IEC 60565-1:2020 specifies methods and procedures for free-field calibration of hydrophones, as well as individual electroacoustic transducers that can be used as hydrophones (receivers) and/or projectors (source transducers). Two general types of calibration are covered within this document: absolute calibration using the method of three-transducer spherical-wave reciprocity, and relative calibration by comparison with a reference device which has already been the subject of an absolute calibration.The maximum frequency range of the methods specified in this document is from 200 Hz to 1 MHz. The lowest acoustic frequency of application will depend on a number of factors, and will typically be in the range 200 Hz to 5 kHz depending mainly on the dimensions of the chosen test facility, The highest frequency of application for the methods described here is 1 MHz.Procedures for pressure hydrophone calibration at low frequencies can be found in IEC 60565 2 [1] . Procedures for hydrophone calibration at acoustic frequencies greater than 1 MHz are covered by IEC 62127-2 [2].Excluded from the scope of this document are low-frequency pressure calibrations of hydrophones, which are described in IEC 60565-2 [1]. Also excluded are calibrations of digital hydrophones and systems, calibration of marine autonomous acoustic recorders, calibration of acoustic vector sensors such as particle velocity sensors and pressure gradient hydrophones, calibration of passive sonar arrays consisting of multiple hydrophones, and calibration of active sonar arrays consisting of projectors and hydrophones.This document presents a description of the requirements for free-field calibration in terms of test facility, equipment and instrumentation, signal processing, and frequency limitations. A description of achievable uncertainty and rules for the presentation of the calibration data are provided. Also included are informative annexes that provide additional guidance on• measurement of directional response of a hydrophone or projector,• measurement of electrical impedance of hydrophones and projectors,• electrical loading corrections,• acoustic far-field criteria in underwater acoustic calibration,• pulsed techniques in free-field calibrations,• assessment of uncertainty in the free-field calibration of hydrophones and projectors,• derivation of the formulae for three-transducer spherical-wave reciprocity calibrations,• calibration using travelling-wave tubes,• calibration of hydrophones using optical interferometry, and• calibrations in reverberant water tanks using continuous signals.IEC 60565-1:2020 together with IEC 60565-2:2019, cancels and replaces the second edition of IEC 60565 published in 2006. This edition constitutes a technical revision.This edition includes the following significant technical changes with respect to the previous edition:1) removal of all descriptions of methods for pressure calibrations of hydrophones – these are now included in Part 2;2) removal of the derivations of formulae for free-field reciprocity calibration (both amplitude sensitivity and phase sensitivity) and placement of these into an informative annex;3) inclusion within the scope of the calibration of the transmitting response of individual source transducers and hydrophones (but not sonar arrays);4) re-ordering of the sections within the document such that the more general procedures for calibration such as guidance on obtaining conditions of acoustic free-field, far-field, and stea