Low-frequency standards for hydroacoustics in the Infra-AUV project

Low frequency Acoustics, Underwater Acoustics and Vibration (AUV) phenomena in water are used to detect major natural events such as earthquakes, tsunamis and volcanic activity, and are also used by the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO) to check compliance with the treaty. Low frequency sound and vibration monitoring technologies are well established; however the lowest frequencies of interest are not sufficiently well covered by current measurement standards, limiting the reliability of data obtained. In addition, monitoring stations are also often located in extreme environments posing additional challenges for assuring the accuracy of measurements recorded by hydrophones. In this poster we describe the work that NPL, TUBITAK and ASN are doing in the Infra-AUV project to develop calibration methods for hydroacoustics in the frequency range from 0.5 Hz to 100 Hz.

The project is establishing both primary standards (based on absolute realisations of the acoustic pascal), and secondary comparison methods to provide routes for effective dissemination and traceability. Two independent primary calibration methods are under development. The first uses a laser pistonphone (NPL and ASN) which uses optical interferometry to determine the motion (and therefore the generated pressure) of a piston driving a small chamber containing the hydrophone under test. The second is the coupler reciprocity method (TUBITAK and NPL) which allows hydrophones to be calibrated in more realistic ocean conditions (increased static pressure). Secondary calibration methods have been developed based on comparison of two devices in a small coupler (NPL and TUBITAK). The resulting calibration capability will underpin new measurement services and improve traceability in low frequency sound measurement and monitoring applications.