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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Time-frequency spectrums of voltage signals obtained from a (a) d31 PZT sensor and (b) d35 PZT sensor for a laminate structure with a notch excited with a d35 PZT actuator at 250 kHz.