Static pressure is ____________ the rpm of the fan.

Static pressure in a duct system is indeed proportional to the square of the fan's rpm. This means that as the speed of the fan increases, the static pressure generated by the fan does not increase linearly but rather increases with the square of the increase in speed.

For instance, if a fan operates at a certain speed and its rpm is doubled, the static pressure it can develop increases by a factor of four (since 2 squared equals 4). This relationship is based in part on the physics of fluid dynamics and the way air is moved by the fan blades.

Understanding this fundamental principle is critical for HVAC professionals when designing systems, sizing equipment, and ensuring that adequate airflow is maintained in various parts of a building.

The other options do not reflect the accurate nature of the relationship between static pressure and fan speed, making them less applicable in the context of how fan performance is influenced by rpm.