Measuring ocean sound
The increasing level of man-made sound in the ocean, whether deliberately generated or not, has led to concern over marine noise pollution. In particular, exposure of marine life to underwater man-made noise is of growing concern as the effects of this exposure begin to be understood. Regulatory requirements are increasing and underwater noise is now explicitly classified as pollution, for example in the EU Marine Strategy Framework Directive (MSFD) which requires member states to achieve Good Environmental Status for European seas.
NPL provides metrology to underpin the protection of the marine environment from anthropogenic sound. This involves establishing in-situ methods for measurement of sound radiated from anthropogenic sources, and for ocean ambient sound. Sources of concern include shipping, oil and gas exploration, and construction and operational noise from marine renewables. There is a need for improved metrology infrastructure in this rapidly evolving area to ensure that decisions are based on robust metrology and sound science, and that the metrology catches up with the rapidly evolving legislative framework.
We provide guidance and expertise in the measurement and assessment of underwater radiated noise and can conduct noise surveys in collaboration with partners. We offer advice and consultancy on in-situ measurement of sound in the ocean and on acoustic propagation modelling, which is required for the environmental impact assessment of underwater noise for offshore developments.
We are developing new optically-based primary standards for the calibration of underwater hydrophones based on heterodyne interferometry and laser Doppler anemometry. The existing primary standard for the free-field calibration of hydrophones is based on the three-transducer reciprocity method for frequencies up to 500 kHz. This method provides the absolute sensitivity of the devices under calibration with traceability through electrical standards. However, there is a requirement for a new method that will enable the calibration of existing hydrophones, and also new devices from emerging technologies, to frequencies up to 1 MHz. We are also researching the development of a new optically-based primary standard that covers frequencies from as low as 10 kHz to as high as 1 MHz. The method is based on laser Doppler effect measured by a custom-design optical heterodyne interferometer.
International standards committees
NPL makes a significant contribution to standardisation and improvement of the metrology for measurement of ocean noise. We have direct input to a number of relevant committees, including ISO Technical Committee 43 Sub-Committee 3, where NPL represents the UK and is highly active on all the Working Groups (WGs) of ISO TC43 SC3 which cover standards for underwater acoustics. NPL is the Convenor of WG3 (Measurement of piling noise) and contributes to WG2 (Terminology) and WG1 (Noise from ships), leading work to develop standards for measuring ship noise in shallow water. NPL led the work to develop ISO 18406:2017 which describes the procedure for measuring noise from marine pile driving. NPL chairs the mirror committee to the above ISO committee, namely BSI EH/1/7 and collates the comments and opinions on proposed standards for the UK underwater acoustics community.