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Nonlinear coupling of a piezoelectric cantilever to an external magnetic force increases the power harvested from a broadband vibration source for relatively large acceleration vibrations. A threshold exists in the minimum acceleration needed for enhancement of the cantilever response. Below this threshold the nonlinear coupling damps the cantilever motion, and the power output is reduced compared to the noncoupled cantilever. By reducing the size of the coupling magnet, the acceleration required to scavenge usable power can be greatly reduced. The smaller diameter magnet decreases the spatial extent of the magnetic force, reducing the acceleration required to surmount the local potential barrier. The experiments result in a six-fold decrease in the acceleration threshold, to levels comparable to those observed in ambient environmental vibrations. The results are in good agreement with numerical calculations. [ABSTRACT FROM AUTHOR]