## 3 Experimental Procedures
A point contact transducer with internal electronics (KRN-BB-PC, 1-mm diameter, KRN Services) was primarily used to detect the vertical motion of passing Lamb waves. Thissensor nominally covers from 10 to 1000 kHz. A broadband PAC Pico sensor (3 mm diameter, Mistras, Princeton, NJ, USA), which has a sensing range from 30 to 800 kHz, was also used. Figure 2d gives the receiving sensitivities of these two sensors to Rayleigh waves [50]. This calibration utilized glass capillary fracture. The unit is in dB in reference to 1 V/nm = 1 mV/pm. For the KRN sensor, the average sensitivity from 10 to 300 kHz is equal to 1.00 mV/pm. That is, its output in mV can be directly converted to displacement output measured in pm. Detected signals were digitized using PicoScope 5242D (15 bit, 8 to 104 ns sampling intervals, input impedance of 1 M\(\Omega\); Pico Technology, St. Neots, UK). During signal acquisition, signal averaging was always used. Averaging counts were always more than 32. This unit also served as a waveform generator, the output of which was amplified to \(\pm\)20 V using an HP-467A amplifier (Hewlett Packard, Palo Alto, CA, USA). All sensor coupling used petroleum gel as a couplant. Figure 2: _Cont._  
For all testing of Lamb-wave attenuation, vibration detection was conducted on both surfaces, as was done in [48]. This necessitated the use of the identical transmitter configuration. The set-up of Figure 2a for the asymmetric mode (and its equivalent for the symmetric mode that uses a C-clamp) can conduct the two-surface sensing by turning over the plate with the transmitter attached. Care was needed to keep the transmitter attachment undisturbed while flipping after the completion of measurements on one side. The receiver was now positioned on the back side and transmitter connection was reestablished, followed by back-side detection. This two-surface method was not used when two point-contact transducers were used as transmitter and receiver. At later stages, a different sensor attachment method allowed the use of the two-surface detection as well. Figure 2: (**a**) Experimental set-up for attenuation measurement of asymmetric mode with a plate sample. A transducer glued to a hex rod is coupled to a plate edge using a pair of jigs. See also enlarged insert. Receiving sensor, a pulser and an oscilloscope are also visible. (**b**) Transmission sensitivity of two transducers, Olympus V101 (blue curve) and NDT C16. (green curve) in dB in reference to 1 nm/V. Normal incidence. Frequency spectrum of an excitation pulse is given in red. (**c**) Displacement output spectra of Olympus V103 (in blue) and NDT C16 (in red). Transducer mounted on a hex rod: 60\({}^{\circ}\) incidence. The dB scale is relative due to partial coupling to a calibrated receiver. (**d**) Rayleigh wave receiving sensitivity of two sensors, KRN-BB-PC (in green) and PAC Pico (in red). The dB scale is in reference to 1 mV/pm. Differences of the detected signals on two sides were used for further signal processing in asymmetric cases, whereas signal summation was used to enhance symmetric wave signals. Time shifting of a few tenths \(\upmu\)s to up to 1.5 \(\upmu\)s was needed to match the signal pair for the same or opposite phase for the symmetric and asymmetric modes, respectively. The value of 1.5 \(\upmu\)s was only required for lower frequency signals below 100 kHz, so it is a small fraction of the wave period. Time shifting was applied at the peak location. This need resulted mainly from the lack of symmetry in the transmitter attachment. In the case of asymmetric wave generation, signal paths were different by design. Complete suppression of unwanted mode cannot be achieved, and time-based discrimination was also applied using the wavelet transform analysis method. For the symmetric wave analysis, signals were truncated when the main asymmetric wave (usually A\({}_{\text{o}}\) mode) arrived and its amplitude grossly exceeded the expected S\({}_{\text{o}}\) mode before applying the summation routine.