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(a,b) Frequency spectrums for electromagnetic and triboelectric output voltage at 0.1 g, 0.2 g and 0.5 g exhibiting multimodal behavior. (c,d) Frequency spectrum for electromagnetic and triboelectric output voltage at accelerations higher than 0.5 g; non-linear stiffening effect can be observed due to engagement of mechanical stoppers for different accelerations from 0.5 g to 2 g.
