## 5 Experimental Results  
The results of the theoretical analysis were verified by experimental investigations. They were carried out with the experimental mica specimen with clearly expressed non-uniform properties (Figure 13). In this sample the sub-sonic A\({}_{0}\) mode was excited and the spatial distributions of  
Figure 18: 2D spatial-temporal spectrum of the B-scan mode in the case of a multiple circular non-uniformities (\(t=20\):\(400\) μs, \(y=3\) mm). the of-plane displacements of the mica paper were measured by the laser interferometer using the experimental setup presented in Figure 19. The experimental setup was the same as used for investigation of PVC samples [29]. For determination of elastic properties of the mica paper a slow sub-sonic A\({}_{0}\) mode guided wave is excited by a contact or by an air-coupled PMN-32%PT ultrasonic transducer (Figure 19). For measurements a pulse-type excitation is used. The off-plane displacements \(\xi\) of the plate surface caused by the propagating wave are picked up along the selected lines, for example, parallel to the coordinate \(x\), and in this way the B-scan is formed \(\xi(x_{j},\,t)\), where \(j=1\,\dots\,\,N\) are the numbers of measurement points. The step between the measurement points is chosen according to the expected dimensions of the spatial non-uniformities, e.g., it should be shorter at least by one order. The obtained primary B-scan is processed by the two proposed signal processing methods described above. Ultrasonic guided waves in the mica sample were excited by the PMN32%PT strip-like transducer with the aperture \(5\times 1\) mm operating in the frequency range 43-48 kHz. The same transducer was used as a dry contact and air-coupled transducer. In the first case the transducer was contacting the mica sample, and in the second case between there was an air gap between the transducer and mica sample of R = 0.5 mm (Figure 20). Figure 19: Experimental setup. In the last case the excitation area is lightly spread in space, but due to the narrow air gap between the tip of the ultrasonic transducer and the sample the amplitude of the lateral air wave is rapidly reducing. During experiments B-scans were registered by a POLYTEC laser interferometer in different zones of the mica sample. The propagation of guided A\({}_{0}\) mode signals is accompanied by the attenuation and scattering of ultrasonic waves. Experiments were performed on the mica A4 dimensions sample with the thickness 35 \(\upmu\)m. The sample together with the contact PMN-32%PT strip-like transducer was scanned with respect to the laser beam with a step of 0.1 mm. Interferometer averaging was set three times. The B-scans were collected in different areas of the sample in order to randomize the influence of the optically darker zones which possess very different acoustic properties. The length of the B-scans was 85 mm. The duration of ultrasonic pulse used for excitation of A\({}_{0}\) mode was six periods, and the central frequency was 44 kHz. The example of the recorded raw B-scan in the case of the contact excitation is presented in Figure 21.  
The 2D spectra of the B-scans obtained in the case of the contact and the air-coupled excitation are presented in Figure 22a,b. Figure 21: The B-scan obtained in the case of the contact excitation (\(y=0\) mm). Figure 20: Excitation of the A\({}_{0}\) mode via air gap. In both cases presence of a few modes with different phase velocities is seen. The largest amplitudes are at a frequency of 46 kHz and distributed along spatial frequencies \(\lambda^{-1}\) in the range of [0.45-0.5] mm\({}^{-1}\). In the case of a contact excitation the \(A_{0}^{\prime}\) and \(A_{0}^{\prime\prime}\) modes are due to the zones with different elastic properties and thickness. The obtained experimental results are very close to the results of the numerical modelling (Figure 18). In the case of the air-coupled excitation in addition to those modes the wave propagating in air originates from the tip of the air-coupled transducer.