0.70 | 0.78 |
| Floor + filter | 0.64 | 0.72 |
| Radiator | 0.62 | 0.72 |
All fans in a setup run at the same fraction of their maximum; the resulting rpm is shown per row. If that would put a fan below its minimum rpm, it runs at minimum instead and the setup delivers more air than asked.
Noise per fan: the aerodynamic term Lmax + 50 · log₁₀(n/nmax) added on an
energy basis to a constant 6 dB(A) motor and bearing floor, so the curve flattens at low rpm instead of running
to zero. The floor is an estimate; fans differ most there.
Sources add on an energy basis: 10 · log₁₀(Σ 10^(L/10)). Doubling the count adds 3 dB.
At fixed airflow, twice the fans at half the speed comes to −12 dB while both are well above the floor.
Levels under about 15 dB(A) are below the noise floor of a quiet room; the chart marks that zone. Manufacturer dB(A) figures are not measured the same way across brands. Cross-brand differences under 3 dB mean nothing.