## 4 Discussion  
The objective of this work was to test the possibility of using FE films to produce water immersion transducers for pulse-echo operation. Two different FE materials were employed: HS03 and HS06. The former has a lower resonant frequency, a more flattened pore structure (that makes the pores and the film more deformable in the thickness direction), and a lower acoustic impedance. Results with these two materials reveal that transducers produced with the HS03 film have a sensitivity about 15 dB better and a wider frequency bandwidth (Figure 6). Considering that the impedance of the HS06 film is better coupled to the water, this better response of the HS03 film could be related with either a better piezoelectric response of HS03 or a more efficient response of the cellular structure of HS03 when operated with a water load. Water is a huge mechanical load for the FE film; hence, thickness resonances are expected to be completely damped out. As a matter of fact, no trace of the film thickness resonance is found in the pulse-echo response in water, where the film could be expected to operate in the lambda/2 thickness mode (Figure 6). This huge load imposed by the water is expected to produce a very large frequency bandwidth and a very low sensitivity. Pulse duration (under spike excitation) is about 2-3 us long (Figures 9 and 10), while the 6 dB frequency band expands from 0.29 up to 2.7 MHz (161%), and the 12 dB band expands up to 4.5 MHz (Figures 6 and 7). The main counterpart is the reduced sensitivity. The observed peak sensitivity values for the different prototypes are in the range of \(-\)114 to \(-\)118 dB (Figures 6-8). Nonetheless, even with this low sensitivity it was possible to observe the front and back echoes of a 53 mm thick steel block (see Figures 9 and 10). Different techniques and materials were tried to provide protection for the radiating surface. The main idea is to provide a uniform and thin coating with acoustic properties similar to the water, so that the effect of this coating on the acoustic performance could be negligible. Measured response in pulse echo mode in water immersion revealed a somewhat better sensitivity (about 5 dB) for transducers with varnish and lacquer compared with PMMA and PTFE coded transducers. This can be due to the fact that the acoustic impedance of PMMA and PTFE coatings is expected to be higher that of the water. However, the SNR of coated transducers seems to be poorer in coated transducers (Figures 7 and 8). This has to be confirmed in further studies. Finally, we have shown that it is possible to produce spherically focused transducers, by using a backing block with a spherical surface and gluing the FE film to it. Adapting the FE film to the curved surface produces some wrinkles in the film, however the effect on the ultrasonic response is reduced (Figures 4 and 7) and no artifacts can be observed in the acoustic fields (Figures 11-13). \(SNS\) and the signal in the time domain (Figure 9) are very similar for flat and focused transducers. The measured acoustic field of the focused transducers revealed a field distribution very close to the expected one. For the transducer with a radius of curvature of 22 mm, the measured focal distance was 19 mm and the focal spot size was 1.8 mm. In this case the calculated focal spot size was 1.67 mm, very close to the measured value. For the transducer with a radius of curvature of 35 mm, the measured focal distance was 32 mm and the focal spot size was 3.7 mm. In this case the calculated focal spot size was 2.88 mm. response must be due to a better piezoelectric response of the HS03 film. As expected, and due to the large load of the water, the thickness resonance of the film is completely damped out, giving rise to a poor sensitivity (around \(-115\) dB) and a very wide band response with a short signal duration.