## 3 Measurements
The input resistance of the matched channel was calculated by  
Figure 13.0  
where \(U_{in1\text{st,\;mch}}\) is the root-mean-square value of the fundamental-frequency component of the matched channel's input voltage, \(I_{in1\text{st,\;mch}}\) is the root-mean-square value of the fundamental-frequency component of the matched channel's input current, and \(\theta_{in1\text{st,\;mch}}\) is the phase angle difference between \(I_{in1\text{st,\;mch}}\) and \(U_{in1\text{st,\;mch}}\). The input reactance of the matched channel was calculated by  
Figure 13.1  
Similarly, the AC-DC converter's input resistance \(R_{in,\;AC-DC}\) and input reactance \(X_{in,\;AC-DC}\) was calculated by  
Figure 13.2  
and  
Figure 13.3  
respectively. The values of \(R_{in,\;mch}\), \(X_{in,\;mch}\), \(R_{in,\;AC-DC}\) and \(X_{in,\;AC-DC}\) at the system's most efficient point (944.4 kHz) can be read from Figure 14c,d. Then the values of the input impedances of the match channel and the AC-DC converter at the system's most efficient point can be calculated as  
Figure 13.4  
and  
Figure 13.5  
respectively. From these values, it can be concluded that both the channel and the AC-DC converter were matched relatively well. As mentioned in Section 1, [PERSON] et al., did not apply any AC-DC conversion circuit in their UTMWPT systems [15; 16]. Systems presented by [PERSON] et al. could at best output only about 1.0 W of AC power [21]. [PERSON] et al. greatly simplified the UTMWPT system model in simulation and did not present experimental results [14]. Only [PERSON] et al. presented a medium-power DC output UTMWPT system, as well as experimental results [17; 18]. Table 3 shows a comparison between our system and the system presented by [PERSON] et al. The maximum power transfer efficiency of the acoustic-electric channel in [PERSON]'s system is 51%, which is 15.3% higher than that of our system. We believe that this is due primarily to two factors. First, the performance of the commercial off-the-shelf transducers we used is lower than that of the custom-designed transducers used by [PERSON]. Second, as mentionedin Section 2.2, the resonant frequencies of the transducers are not best matched with the thickness of the steel plate in our system [17]. These two problems can both be solved by custom designing new low-loss transducers. However, in spite of the disadvantages, the maximum DC output efficiency of our system (27.7%) is still 8.7% higher than that of [PERSON]'s system (19%), while the maximum DC output power of our system (15.7 W) is commensurate with that of [PERSON]'s system (19 W). Furthermore, the output voltage of [PERSON]'s system is not regulated, which greatly limits its usefulness. Therefore, the UTMWPT system which we present in this paper is superior to previous ones. More importantly, the effectiveness of this system has been successfully demonstrated using three ARM(r) Cortex(tm)-A8 evaluation boards with LCD panels, as shown in Figure 13.6 The ARM boards are used as the load circuit in Figure 1, and the power input ports of them are connected in parallel. The peak total power consumption of the ARM boards when loading operating systems is about 17 W. After booting up, the dynamic power consumption of the boards fluctuates between 13 W and 17 W. The demonstration experiment lasted for an hour, the circuits and the transducers did not overheat and the ARM boards worked continuously and stably during the experiment. Therefore, this system can certainly be applied to almost all medium-power electronic/electric appliances.