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IEC 60565-1-2020 pdf free download
IEC 60565-1-2020 pdf free download.Underwater acoustics — Hydrophones — Calibration of hydrophones — Part 1: Procedures for free-field calibration of hydrophones
Acoustique sou s-marine — Hydrophones — Etalonnage des hydrophones — Partie 1: Procedures d’étalonnage en champ libre des hydrophones.
5.2.1 Continuous signals
With continuous signals, to limit the fluctuations in signal amplitude due to interference from boundary reflections, the minimum distance from the transducers to nearest medium boundary shall be such that the amplitude of the reflected signals is no more than 3 % of the amplitude of the direct signal.
NOTE 1 For a given measurement configuration, the minimum distance to the boundaries to satisfy the above requirement depends on the directivity of the transducers, the reflection coefficient of the boundary surfaces and on the signal frequency (at high acoustic frequencies, absorption in the water will increase the propagation loss, and reduce the minimum distance needed). An individual assessment of these factors can be undertaken before deciding on the minimum distance to the boundaries to meet the requirement for continuous signals. Often the water surface is the nearest boundary. As an example, for two omnidirectional transducers positioned at 5 m separation, for the amplitude of reflections to be within the above requirement, the transducers are positioned at a depth of approximately 16 m (assuming total sound reflection occurs at the water surface, and that the acoustic frequency is sufficiently low that absorption is not significant)  .
NOTE 2 An alternative means of achieving acoustic free-field conditions for continuous signals is by use of an anechoic acoustic lining deployed on the inner surface of a test tank. In this case, the performance of the anechoic material (in terms of reflection loss as a function of angle) needs to be sufficient to achieve the above requirement for reflected signal amplitude at the hydrophone over the frequency range of interest. Note that the water surface is a strong reflector of sound, and a fully anechoic test tank will require the anechoic material to be suspended just below the water surface, in addition to lining the walls and floor of the tank .
NOTE 3 Some calibration methods using reverberant water tanks make use of continuous signals (or at least signals that are significantly longer than the echo-free time of the tank). In such methods, pseudo-free-field conditions can be achieved by use of signal processing techniques which compensate for the effect of reflections from the tank boundaries, See Annex J for examples of such methods.
5.2.2 Time-limited signals
When time-limited signals are employed, such as pulses or bursts of single frequency (toneburst signals). IEC 60565-1-2020 pdf free download.
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