## 2 Materials and Methods  
### The Importance of Reliability and Repeatability of Underwater Noise Research  
The literature analysis provides information that the changes in the sensitivity of hydrophones (dynamic pressure sensors) are mainly caused by the following:  
* Adulteration (voltage sensitivity drift). The German Technical Center for Ships, Naval Weapons, Technology, and Maritime Research (WTD71) describes the research carried out on the change in voltage sensitivity with the age of the tested hydroacoustic sensor in [12]. The article presents the calibration characteristics (dependence of voltage sensitivity on frequency) of the same hydrophone, which was carried out four times in the years (2002, 2004, 2006, and 2012). The characteristics show that there are frequencies for which the voltage sensitivity is unchanged and those for which the sensitivity differs in extreme cases by 1.5 dB re 1 V/uPa. The author's comment is as follows: \"It shows variations less than 1.5 dB which is within the uncertainty range of the underwater measurement. Therefore, this hydrophone did not change the frequency dependent sensitivity. In case of a wide spread or a tonal deviation the hydrophone would not be used further in the measurement chain. New hydrophones will be calibrated at this facility as well and compared with the manufacturer calibration\". The described facility is accredited facility in the Ploner lake. Similar research [13] was carried out at the Polish Naval Academy as part of the EDA SIRAMIS project. Drift voltage sensitivity hydrophone Reson TC-4032 between 2010 and 2013 reached 2.2 dB re 1 V/uPa (Figures 3 and 4). It should be emphasized that both tests were conducted in the frequency ranges from 4 kHz (WTD-71), 5 kHz (PNA), to over 100 KHz. However, similar tests are not carried  
Figure 2: Challenges in the research of marine ecology system and ways to solve the formulated problem. out for lower frequencies. It seems that this type of research containing the trend of changes in sensitivity with the aging of measurement sensors should be included in general practice. * Immersion of the sensors during measurements (different measurement depths should result in sensitivity correction in the measurement results). Document [14] presents research on changes in the voltage sensitivity of hydrophones depending on changes in the pressure in which they are placed. The hydrophone was subjected to pressure changes from 0.5 MPa to 3.5 MPa in the frequency range from approximately 50 Hz to 1600 Hz. Pressure changes can be interpreted as changes in the hydrophone's immersion in the sea, which corresponded to changes from approx. 50 m to approx. 350 m. With such a pressure change, for a frequency of 800 Hz, the largest change in voltage sensitivity was obtained, which was approx. 1.4 dB re 1 V/\(\upmu\)Pa, with smaller pressure differences; these changes were correspondingly smaller. The question arises whether the sensors should be calibrated taking into account the target operating pressure. * The underwater structure on which the sensor is mounted (directly or at a distance). Voltage sensitivity tests depending on the frequency and directional characteristics were carried out in a large open water reservoir. Two configurations of underwater recorders were tested. In one, the hydrophone was mounted directly to the recorder structure; in the other, it was located 2.5 m away on a cable. The tests were carried out in the frequency range from 1 kHz to 100 kHz, and even for the lowest frequency of 1 kHz, differences of several decibels in the voltage sensitivity of hydrophones mounted directly on the cable [15]. * Frequency range used (special attention should be paid to calibration in the frequency range used). Particular attention should be paid to the frequency range of calibration. Measuring equipment should only be used to the extent to which it has been calibrated. This also applies to the software in which the signal is processed.