Hydrophone Calibration Standards
A number of field quantities may be used to describe a sound wave, but the most common is sound pressure. A hydrophone is a device used underwater to measure sound pressure. Hydrophones respond directly to sound pressure, as do the hearing organs of many species. It is noted that many aquatic species are also sensitive to particle motion.
The unit of sound pressure, as defined by the International System of Units (S.I.) , is the pascal (Pa), which is equivalent to one newton per metre squared, or N/m2.
Primary calibration methods for a measurement hydrophone are absolute calibration methods which realise the acoustic pascal in the most accurate manner available. This is most often achieved using calibration methods which depend on the principle of reciprocity.
The primary method for free-field calibration of hydrophones at kilohertz frequencies is that of three-transducer spherical-wave reciprocity . This classic method requires the use of three hydrophones, at least one of which must be a reciprocal transducer; that is, its transmitting and receiving sensitivities are related by a constant factor. The hydrophones are paired off in three measurement stages, during each of which one device is used as a transmitter and the other as a receiver. For each pair of hydrophones, a calculation is made of the ratio of the voltage across the terminals of the receiving device to the current driving the transmitting device. Using the reciprocity principle as applied to the reciprocal hydrophone, the sensitivity of any one of the hydrophones can be determined from the purely electrical measurements outlined above. The achievable uncertainty for a hydrophone sensitivity using this method is typically 0.5 dB expressed at a 95% confidence level. At frequencies lower than 1 kHz, the reciprocity calibration method is typically applied to measurements made in a small chamber with dimensions smaller than the acoustic wavelength. This method is described as the coupler reciprocity method .
Such primary calibration methods have successfully provided primary calibration standards for many decades, but the methods have some weaknesses, particularly their dependence on the performance of a specific artefact (the reciprocal transducer). More recently, research has focused on novel calibration methods using optical techniques to realize the acoustic pascal , and these show considerable promise for the provision of standards into the 21st century.
Validation of the primary standard for hydrophone calibration
Whilst traceability of measurements to primary standards provides some quality assurance, there remains the question of how to determine that the primary standard itself is accurate. Since it provides the benchmark for secondary and tertiary calibrations, the primary standard cannot simply be compared to a method that is lower down the standards hierarchy. Instead, an assessment of the absolute accuracy must be made, which can be achieved in three ways:
- systematic study of the uncertainties in the primary standard calibration method. This may involve performing experiments to determine the size of the uncertainty contributions, and span many years of study ;
- where possible, a comparison is made with another independent absolute calibration method, preferably one based on a different physical principle (and therefore with few common sources of uncertainty) ;
- comparisons are conducted with the NMIs in different countries. Such comparisons help to harmonise standards across national boundaries and can lead to the discovery of previously unknown sources of error .
Figure 1 shows the results of a hydrophone calibration by two independent methods, namely free-field reciprocity and optical interferometry. The close agreement between the results (within the uncertainties of both methods – error bars not shown) illustrates the value of such comparisons in providing further confidence and validation. Note that the two methods are based on completely different physical principles and share few sources of uncertainty [3,5].
Figure 1. Comparison of calibration result for a B&K8103 hydrophone by free-field reciprocity (squares) and high-frequency optical interferometry (circles). The plot is reproduced from , with permission.
Figure 2 shows some of the results of the Key Comparison of hydrophone calibration organised by the Consultative Committee for Acoustics, Ultrasound and Vibration (CCAUV), which involved the NMIs of seven countries calibrating the same hydrophones . The participating countries in Key Comparison CCAUV.W-K1 were UK, Germany, USA, Russia, China, Canada, and South Africa. The results are available in full at: www.bipm.org.
Figure 2. Degree of equivalence, with the uncertainties, obtained through a comparison of hydrophone calibration results of UK, Germany, USA, Russia, China, Canada and South Africa from Key Comparison CCAUV.W-K1. The plot is reproduced from , with permission.
The results may be expressed in terms of the degree of equivalence between the different countries along with the uncertainties. This type of exercise provides greater confidence in the individual results and allows for mutual recognition of calibrations undertaken in different countries.
Specification standards for hydrophone calibration
At the international level, there is a key standard covering the calibration of hydrophones:
IEC 60565:2006, Underwater acoustics - Hydrophones - Calibration in the frequency range 0.01 Hz to 1 MHz, IEC 60565-2006.
This standard has comprehensive coverage of techniques for calibration of hydrophones in free-field conditions, and at low-frequencies in specialised couplers. It is currently under revision by IEC Technical Committee 87 (TC87) – Working Group 15 (WG15) and is likely to be divided into two parts: one for free-field techniques and one for low-frequency pressure calibration techniques. This standard is available from the IEC web-store at: http://www.iec.ch/.
In the USA, ANSI has a similar standard relating to the calibration of hydrophones:
ANSI S1.20-2012, Procedures for Calibration of Underwater Electroacoustic Transducers.
ANSI S1.20 provides a broader coverage than IEC 60565 as it considers the calibration of other types of electroacoustic transducers (not just hydrophones). ANSI S1.20 also provides information on a standard-target method for calibrating active sonars. This document is available at: http://webstore.ansi.org.
Hydrophone usage guidelines
There are practical considerations about electrical loading, mounting, and wetting, for which guidelines have been prepared. Links are provided in the following.
Hydrophone electrical loading corrections [http://resource.npl.co.uk/acoustics/techguides/loading/]
Hydrophone mounting [http://www.npl.co.uk/acoustics/underwater-acoustics/research/hydrophone-mounting]
Hydrophone wetting [http://www.npl.co.uk/acoustics/underwater-acoustics/research/hydrophone-wetting]
 The International System of Units (SI), Bureau International des Poids et Mesures (BIPM), Paris. Brochure available from www.bipm.org
 R. J. Bobber, Underwater Electroacoustic Measurements, Naval Research Laboratory, Washington, DC, 1970 [Peninsula Press, USA, (2nd edition), 1989].
 P. D. Theobald, S. P. Robinson, A. D. Thompson, R.C. Preston, P. A. Lepper and W. Yuebing, “Technique for the calibration of hydrophones in the frequency range 10 to 600 kHz using a heterodyne interferometer and an acoustically compliant membrane,” J. Acoust. Soc. Am. 118(5), pp. 3110-3116, 2005.
 P. M. Harris, G. Hayman, S. Robinson and I. Smith. “Uncertainty evaluation of an underwater acoustic transducer by the method of free-field reciprocity,” NPL Report MS9, October 2010. Available from www.npl.co.uk
 S. P. Robinson and P. D. Theobald, “Validation of primary hydrophone calibrations by inter-laboratory comparisons and by independent calibration methods,” J. Acoust. Soc. Am., 123, p3346, 2008. Full paper reproduced in Proceedings of the 9th European Conference on Underwater Acoustics (ECUA2008), ed. M. Zakaria, pub. Société Française d’Acoustique, vol 1, pp125-130, July 2008.
 S. P. Robinson, P. M. Harris, J. Ablitt, G. Hayman, A. Thompson, A. L. Van Buren, J. F. Zalesak, R. M. Drake, A. E. Isaev, A. M. Enyakov, C. Purcell, H. Zhu, Y. Wang, Y. Zhang, P. Botha, and D. Krüger, “An international key comparison of free-field hydrophone calibrations in the frequency range 1 kHz to 500 kHz,” J. Acoust. Soc. Am., vol 120 (3), p 1366 – 1373, 2006.