## 2 Materials and Methods
### Calibration Recommendations
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). \begin{table}
\begin{tabular}{c c c} \hline \hline
**No** & **Standard** & **Short Description** \\ \hline \multirow{4}{*}{1} & & 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). \\ \hline \multirow{4}{*}{2} & & 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.) \\ \hline \multirow{4}{*}{3} & & 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). \\ \hline \multirow{4}{*}{4} & & 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. \\  & ANSI/ASA S1.20-2012 & 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). \\ \cline{1-1}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A list of standards of hydrophone calibration. 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.