## 2 Materials and Methods
### Calibration Recommendations
In accordance with Article 5 of that decision, 'threshold value' means a value or range of values, which makes it possible to assess the level of quality achieved for a given criterion, thereby contributing to the assessment of the extent to which good environmental status is achieved [20]. It was mentioned in [21] that, among others, the basis for determining threshold values is the joint monitoring of ambient noise in the Baltic Sea (through the completed some time ago BIAS (the Baltic Sea Information on the Acoustic Soundscape) project [22]). The BIAS standards for noise measurements [23] contain instructions for all phases of the field work, spanning from the preparatory work until the final retrieval of the systems: essential function tests and calibration of the instrument chain. The TG Noise in the methodology report [24] defined the Level of Onset of biologically Significant Effects (LOSEs), which represents a sound level above which effects on indicator species can be expected to affect their well-being, survival, and reproduction. Together with below Ceiling of Reference Condition (CRC), LOSE defines three states of the current condition. CRC: the sound level is within the range of levels animals can be expected to encounter under natural (reference) conditions (zone of natural variation). Below LOSEs' sound levels may impact animals, but not to a degree where their long-term survival and reproduction is affected (zone of non-significant effects). This, however, is the case for levels above LOSEs (zone of significant adverse effects). In EU-financed projects BIAS, JOMOPANS (the Joint Monitoring Programme for Ambient Noise in the North Sea), JONAS (the Joint Framework for Ocean Noise in the Atlantic Seas), RAGES (the Risk Based Approach for Good Environmental Status), QUIETMED (a joint program on underwater noise (D11) for the implementation of the MSFD in the Mediterranean Sea) and QUIETMED2 (a joint program on underwater noise (D11) for the implementation of the Second Cycle of the MSFD in the Mediterranean Sea) it is recognized that temporal scales, spatial scales, selection of marine species, and statistical properties of the sound must be part of the framework [25]. The joint program on noise (D11) for the implementation of the second cycle of the MSFD in the Mediterranean Sea--QUIETMED--is presented in the report: Deliverable D3.1. Best practices guidelines on sensor calibration for underwater noise monitoring in the Mediterranean Sea: \"Establish guidelines on how to perform sensor calibration and mooring to avoid or reduce any possible mistakes for monitoring ambient noise (D11C2). These common recommendations should allow traceability in case the sensor give unexpected results and help to obtain high quality and comparable data\" [26]. HLECOM--Baltic Marine Environment Protection Commission--belongs to the most important organizations taking a stand on Baltic Sea marine environment. Within the frame of this organization, the HELCOM Expert Group of Underwater Noise (EG Noise) has been created. An expert group has been raised through the need to,  
* Facilitate the implementation of the Regional Action Plan on Underwater Noise;
* Develop regional core indicators on underwater noise;
* Carry out regional assessments on occurrence and impacts of underwater noise in the Baltic Sea. In the document _Terms of Reference for the HELCOM Expert Group on Underwater Noise_ (EG Noise) 2022-2024 [27] it is described that the HELCOM EG Noise will support lead countries in the further development of HELCOM indicators on underwater noise, taking account, as appropriate, of developments on topics under the EU MSFD for the EU member states connected to \"continue working on the analysis of sensors, devices and methods used to measure underwater noise to ensure the reliability and repeatability of the collected data\".