Designing a comparison coupler for low-frequency calibration of hydrophones in air

Requirements of traceability for measurements of sound pressure in the maritime environment demand that hydrophones are calibrated to frequencies down to 20 Hz. A recently completed European project entitled “Underwater Acoustic Calibration Standards for Frequencies Below 1 kHz (UNAC-LOW)”, within the European Metrology Programme for Innovation and Research (EMPIR) presented a proposal for some methods of providing traceability at that frequency range. One of these methods was based on direct comparison to reference microphones in an air-filled coupler. A design of the coupler was proposed by TÜBITAK, and used by some participants in the project, including DFM. Preliminary results of the sensitivity of the hydrophone determined from measurements in the coupler showed that there was a growing deviation from the sound pressure uniformity assumption as the frequency increased. This limitation was solved using an ad-hoc correction based on known sensitivity values of a hydrophone at 1 kHz. Due to the hydrophone-specific nature of the correction, the design of the coupler was revisited so such a correction was not needed. This paper presents the new design of the coupler, and results of its application. © 2019 IMEKO TC19 International Workshop on Metrology for the Sea: Learning to Measure Sea Health Parameters, MetroSea 2019. All rights reserved.