2025_programme: Free-field primary calibration techniques for hydrophones at NPL using optical methods

• Day: June 17, Tuesday
    Location / Time: A. TERPSIHORI at 17:00-17:20
• Last minutes changes: -
• Session: 20. Underwater acoustic calibration, measurement, and standards
  Organiser(s): Stephen Robinson, Will Slater
  Chairperson(s): Stephen Robinson, Will Slater
• Lecture: Free-field primary calibration techniques for hydrophones at NPL using optical methods [Invited]
  Paper ID: 2265
  Author(s): Christopher Fury, Ben Ford, Stephen Robinson, Justin Ablitt, Raphaela Baesso, Srinath Rajagopal
  Presenter: Stephen Robinson
  Abstract: Primary free-field calibration of hydrophones is most commonly achieved using methods based on the principle of reciprocity. However, primary calibration is also possible using optical methods to interrogate the acoustic field to which the hydrophone is exposed. Such methods typically use optical interferometry to determine the acoustic particle velocity, or displacement, at a point in the field using the light reflected from a thin acoustically-transparent but optically-reflecting membrane. Methods based on optical interferometry provide calibrations traceable to primary standards of length (via the wavelength of laser light) and have an advantage that they directly realise the acoustic pascal, and do not depend on properties of the hydrophone (such as the requirement that the device be reciprocal). \nThe UK National Physical Laboratory has established a primary method for the free-field realisation of the acoustic pascal using optical interferometry covering the frequency range from 100 kHz to 40 MHz. The method may be used as the primary standard for calibration of hydrophones used in metrology for underwater acoustics at hundreds of kilohertz, and for medical ultrasound applications at megahertz frequencies. A description of the method and its implementation for underwater acoustic hydrophones are presented, followed by results of calibrations undertaken and a discussion of the main sources of uncertainty. Validation of the method is undertaken by comparison with the method of three-transducer spherical-wave reciprocity, an independent absolute calibration method based on a different physical principle (and therefore with few common sources of uncertainty).
• Corresponding author: Dr Christopher Fury
  Affiliation: National Physical Laboratory
  Country: United Kingdom