## 4 Noise Levels in the Ocean Generated by Hom and Non-Horn Marinised Transducers  
The objective of this section is to evaluate the noise level produced by the steel and aluminum marinised transducers with and without a horn. As the cavitation produced by the transducers is local to the transducer position [46, 47], the ladder should be instrumented with a number of HPUTs to cover the whole ladder for biofouling prevention. Hence, the noise produced by a different number of sensors is assessed in the measurements described in this section. The test rig consisted of a ladder attached to a number of transducers. The transducer location and the grid of measurement points are illustrated in Figure 11. Transducer 1 has an aluminum horn, and Transducer 2 has a steel horn; both are rear-mounted to the front face of the ladder. Transducers 4-6 have no horns and are front-mounted to the ladder. Transducer 4 is the only transducer that is attached to the rear face of the ladder. As shown in Figure 11c, the ladders were suspended into the dock from a telehandler on the dock side. The experimental rig consists of an 80 \(\times\) 20 \(\times\) 3 m deck. During the measurements, all HPUTs were fully submerged in the water, and the ladder was positioned approximately 1 m away from the quay walls. The measurement grid consists of five rows spaced 14 m apart and four columns spaced 4 m apart, shown in Figure 12a. A hydrophone survey was undertaken in the following stages; stage 1--running just transducer 1, stage 2--running transducer 2, stage 3--running transducer 3, stage 4--running transducer 4, stage 5--running transducers 3 and 4, stage 6--running transducers 5 and 6, stage 7--running transducers 3, 4, 5, and 6. The produced noise from each stage is recorded through a Porpoise hydrophone. The standard porpoise hydrophone sensitivity is \(-\)204 dB. A Porpoise automatically adds approximately 10.62 dB gain (Gain 0 = 10.62 dB). This would make the sensitivity \(-\)193.38 dB without adding any gain (Gain 1, Gain 2, Gain 3). The highest gain setting is approximately Gain 3 = 45.74 dB. Therefore, the sensitivity would be approximately \(-\)158.26 dB using Gain 3. Figure 11: Preparing the test rig, (**a**) attached transducer to the ladder with (**b**) their corresponding number and (**c**) suspending the ladder into a pool—courtesy of Offshore Renewable Energy, Catapult. To estimate the impact of the produced noise on marine life, the vibration outputs from the HPUs' arrays are analyzed based on power spectrum density and the sound equivalent value. The PSD can be estimated by  
\[\mathrm{PSD}(f)=\lim_{T\to\infty}\frac{1}{T}E\left[\left|\int\limits_{0}^{T}x(t )e^{j2\pi ft}dt\right|^{2}\right], \tag{1}\]  
where \(x(t)\) is the time histories of the signals and \(E[]\) is the mathematical expectation in the interval of \([0,T]\). Sound exposure level (SEL) is the integration of squared sound pressure over a period of time, \(T\), and is estimated by  
\[\mathrm{SEL}(f)=\mathrm{PSD}+10\log_{10}^{t} \tag{2}\]  
Table 1 gives the minima and maxima associated with the noise produced by the transducers for each stage. It includes the sound exposure level (SEL) and sound pressure level (SPL) values associated with each step. As shown in Table 1, the noise was reduced by approximately 5 (dB) in SEL and SPL for the front mount without a horn compared with the rear mount with a horn. The aluminum horn transducer was not vibrating at its resonance frequency; hence, a lower noise level was expected. There is a minor difference between the noise level at stage 7 compared with stage 6. Therefore, noise cancellation works. The measurement field is highly reverberant, and the presented maximum value contains reflections of signal from the rear walls. The background noise, called noise level, and the noise produced by the transducers at each stage are presented in Figures 13 and 14. The peaks in the noise level plot are associated with the sudden operating external sound source close to the pool.