Pick fans, choose what they should have in common — rpm, delivered airflow, static pressure or noise — and read the rest off the overlay. Same model and bands as the database. Not measurements.
Same airflow: each fan is set to the rpm that delivers that much air behind the chosen mounting; a fan under its minimum rpm is held there and delivers more. Same pressure: airflow each fan pushes against that static pressure at max rpm, from the parabolic fan curve. Same noise: the rpm at which each fan reaches that level, then its airflow. ΔP is the pressure the fan develops at its operating point — the drop across the mounting.
Delivered airflow is CFMfree air · derate · rpm/rpmmax. The free-air figure is the
manufacturer's. The derate is the share of it that survives the mounting; it does not depend on rpm because fan
pressure and flow resistance both scale with rpm². Airflow is therefore linear in rpm.
| Mounting | Airflow blades | Pressure-optimized blades |
|---|---|---|
| Open grille | 0.88 (0.80–0.95) | 0.88 (0.80–0.95) |
| Dust filter | 0.70 (0.60–0.80) | 0.78 (0.70–0.86) |
| Radiator 30 mm | 0.60 (0.47–0.76) | 0.70 (0.58–0.83) |
| Solid or glass front (side slits) | 0.50 (0.40–0.62) | 0.58 (0.48–0.68) |
The bands are the plausible spread of filter mesh density, radiator fin density and grille geometry. They are not statistical confidence intervals from measurements; they are the range the same fan lands in across typical cases.
Noise is the aerodynamic term Lmax + 50 · log₁₀(rpm/rpmmax) 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 band uses slopes of 40 and 60 dB per decade, ±1.5 dB on the rating and a 3 to 9 dB(A) floor. Manufacturer dB(A)
figures are not measured the same way across brands; differences under 3 dB between brands mean nothing.
Static pressure, where the datasheet gives it, is drawn as a parabola from the shut-off pressure to free delivery and scaled by the fan laws: flow ∝ rpm, pressure ∝ rpm². It is a shape between two datasheet points, not a measured curve; real axial fans dip in the middle.
Nothing here is measured. The values are what the fan laws and typical case resistances predict from the datasheet. Use them to compare and to size; expect a real build to land inside the band, not on the line.