IEC 60565-1:2020 pdf free.Underwater acoustics – Hydrophones – Calibration of hydrophones – Procedures for free-field calibration of hydrophones.
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.
When time-limited signals are employed,such as pulses or bursts of single frequency (tone-burst signals), reflections shall be eliminated by choosing the minimum distance from thetransducers to each boundary surface (water surface, tank bottom and side walls) such thatthe time delay of the reflected signals with respect to the direct signal is greater than theduration of the direct signal.IEC 60565-1 pdf download.