## 2 Electrical Equipment and Assembly to Power the Transducer
### Marinised Power Ultrasonic Transducers  
To perform biofouling prevention with ultrasonic, it is required to generate ultrasound cavitations using HPUTs that are marinised for underwater operation. This is necessary to prevent water from entering the internal of the transducer and prevent water from contacting the electrical transducer terminals. As the system is intended to be used underwater for extended periods, the marinisation process should last for a long time and be able to operate at depths of up to 10 m. Figure 2 shows the marinised and non-marinised transducers used in these experiments. The transducers are the 28 kHz 100 W piezoelectric ultrasonic transducers (PZT-4) provided by Beijing Ultrasonic Ltd. in Beijing, China [45], which are designed for ultrasonic cleaning applications and have a frequency of 28 kHz and a power output of 100 W (without marinisation). The \"PZT-4\" designation refers to the piezoelectric material used in the transducer, lead zirconate titanate (PZT). The transducer's parts, frontal and back mass and piezoceramics are attached with a prestressed bolt. The frontal radiating surface has a diameter of 68 mm and involves a built-in 10 mm thread for horn attachment. However, marinisation has one main disadvantage: the mechanical performance of the transducer is degraded. There are two parameters that show this performance loss. The electrical impedance of the transducer is affected by the marinisation, as shown in Figures 3 and 4. As shown in Figures 3 and 4, at the resonance point of the transducer, electric impedance increases for the marinised transducers from 37.8 to 129 ohms. The effect of marinisation on the transducer is increased damping and a reduction in the effective Q value at resonance. The implication is that the transducers need to be driven with higher voltage to achieve the rated power and exhibit more heat losses. Tests were performed to measure the mechanical performance of the marinised transducers. In addition, they were compared with a non-marinised version using a laser vibrometer to measure displacements on a plate driven by the transducer. The experimental rig and configuration are illustrated in Figure 5.  
Three plain mount configurations were used magnetic holder for straight, straight with resonant horn, and straight screw mount. The Polytec 3D-CLV Vibrometer can measure two types of displacements: average (FFT method) and Peak to Peak (Time domain method). The displacements were taken at 20, 30, 40, and 50 cm distances from the transducer mounting position. The displacement values in picometers obtained for different mount configurations can be seen in Figure 6.  
This indicates that the screw mount configuration, on average, is best for maximizing mechanical power from the transducer to the plate, while the horn mount shows more power being delivered at a short distance. Figure 3: Non-marinised transducer, main resonance frequency is 28.1 kHz and 33.4 Ohms. Figure 2: Transducers used in tests: (**a**) plain and (**b**) marinised transducer. The graph in Figure 7 shows the maximum averaged peak-to-peak displacements values obtained using a marinised and non-marinised version of the same transducer in different configurations. The measurement was done in by 16 times averaging to reduce the inclusion of the error. Figure 4: Marinised transducer, main resonance frequency is 27.5 and 119.8 Ohms. Figure 5: Experimental set-up plate and transducer with horn and magnetic mounting system. These results show a loss in mechanical energy of the marinised transducers. To counter the effect of the mechanical losses, the electrical power to the transducer should be raised. As shown in Figure 4, the marinised transducer presents higher impedance at resonance. This means that the power used by the transducer is lower when driven with the same voltages used for the non-marinised transducer. The power can be raised by increasing the signal voltage accordingly for marinised transducers.