Frequency-sweep tests were used to characterize the broadband harvesting performance, while PSD-based spectral power analysis was used as a complementary frequency-domain energy metric. [1][3][6][7] Since the sweep excitation was treated as a nonstationary test condition in this analysis, the PSD-based result was interpreted as a spectral power distribution rather than a strict steady-state power-frequency response.P(t) = Vrms(t)^2 / RP(f) ผ่านฟังก์ชัน f(t) ที่ทราบ