A compact setup for low-frequency free-field calibration of hydrophones and autonomous underwater recorders in lakes

A new setup is described for low-frequency, free field calibration by comparison of hydrophones and other receiving devices such as autonomous recorders. This setup is suitable to be employed onboard small vessels in open water sites such as lakes due to compact size and ease of operation of its components. A particular geometry is adopted so that sound waves produced by the source which are reflected by the free surface are added coherently to the direct wave by varying source depth with frequency, enabling propagation conditions at shallow depths equivalent to free-field and simplifying constraints on device handling and positioning. Results are presented of the calibration of autonomous recorders in a lake with approximately 100 m water depth, which prove the method to be reliable for frequencies ranging from 63 Hz up to 20 kHz.