Open-water free-field calibration of an autonomous underwater noise recorder for frequencies below 1 kHz

The results of the free-field calibration of an autonomous underwater noise recorder performed in an open-water test site are presented, as part of the output of the “UNAC-LOW” metrology research project which aimed at developing low-frequency calibration methods for hydrophones and autonomous recorders. The calibration was done for frequencies ranging from 200 Hz to 2 kHz by comparison with a reference hydrophone which was calibrated by another project partner using a closed-chamber pressure method. Calibration uncertainties were between 0.7 dB and 1.7 dB, and good agreement up to about 550 Hz was found with results obtained by another project partner using a different setup. Two different mooring solutions are presented and discussed for the device under test. Possible ways to extend the low frequency limit are discussed, to comply with requirements set in current EU regulation for monitoring of low-frequency continuous underwater noise.