## 3 Results and Discussion  
The importance of studying underwater noise at frequencies 10-1000 Hz and, consequently, on the calibration of underwater sensors and recorders (preferably together in a set) results from the frequency range of sources of anthropogenic noise such as underwater mines and torpedoes explosion, air-gun arrays (seismic research), low frequency sonars, pile-driving hammers, ship traffic, operating windmill turbines, etc. [34]. In this respect, the impact of underwater noise on living organisms in the sea is also significant [7; 9; 10; 27]. Relating the obtained results of underwater noise monitoring to those obtained by other centers requires periodic calibration of sensors and measuring equipment as presented in the previous chapter. The number of calibrations of hydrophones and underwater recorders that can be carried out in well-known centers such as the National Physical Laboratory (NPL) is limited and very time-consuming (it can even take several months if we take into account transport back and forth and customs activities at the border between the European customs area and other countries). This leads to the conclusion that the number of places (institutions, companies, etc.) where regular calibration of measuring equipment can be carried out is limited. Regular calibration is particularly important when we compare the obtained data or place them in a common database, which is the basis for spatial and temporal modeling of underwater noise. In the authors' opinion, the The European Association of National Metrology Institutes (Euramet) 15 RPT02 UNAC-LOW project entitled underwater acoustic calibration standards for frequencies below 1 kHz is the most significant recent project regarding the calibration of underwater noise sensors and recorders [35; 36]. According to project's objectives, \"This project developed new and extended existing calibration capabilities for hydrophones and autonomous underwater acoustic noise recording systems at acoustic frequencies below 1 kHz. After successful development of the calibration capabilities, all related partners participated in round robin calibrations of commercially available hydrophones and underwater autonomous noise recorders to validate their calibration infrastructure. Outputs from the project have been used by partners to establish calibration services for hydrophones and autonomous noise recorders. Calibration has been performed for manufacturers, defense contractors, regulators, government institutes, and end users\". The results which authors believed to be the most important for the European comparisons of the sensors calibrations are presented in Table 2, and [37; 38] mentioned other (outside Europe too) calibration infrastructures in the NMI's. Digital hydroacoustic sensors are also emerging and with them unprecedented problems related to their calibration [39]. Accordingly, at the end of 2017, the Central Office of Measures (Polish abbreviation GUM) decided to start work on the creation of metrological infrastructure in the area of underwater acoustics in GUM. In Poland, GUM serves as the National Metrology Institute (NMI). In order to carry out this task, the Working Group for Underwater Acoustics was established at the GUM and an expert in this field was employed. A detailed description of the beginnings of the activity is described in [40]. As part of one of the first tasks, work began on a design of an acoustic coupler for calibrate hydrophones at low frequencies. The design assumptions were described and published in [37]. Figure 5 shows the acoustic coupler implemented at GUM for calibrating hydrophones at low frequencies: from 10 to 2000 Hz. Implementation of this acoustic coupler has taken place in the District Office of Measures in Gdansk, branch in Gdynia. Work is currently underway to launch the coupler, determine the measurement uncertainty budget and its components, and prepare calibration instructions in accordance with strict procedures used at GMU.