Underwater acoustic calibration standards for frequencies below 1 kHz

The EU Marine Strategy Framework Directive (MSFD) is in place to protect marine biodiversity and ecosystems.  It includes limits on underwater noise generated by human activity which can have a profound effect on marine fauna.

Sound energy with a frequency of 20 Hz to 1 kHz is of greatest environmental concern. However, there was a lack in traceable measurement standards in Europe for accurately detecting underwater sound using hydrophones in this range.

New measuring instrument technology such as ‘autonomous noise recorders’, that combine hydrophones, data acquisition and storage capabilities, required specification standards and traceable calibration methods. To address these issues metrology strategies were needed to develop long-term capabilities in Europe.

This project developed a metrology framework to underpin accurate measurement of underwater noise.  Six different methods for the calibration of hydrophones were developed based on the standard IEC 60565:2006. Two hydrophones, B&K 8104 and B&K 8106, were used in an inter-comparison for calibrations at one third octave frequencies. All methods were in good agreement and the pressure coupling chamber method proved especially useful for calibrating hydrophones at frequencies from 1-2 Hz up to 1-2 kHz with uncertainties less than 1dB.  

Methodology for calibrating autonomous underwater acoustic noise recording systems was also established for 20 Hz to 1 kHz.

Two round-robin calibration campaigns were performed in two open water sites using a SM4M acoustic recorder, with a common range from 200 Hz to 5 kHz, results from both giving differences smaller than 2 dB.

Project outputs have been incorporated into the latest version of IEC 60565 (part-1 and part-2) and communicated with technical standards committees including ISO TC43 SC3 (Underwater acoustics) and IEC TC87 (Ultrasonics) for preparation of related standards.

The improved traceability for underwater hydrophones developed allowed minimal disruption to marine species during the construction of the Dardanelle bridge in Turkey. In addition project outcomes have inputted directly into ocean noise monitoring projects such as OSPAR, JOMOPANS and HELCOM.

Individual strategies for long-term measurement capabilities and calibration of hydrophones and noise recorders are available from 20 Hz to 1 kHz. This will enable manufactures of recorder systems to demonstrate compliance to EU directives and help protect vulnerable marine species.