Calibration standards for hydrophones and autonomous underwater noise recorders for frequencies below 1 kHz: current activities of EMPIR “UNAC-LOW” project

Abstract

The project entitled “UNderwater Acoustic Calibration standards for frequencies beLOW 1 kHz” (“UNAC-LOW”), currently active within the European Metrology Programme for Innovation and Research (EMPIR), is presented by describing its objectives and current activities. The project aims at developing the metrological capacity of the European Union (EU) for the calibration of hydrophones and autonomous recording systems for the frequency range between 20 Hz and 1 kHz, for which traceability is presently not fully available. In this way, EU metrological capacities for absolute measurement of underwater sound will be improved, with a direct effect on the implementation of regulation and EU Directives that require underwater acoustic measurements to be traceable. After having completed the initial project tasks regarding the review of existing methods and the design of the experimental setups, comparison measurements between the project partners are currently under way and their results will be validated and presented upon project end after first quarter of 2019. To ensure long-term operation of the calibration capabilities by each partner, a coherent EU metrology strategy for underwater acoustics will be developed as one of the main project outcomes. Current activities include the implementation of the calibration setups developed in earlier stages of the project for both hydrophones and autonomous recorders. The methods that shall be used for hydrophones are the pressure method in a closed chamber and the standing wave tube method. For autonomous recorders, in addition to the above methods, calibrations will be performed using free-field methods in different open-water test sites possessing suitable characteristics for low frequency measurements.

Development of the Autonomous Underwater Noise Recorder

“…The Figure 4 shows the voltage amplitude recorded of the square wave signal of 1 kHz, at 44.1 kHz sampling frequency, 16-bit resolution, of the 99 dBA tone signal sound pressure level (underwater standard SPL 160.5 dB re.1µPa 2 ). Not all the commercially available systems would be suitable for use in the measurement of very low-level ambient noise in conditions near Sea State of Zero (Biber et al, 2018). To test the self-noise of the recording chain under consideration the results were compared against the Sea state of Zero noise levels (Eq.…”

“…This deviations can be attributable to the testing setting, i.e. the testing of the hydrophones and recorders are recommended to be implemented in the coupling chamber filled with water either gas or standing wave tube (Biber et al, 2018). The sensitivity of the recording chain was tested as well, that reached the -197.6 dB re.1V/µPa in 1 kHz band (compared to hydrophone sensitivity equal to -180 dB re.1V/µPa at lower frequencies).…”

“…Generally, the acoustic recorders to be used for underwater noise monitoring is recommended to have the sensitivity within the range of -165 to -185 dB re.1V/µPa (van der Graaf et al, 2012) and hold the flat response in the frequency range of interest. To fulfil a full calibration of the device the second features of the recorder should be known (Biber et al, 2018;Hayman et al, 2016) − The system testing was performed in the laboratory premises, although calibration of the recording systems is recommended to be done in the coupling chamber filled with water either gas or standing wave tube, where the development of the special chamber in Lithuania should be considered; − Finally, the equipment tests were made using the reference Sound Level Meter with condenser microphone, that has an accuracy of  3.5 dB, where the more accurate results can be achieved using more accurate equipment. triukšmo matavimai jūrose.…”

“…where: V h is the read out of the voltage and P h is the pressure in µPa (National Physical Laboratory, 2018;Biber et al, 2018). The frequency response of the recording chain was tested at the premises of the Marine Research Institute of the Klaipėda University, using the Multi-Function Environment Meter (Sound Level Meter) Model-8820, with electric condenser microphone, having the frequency range of 30 Hz-10 kHz, accuracy of ±3.5 dB at 94 dBA sound level of 1 kHz sine wave, provided by the Waterborne Transport and Air Pollution Laboratory of the Klaipėda University, that was used as the reference microphone.…”

Data_Sheet_1.docx

“…For all PAM technologies, the hydrophones and related hardware need to be calibrated (every 3 to 5 years) and their performance systematically measured and optimized within frequency bandwidths of interest for the particular activity, species, and environment. Calibration data, and relevant settings and sensitivities should be noted for all hardware used in recording/monitoring to ensure consistency among measurements for particular hardware and software [more detail can be found in Biber et al (2018)]. Array synchronization information (where relevant) should also be documented.…”