Study on the Ensuring of Reliability and Repeatability of Research in the Area of Marine Ecology through Calibration of Underwater Acoustics Devices

Abstract

This paper emphasizes the crucial needs and reasons to ensure the reliability and repeatability of underwater acoustic measurements. As an exemplification of the aforementioned acoustic measurements challenges, the development of metrological infrastructure in the area of underwater acoustics in GUM (Central Office of Measures, Poland) is presented. An analysis of presented solutions was carried out mainly in the low frequency range, in comparison with other solutions recently developed worldwide. Moreover, factors influencing the sensitivity of hydroacoustic measuring devices are discussed. The summary of this discussion outlines the further works aimed at ensuring the reliability and repeatability of underwater acoustic measurements. The conclusions present the current state of the calibration infrastructure of underwater acoustic devices, with particular emphasis on marine ecology and fisheries sciences.

1. Introduction

In recent years, the intensification of human activity in marine areas has been related to, among others, broadly understood economic development and maritime research, such as maritime transport [1], gas and oil platforms, wind farms [2], underwater cables, port expansion [3], and fishing [4]. Underwater acoustic monitoring of the marine environment with the extensive use of underwater sensors and acoustic devices to protect marine fauna and flora has become a fundamental necessity resulting from the requirements of Directive 2008/56/EC of the European Parliament and of the Council, known as the Marine Strategy Framework Directive [5,6]. The causes and effects relations in marine ecology area including the place and role of calibration of underwater acoustics sensors and measuring devices are presented in Figure 1. The content in the diagram reflects the main issues and does not exhaust all detailed relationships. Anthropogenic underwater noise and natural underwater noise, which are the main sources of underwater noise, affect not only living organisms in the sea but also each other [7]. The registered threats for marine ecology system impaired echolocation caused by high levels of underwater noise from, for example, an underwater volcanic eruption or high-energy sonar used by active warships, or more widely with increased military operations. As a detailed example, underwater mine explosions, in addition to noise, may cause additional waves, and increased waves and wind strength may increase the demand for propulsion power on ships and thus increase the generation of noise from ships’ power plants into the water environment. The main effects provoked by underwater noise and causing degradation of the marine ecosystem are presented in the second column. Currently, this impact on all living species is not sufficiently studied and requires further research [8,9]. The third column presents the main areas for improving the situation in marine ecology. Not only does the European Union have a Marine Strategy in the area of marine ecology, but similar initiatives are implemented all over the world [7,10], also as part of the activities of the United Nations under the United Nations Framework Convention on Climate Change (UNFCCC) [11]. The flow of present challenges in the research of marine ecology system and ways to solve the formulated problem is illustrated in Figure 2. The aim of this article is to systematize the knowledge on the influence of factors changing the sensitivity of measurement sensors on the need to provide calibrated sensors and devices in measurements in the area of underwater acoustics for the protection of fauna and flora in the marine environment and the participation of Polish scientific institutions, including GUM (Polish NMI), in the process of maintaining appropriate measurement standards ensuring the repeatability and reliability of underwater noise measurements. A problem to solve was appointed as a necessity to protect marine fauna and flora in the face of existing operating conditions and threats characterizing today marine ecology systems. A main thesis is that the ensuring (leading to improvement) of reliability and repeatability of research in the area of marine ecology through traceable calibration of hydrophones and measuring instrumentation, which enables the identification of the critical parameters in the analyzed marine system, and, in consequence, makes it possible to undertake the counter-measures to effective protection of the selected objects of fauna and flora in this system. Individual elements of actions aimed at improving the state of the natural environment, presented in Figure 2 as blocks, will be successively discussed later in this article. For Figure 1 and Figure 2, the solid single direction arrows mean the relationship of the block near the arrow relative to the block at the opposite end; the solid double direction arrows mean that both blocks are dependent on each other. The dashed line means that a path where there is no proper sensor calibration process. The paper is organized as follows: In the introduction and the second chapter we provide a short description of the crucial needs and reasons to ensure the reliability and repeatability of underwater acoustic measurements, taking into account legal aspects and threats for marine fauna and flora. The next chapter presents Materials and Methods including the importance of reliability and repeatability of underwater noise research and the calibration recommendations for underwater acoustic sensors and measuring devices. In the third chapter, the results and discussion with the low frequency underwater acoustic devices that are fundamental for this article, shown in the wake of the factors influencing the sensitivity of hydroacoustic measuring devices and their influence on the reliability and repeatability of underwater acoustic measurements are presented. This chapter concerns the low frequency calibration systems worldwide with a case study from Poland.

Finally, the last chapter concludes the current state of the calibration infrastructure of the underwater acoustic devices, with particular emphasis on marine ecology and fisheries sciences, outlining the counter-measures to effective protection of selected objects of fauna and flora in the considered marine system.

2. Materials and Methods

2.1. 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/µPa. 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 Plöner 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/µPa (Figure 3 and Figure 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 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/µPa, 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. Various signal processing procedures may introduce additional measurement uncertainties into the measurement system.
• Waves/tides for very low frequency ranges. As shown above, fluctuations in water level changes (caused e.g., by tide) in the case of installing sea noise recorders on bottom structures may cause changes in sensitivity. For example, in the English Channel, near Saint Malo, tide differences can reach several meters
• Other (e.g., method of calibration, attenuation of the electronic channel).

The authors want to indicate that unknown sensitivity (this has place when hydroacoustic sensors have no regular calibration) provide unknown uncertainty values of measurement results. This requires special attention during evaluation of the levels of underwater noise measurements. A logical consequence of analysis of the changes in voltages’ sensitivity of hydroacoustic sensors are presented, among others, in this article. The measurement results should be treated with caution if directly before or after the measurements (in the case of short-term measurements) or directly before and after (in the case of long-term measurements) sensors used during the measurements, or they were not calibrated with an appropriate method in the frequency range under consideration.

2.2. Calibration Recommendations

The Marine Strategy Framework Directive (MSFD) requires European Member States to develop strategies for their marine waters, which should lead to research programs aimed at achieving or maintaining a Good Environmental Status (GES) in European seas.

The European Commission Decision (EU) 2017/848 of 17 May 2017 clarified descriptor 11 regarding the introduction of energy, including underwater noise, at levels that do not result in an adverse impact on the marine environment [16]:

• (D11C1) Primary: The spatial distribution, temporal extent, and levels of anthropogenic impulsive sound sources do not exceed levels that adversely affect populations of marine animals. Member states shall establish threshold values for these levels through cooperation at union level, taking into account regional or subregional specificities.
• (D11C2) Primary: The spatial distribution, temporal extent, and levels of anthropogenic continuous low-frequency sound do not exceed levels that adversely affect populations of marine animals. Member states shall establish threshold values for these levels through cooperation at Union level, taking into account regional or subregional specificities.

As a first step towards achieving good environmental status, member states have established programs to monitor the state of the marine environment, which is regularly assessed. Significant progress has been made in establishing joint monitoring programs in recent years. Continuous and impulse monitoring of underwater noise has been initiated (through joint programs) for the Mediterranean, Baltic Sea and north-east Atlantic regions. Important institutions and organizations responsible for environmental protection establish expert groups. Members usually work in various executive institutions and governmental and non-governmental organizations, including those related to the calibration of sensors and measurement equipment in the area of underwater noise. The Technical Group on Underwater Noise (TG Noise) has been established at the European Commission, Directorate General for Environment, Directorate C—Zero Pollution, Unit C2 Marine Environment, and Clean Water Services. The Progress Report on Underwater Noise Monitoring [17] established that a standard for underwater noise monitoring methods should be developed that includes detailed monitoring methods: measurements, modeling, and combining model predictions with measurements: “Standardize modelling and measurement of underwater sound for purpose of noise monitoring”. The measurements have two purposes: to provide appropriate acoustic measurements for validation or an accurate model, and supporting data solutions, such as source level and environmental characteristics. The standard should also include equipment calibration methods. The report also draws attention to the harmonization of IEC and ISO standards and refers to the Australian experience, Integrated Marine Observing System (IMOS), and the U.S., National Oceanic and Atmospheric Administration (NOAA). The Monitoring Guidance for Underwater Noise in European Seas, Part II: Monitoring Guidance Specifications [18] report contains a Technical Specifications of Measuring Equipment chapter and an Equipment Calibration subchapter, which contains the basic requirements for measuring devices and their calibration in accordance with existing standards Table 1, line 4, and the standard IEC 60565:2006 [19], which is an earlier version of the standards placed in lines 1 and 2. TG Noise also participates in the development of the so-called the threshold values. The concept of threshold values was introduced by Commission Decision (EU) 2017/848 of 17 May 2017 establishing criteria and methodological standards for good environmental status in marine waters, as well as specifications and harmonized methods for monitoring and assessment. Pursuant to Art. 2 section. 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”. A project carried out in recent years in the field of underwater acoustics sensor and underwater registers calibrations is the EURAMET EMPIR project UNAC-LOW “Underwater Acoustic Calibration Standards for Frequencies Below 1 kHz” [28]. The range of calibration devices and result of their comparisons in this project will be presented in a later chapter. An important part of the project was the update of the IEC 60565:2006 standard, which resulted in the creation of two standards: separately for the calibration of sensors in the free field (Table 1, line 1), and separately for the calibration of sensors at low frequencies (Table 1, line 2).

Table 1. A list of standards of hydrophone calibration.

No	Standard	Short Description
1	IEC 60565-1:2020 [29]	Underwater acoustics—hydrophones—calibration of hydrophones—Part 1: Pro-cedures for free-field calibration of hydrophones (The maximum frequency range of the methods specified in this document is from 200 Hz to 1 MHz. In this standard are excluded the calibration of digital hydrophones and systems, the calibration of marine autonomous acoustic recorders, the calibration of acoustic vector sensors such as particle velocity sensors and pressure gradient hydrophones, the calibration of passive multi-hydrophone sonar arrays, and the calibration of active sonar arrays consisting of projectors and hydrophones).
2	IEC 60565-2:2019 [30]	Underwater acoustics—hydrophones—calibration of hydrophones—Part 2: Pro-cedures for low frequency pressure calibration (frequencies from 0.01 Hz to sever-al kilohertz depending on calibration method). In the standard added: (1) A relative calibration method has been added to Clause 8: Calibration by piezoelectric compensation. (2) A relative calibration method has been added to Clause 11: Calibration by vibrating column. (3) Clause 12: Calibration by static pressure transducer, has been added. (4) Annex A: Equivalent circuit of the excitation system for calibration with a vibrating column, has been deleted. (5) Subclauses 9.6, 9.7 and 9.8 have been moved to form a new Annex A: Advanced acoustic coupler calibration methods.)
3	IEC 60500:2017 [31]	Underwater acoustics—hydrophones—properties of hydrophones in the frequen-cy range 1 Hz to 500 kHz (New edition includes the following significant technical changes with respect to the previous edition: - the format and scope of IEC 60500 have been changed to be compatible with other IEC standards; - the upper limit of the frequency range of hydrophones has been expanded).
4	ANSI/ASA S1.20-2012 (R2020) [32]	Procedures for calibration of underwater electroacoustic transducers (both primary and secondary calibration procedures are specified for frequencies from a few hertz to a few megahertz. Procedures are specified for determining the measurable characteristics of free-field receive voltage sensitivity, transmitting response, directional response, voltage coupling loss, impedance, and equivalent noise pressure. Measurement uncertainty analysis is introduced for these measurement types, with identification of common error sources).

Table 1. A list of standards of hydrophone calibration.

No	Standard	Short Description
1	IEC 60565-1:2020 [29]	Underwater acoustics—hydrophones—calibration of hydrophones—Part 1: Pro-cedures for free-field calibration of hydrophones (The maximum frequency range of the methods specified in this document is from 200 Hz to 1 MHz. In this standard are excluded the calibration of digital hydrophones and systems, the calibration of marine autonomous acoustic recorders, the calibration of acoustic vector sensors such as particle velocity sensors and pressure gradient hydrophones, the calibration of passive multi-hydrophone sonar arrays, and the calibration of active sonar arrays consisting of projectors and hydrophones).
2	IEC 60565-2:2019 [30]	Underwater acoustics—hydrophones—calibration of hydrophones—Part 2: Pro-cedures for low frequency pressure calibration (frequencies from 0.01 Hz to sever-al kilohertz depending on calibration method). In the standard added: (1) A relative calibration method has been added to Clause 8: Calibration by piezoelectric compensation. (2) A relative calibration method has been added to Clause 11: Calibration by vibrating column. (3) Clause 12: Calibration by static pressure transducer, has been added. (4) Annex A: Equivalent circuit of the excitation system for calibration with a vibrating column, has been deleted. (5) Subclauses 9.6, 9.7 and 9.8 have been moved to form a new Annex A: Advanced acoustic coupler calibration methods.)
3	IEC 60500:2017 [31]	Underwater acoustics—hydrophones—properties of hydrophones in the frequen-cy range 1 Hz to 500 kHz (New edition includes the following significant technical changes with respect to the previous edition: - the format and scope of IEC 60500 have been changed to be compatible with other IEC standards; - the upper limit of the frequency range of hydrophones has been expanded).
4	ANSI/ASA S1.20-2012 (R2020) [32]	Procedures for calibration of underwater electroacoustic transducers (both primary and secondary calibration procedures are specified for frequencies from a few hertz to a few megahertz. Procedures are specified for determining the measurable characteristics of free-field receive voltage sensitivity, transmitting response, directional response, voltage coupling loss, impedance, and equivalent noise pressure. Measurement uncertainty analysis is introduced for these measurement types, with identification of common error sources).

The main methods for validating primary calibrations are calibrations by inter-laboratory comparisons and by independent calibration methods [33], which are described in detail in the standards listed in Table 1. A list of standards of hydrophone calibration.

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) 15RPT02 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. A list of calibration facilities for frequencies below 1 kHz.

No	Institution/Source of Guideline	Calibration Uncertainty [dB]	Range of Frequency [Hz]	Method of Calibration	Project/References
1	TÜBİTAK Scientific and Technological Research Council of Türkiye	B&K 8104: 0.2–0.7 B&K 8106: 0.2–0.5 Acoustics SM4M recorder 1.0	20–2000 20–1000	Pressure chamber Pressure chamber	EURAMET 15RPT02 UNAC-LOW [27]
2	NPL	B&K 8104: 0.5 B&K 8106: 0.5 1.0-0.5 Wildlife Acoustics SM4M recorder 0.5	5–400 5–400 250–2000 5–315	Pressure chamber Pressure chamber Free-field Pressure chamber	EURAMET 15RPT02 UNAC-LOW [27]
3	DFM Danish National Metrology Institute FOI Swedish Defence Research Agency	B&K 8104: 0.2–0.5 B&K 8106: 0.3–1.0	20–2000 5–1600	Pressure chamber	EURAMET 15RPT02 UNAC-LOW [27]
4	FOI Swedish Defence Research Agency	B&K 8104: 0.8 B&K 8106: 0.8 Wildlife Acoustics SM4M recorder 1.0–1.5	400–1000 400–1000 85.3–1200	Standing wave tube Reciprocity calibration, secondary standard	EURAMET 15RPT02 UNAC-LOW [27]
5	CNR/ISPRA National Research Council of Italy/Italian National Institute for Environmental Protection and Research	Wildlife Acoustics SM4M recorder 0.7–1.2	200–2000	Reciprocity calibration, primary standard	EURAMET 15RPT02 UNAC-LOW [27]

, 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.

In 2021 and 2023, the Minister of Education and Science established the Polish Metrology program. The basic principles of the program are as follows:

• Support for the conduct of scientific research or development work in areas related to metrology by entities of the higher education and science system, in cooperation with the President of the Central Office of Measures, hereinafter referred to as “GUM”.
• Supporting the implementation of projects aimed at increasing the level of research capabilities of metrological institutions, strengthening intellectual capital, increasing the competitiveness of the Polish economy in strategic areas for the country, developing modern technologies, and stimulating the development of metrology, in particular in the areas of health, environment, energy, and advanced measurement techniques, as well as development of digital technologies.

The consortium consisting of Maritime University of Gdynia (leader), Gdansk University of Technology, and the University of Gdansk is currently conducting the second project related to the development of the “concept of building metrological infrastructure in the area of underwater acoustics at GUM”. The development of underwater acoustics is closely related to the intensification of work in the marine areas of the Baltic Sea, correlated with the broadly understood economic development of Poland and marine research. The list of planed infrastructure is presented in

Table 3. Planned metrological infrastructure for research in the area of underwater acoustics at GUM.

No.	Device/Object	Application	Frequency Band/Method
1	Low frequency acoustic coupler [33]	Calibration of underwater acoustic sensor	10–2 kHz vibrating water column method—in progress from GUM’s own funds
2	Very low frequency acoustic coupler	Calibration of underwater acoustic sensor	0.01–10 Hz/acoustic coupler
3	Small measuring tank 3 × 3 × 4 m	Calibration of underwater acoustic sensor and devices	kHz–1 MHz/Free field
4	Large measuring tank 10 × 15 × 10 m	Calibration of underwater acoustic sensor and devices	several hundred Hz–1 MHz/Free field
5	Marine reservoir with research infrastructure in the silence zone (actual conditions of operation of devices)	Calibration of underwater acoustic sensor and devices, adjustment of SONARs and underwater communication devices, measurement of shipping noise	several hundred Hz–1 MHz/Free field
6	Large water reservoir) with research infrastructure in the silence zone (model conditions for the operation of devices)	Calibration of underwater acoustic sensor and devices, adjustment of SONARs and underwater communication devices	several hundred Hz–1 MHz/Free field

. Figure 6 presents a general view of buildings intended as metrological infrastructure in the area of underwater acoustics.

Benefits of management by a consortium of universities/institutions in cooperation with GUM:

• Proven procedures for establishing state standards:
• No fees for the area and use of the existing building as part of the future metrological infrastructure;
• Self-sufficiency of national institutions when it comes to calibration of sensors and devices in the area of underwater acoustics—saving costs and time;
• Access to full metrological infrastructure while reducing maintenance costs for participating institutions;
• Possibility of using infrastructure and equipment in teaching (lectures and laboratories) related to metrology in underwater acoustics and, after expansion, in the offshore area;
• Possibility of using the infrastructure to conduct scientific research and test new products related to underwater/offshore acoustics;
• Having appropriate certificates for devices may be important/helpful in obtaining orders resulting from tenders and research grants;
• It is planned that the future Underwater Acoustics Laboratory will be managed on the basis of cooperation/establishing a consortium with interested institutions and with the participation of the Central Office of Measures as the National Metrological Institute.

4. Conclusions

The current state of the calibration infrastructure of the underwater acoustic devices, with particular emphasis on marine ecology and fisheries sciences, outlining the counter-measures to effective protection of selected objects of fauna and flora in the considered marine system may be summarized as follows:

• The ensuring of reliability and repeatability, leading to improvement of research in the area of marine ecology through traceable calibration of hydrophones and measuring instrumentation enables the identification of the critical parameters in the analyzed marine system and, in consequence, makes it possible to undertake the counter-measures for effective protection of the selected objects of fauna and flora in this system. A complexity of the presented subject is illustrated in Figure 1, explaining the causes and effects relations in marine ecology with stressing the place and role of calibration underwater acoustics sensors and measuring devices, as well as in Figure 2, showing challenges in the research of marine ecology system and ways to solve the formulated problem.
• The importance of reliability and repeatability of underwater noise research results from the changes in the sensitivity of hydrophones, that is, dynamic pressure sensors, which is mainly caused by adulteration (voltage sensitivity drift), immersion of the sensors during measurements, underwater mounting conditions of the sensors, as well as the waves/tides phenomena, and is presented and discussed in chapter 2.
• Calibration recommendations concerning hydrophones, based on research studies presented in Figure 3 and Figure 4, cover the analysis of the previously described influencing factors on the reliability and repeatability of underwater acoustic measurements. These recommendations are strictly connected with the Marine Strategy Framework Directive (MFSD) and included in appropriate legal standards and related documents presented in Table 1.
• The analysis of the low frequency calibration systems worldwide, illustrated in Table 2, shows that commonly accepted and used by numerous well recognized institutions, and confirmed by existing standards, frequency range of calibrations of underwater acoustics devices used in measurement and modelling phenomena in area of marine ecology is defined as a frequency below 1 kHz.
• The Polish case-study based on the development of metrological infrastructure in the underwater acoustic measurements in GUM (Central Office of Measures, Poland) shows that ensuring the reliability and reliability of measurements in the field of underwater acoustics in civil and military applications with the participation of the Central Office of Measures in Warsaw (Polish NMI) is crucial and will be further developed.