National Physical Laboratory

Calibration of hydrophones and projectors

Calibration of  hydrophones and projectors 

Open Tank Facilities

NPL's unique underwater acoustics measurement capability includes two open test tanks that provide a flexible approach to the most demanding testing requirements. Both laboratory tanks enable free-field calibration of hydrophones and the acoustic characterisation of sonar transducers.

The larger test tank is constructed of wood and is 5.5 m in diameter and 5 m deep. It has a high-precision positioning system consisting of vibration-isolated beams supporting three carriages, two with XYZ and θZ motion and one with Z and θZ alone. The maximum travel in the XYZ axes is 1 m with a linear resolution of 5 µm (0.005° in rotation), but extended travel is available in the X direction by rolling the carriages along the beams. The maximum in-air load capability is 50 kg (20 kg for the XYZ carriages). Remote computer control of all axes is available including simultaneous movement of all axes.

The smaller tank consists of a 2 x 1.5 x 1.5 m GRP tank with a two-carriage positioning system (XYZ and θZ motion with 10 ìm resolution), and features a glass window to allow optical interrogation of the acoustic field.

Full acoustic test instrumentation is provided with both open tanks including impedance analyser, data acquisition and analysis systems, all under computer control. NPL can determine all the parameters that characterise an acoustic transducer such as transmit and receive sensitivities, and beam-patterns (including the very narrow beams created by high-frequency sonars).

NPL provides a free-field calibration service for hydrophones and transducers which is accredited to ISO 17025 by UKAS in the frequency range 1 kHz to 20 MHz.

For the determination of transducer directional response [beam plots] NPL offers a service that is also accredited to ISO17025 by UKAS. The frequency range of this service is 1 kHz to 1 MHz.

Acoustic characterisation using near-field methods

NPL has the capability to use near-field measurement methods for characterisation of sonar transducers using either Fourier or boundary element techniques. Such measurements may be performed in the acoustic near field of a source in cases where far-field measurements are not possible.

The precision positioning systems allow scanning of a small hydrophone over a designated surface whilst measuring the amplitude and phase of the acoustic pressure. These measurements can then be used to predict the far-field response, or to back-propagate to predict the pressure distribution at the source.


The National Physical Laboratory provides a unique a range of facilities and measurement services to meet the individual measurement needs of the customer.

Accredited Calibration Services (accredited to ISO17025 by UKAS).

NPL currently provides the following underwater acoustics calibration services accredited to ISO17025 by the United Kingdom Accreditation Service (UKAS):

  • Free-field calibration of hydrophones and projectors by the reciprocity method in the NPL open tank facility from 1 kHz to 500 kHz.
  • Free-field calibration of reference measuring hydrophones by a comparison method in the NPL open tank facility from 1 kHz to 1 MHz.
  • Free-field calibration of miniature ultrasonic hydrophones by a comparison method from 1 MHz to 20 MHz.
  • Air pistonphone calibration of reference measuring hydrophones by comparison method from 25 Hz to 400 Hz.
  • Directional response measurement of hydrophones and transducers in the frequency range 1 kHz to 1 MHz over 360 degrees of rotation in both the horizontal and vertical plane in the NPL open tank facility.

Other measurements include, for example, transducer electrical impedance/admittance in the frequency range 1 Hz to 20 MHz, directional response/beam patterns of transducers and transducer efficiency.

Our experienced team of scientific staff offer a flexible cost-effective approach to test work with a choice of service arrangements

  • Fixed price - Calibration and tests undertaken by NPL staff on customer devices to a schedule agreed with the customer, with results presented in a report.
  • Day rates - Calibration and tests undertaken by NPL staff in collaboration with customer staff on customer devices to a schedule agreed with the customer.
  • Facility hire rates - Calibration and tests undertaken by customer staff previously trained to use the facility and under NPL facility management during the work.

Other options may also be considered.


Customer Services tel: +44 20 8943 8631

Last Updated: 25 Mar 2010
Created: 6 Jun 2007


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