Hydrophone Calibration in Ultrasonic Measurement Systems

Ultrasonic measurement systems rely on hydrophones to convert acoustic pressure fields into electrical signals for applications ranging from medical diagnostics and high-intensity therapeutic ultrasound to underwater acoustics and industrial non-destructive testing. Robust hydrophone calibration establishes the relationship between incident sound pressure and output voltage, accounting for frequency-dependent sensitivity, directional response and spatial-averaging effects. Conventional approaches include free-field calibration in open-water facilities and comparison methods using calibrated couplers, while reciprocity techniques serve as primary standards in many national metrology institutes. In laboratory tanks, boundary reflections and transducer start-up transients impose limits on echo-free time, necessitating advanced signal-modelling or inversion algorithms to isolate the direct acoustic wave. Temperature fluctuations, non-linear propagation in high-pressure fields and finite element size introduce additional uncertainties. Emerging strategies such as laser-generated ultrasound sources and Two-Equations Two-Unknowns inversion schemes enable calibration across extended frequency and temperature ranges. Continuous refinement of dynamic measurement uncertainty and international standardisation efforts aim to improve inter-laboratory consistency and support the global deployment of safe, accurate ultrasonic technologies.

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Hydrophone Calibration in Ultrasonic Measurement Systems publication trend

The graph below shows the total number of articles in hydrophone calibration in ultrasonic measurement systems across all publications each year (not limited to Nature Index journals).

Technical terms

Hydrophone calibration: Process of determining the frequency-dependent sensitivity and directional response of an acoustic receiver to ensure accurate pressure measurements.

Free-field conditions: Acoustic environment without reflections, enabling direct sound-wave measurements.

Reverberant test tank: Laboratory tank in which multiple reflections occur, requiring specialised signal processing to isolate direct acoustic signals.

Two-Equations Two-Unknowns (2E-2U) method: Inversion technique that uses paired equations to separate direct and reflected acoustic components for low-frequency calibration.

Laser-generated ultrasound (LGUS): Calibration source produced by pulsed laser interaction with water, allowing broadband and temperature-varying acoustic emissions.

Spatial averaging: Effect of finite hydrophone element size that averages pressure over its sensing aperture, requiring correction for accurate measurements.

References

1. Signal-modelling methods applied to the free-field calibration of hydrophones and projectors in laboratory test tanks. Measurement Science and Technology (2018).
2. Investigation of the repeatability and reproducibility of hydrophone measurements of medical ultrasound fields. The Journal of the Acoustical Society of America (2019).
3. Evaluation of dynamic measurement uncertainty – an open-source software package to bridge theory and practice. Journal of Sensors and Sensor Systems (2017).
4. Characterisation of hydrophone sensitivity with temperature using a broadband laser-generated ultrasound source. Metrologia (2023).
5. Application of 2E-2U method for free-field underwater calibrations of hydrophones and projectors in a reverberant laboratory test tank. Measurement Science and Technology (2024).