A pair of identical piezoelectric (PZT) wafers collocated on a beam enable extraction of individual Lamb wave mode signals due to their polarization characteristics. Among these extracted Lamb wave mode signals, mode-converted ones proved to be promising for reference-free crack detection in a beam. This paper investigates the mode-converted Lamb wave signal induced by a notch on the beam in the frequency domain. Through the FFT of the mode-converted Lamb wave signals truncated by a series of time windows, the convergence of the temporal spectrums are demonstrated resulting in the resonance of the beam within the driving frequency range. The root mean squares (RMS) of the temporal spectrums indicate that the signal to noise ratio associated with damage is amplified as the time window size increases. Based this observation, it is concluded that the electro-mechanical (EM) impedance signal is more promising than the transient Lamb wave signals for reference-free damage diagnosis in an overall sense.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.