Bernanke made his observation in his fourth and final lecture to George Washington University students. 

The lectures this month have been intended to both demystify the Fed and defend the steps it took to confront the 2008 financial crisis and the Great Recession.

The Fed chairman showed the students a chart illustrating that annual U.S. economic growth over the past century has been about 3 percent. Since the recession ended in 2009, the economy has averaged about 2.5 percent growth.

Bernanke didn't say when he thought the U.S. economy would return to its normal growth rate.