The project entitled “Metrology for Low Frequency Sound and Vibration” (“INFRA-AUV”), currently active within the European Metrology Programme for Innovation and Research (EMPIR). Measurement of seismic and infrasound activity is very important for monitoring natural extreme events. Low frequency sound and vibration monitoring technologies are well-established, but the sensors are often used for specific and local applications only and lack traceability. This project aims to establish both the first primary measurement standards for low frequency sound (in both air and water) and vibration. The aim of the task “Development and Evaluation of Primary Calibration Methods in Underwater Acoustics in a Frequency Range from 0.5 Hz to 100 Hz” is to develop and evaluate candidate calibration techniques in underwater acoustics for use as primary standards in frequency range of interest. These methods will be evaluated in terms of their limitations, uncertainty contributions and calibration results. The pressure coupler reciprocity method is based on the hydrophone, a transducer and projector which are calibrated inside a relatively small coupling chamber where the acoustic field is homogenous. Through a series of electrical measurements of voltage and current, the acoustic sensitivity of the three devices may be calculated traceable to electrical primary units.