Anderson acceleration (AA) is a technique for accelerating the convergence of fixed-point iterations. In this paper, we apply AA to a sequence of functions and modify the norm in its internal optimization problem to the H^s norm for some integer s to bias it towards low-frequency spectral content in the residual. We analyze the convergence of AA by quantifying its improvement over Picard iteration. We find that AA based on the H^{-2} norm is well-suited to solve fixed-point operators derived from second-order elliptic differential operators, including the Helmholtz equation.