IEC 60565-1:2020 pdf free download – Underwater acoustics – Hydrophones – Calibration of hydrophones
1 Scope
This part of IEC 60565 specifies methods and procedures for free-field calibration ofhydrophones,as well as individual electroacoustic transducers that can be used ashydrophones(receivers) and/or projectors (source transducers). Two general types ofcalibration are covered within this document: absolute calibration using the method of three-transducer spherical-wave reciprocity, and relative calibration by comparison with a referencedevice 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 to1 MHz.The lowest acoustic frequency of application will depend on a number of factors, andwill typically be in the range 200 Hz to 5 kHz depending mainly on the dimensions of the chosentest 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 inIEC 60565-2[1]1.Procedures for hydrophone calibration at acoustic frequencies greater than1 MHz are covered by IEC 62127-2[2].
Excluded from the scope of this document are low-frequency pressure calibrations ofhydrophones,which are described in lEC 60565-2[1].Also excluded are calibrations of digitalhydrophones and systems,calibration of marine autonomous acoustic recorders,calibrationof acoustic vector sensors such as particle velocity sensors and pressure gradienthydrophones,calibration of passive sonar arrays consisting of multiple hydrophones,andcalibration of active sonar arrays consisting of projectors and hydrophones.
5.2.1Continuous signals
With continuous signals, to limit the fluctuations in signal amplitude due to interference fromboundary reflections, the minimum distance from the transducers to nearest medium boundaryshall be such that the amplitude of the reflected signals is no more than 3 % of the amplitudeof the direct signal.
NOTE 1 For a given measurement configuration, the minimum distance to the boundaries to satisfy the aboverequirement depends on the directivity of the transducers, the reflection coefficient of the boundary surfaces andon the signal frequency (at high acoustic frequencies, absorption in the water will increase the propagation loss, andreduce the minimum distance needed). An individual assessment of these factors can be undertaken before decidingon the minimum distance to the boundaries to meet the requirement for continuous signals.Often the water surfaceis the nearest boundary. As an example, for two omnidirectional transducers positioned at 5 m separation, for theamplitude of reflections to be within the above requirement,the transducers are positioned at a depth ofapproximately 16 m (assuming total sound reflection occurs at the water surface, and that the acoustic frequency issufficiently low that absorption is not significant)[4][5].
NOTE 2 An alternative means of achieving acoustic free-field conditions for continuous signals is by use of ananechoic acoustic lining deployed on the inner surface of a test tank. In this case, the performance of the anechoicmaterial (in terms of reflection loss as a function of angle) needs to be sufficient to achieve the above requirementfor reflected signal amplitude at the hydrophone over the frequency range of interest. Note that the water surfaceis a strong reflector of sound, and a fully anechoic test tank will require the anechoic material to be suspended justbelow the water surface, in addition to lining the walls and floor of the tank [4].
NOTE 3 Some calibration methods using reverberant water tanks make use of continuous signals (or at leastsignals that are significantly longer than the echo-free time of the tank). In such methods， pseudo-free-fieldconditions can be achieved by use of signal processing techniques which compensate for the effect of reflectionsfrom the tank boundaries. See Annex J for examples of such methods.