And why doubling a fan's speed doesn't just double the airflow.
Every duct system carries two different kinds of pressure at once, and a single gauge reading "0.8 in. w.g." on a system means nothing until you know which one it's measuring. One component does the actual work of pushing air through resistance. The other exists purely because the air is moving. Confusing the two — or assuming that speeding up a fan a little only costs a little more energy — is one of the most common and most expensive misunderstandings in duct design and fan selection.