Performance of commercially available hydrophones with temperature
Background
The following information shows performance data for a number of commercially available hydrophones that have been characterised at various simulated ocean conditions.
Contents
Introduction
Data describing the variation in performance of commercially available hydrophones with temperature and depth have until now been difficult to obtain and are not readily available from hydrophone manufacturers. The performance parameters, such as frequency response, electrical impedance and directional response, are important when one wishes to relate measurements made using the hydrophone in the marine environment to the free-field calibration obtained in a laboratory tank.
Using the NPL Acoustic Pressure Vessel (APV), free-field calibrations of hydrophones may be undertaken at conditions that simulate those existing in the marine environment. It was using this facility that the hydrophone performance data shown below was obtained.
The data shown here are derived from testing one or two examples of each of the hydrophone models listed. In order that the data can be considered as definitively representing the performance of that hydrophone model, a larger sample of each hydrophone model would be required than was possible within the constraints of the work undertaken here. However, it is believed that the data provides a good indication of the likely behaviour.
Measurement Methodology
To determine the absolute sensitivity of a hydrophone or projector, a number of techniques may be used. Calibrations could be undertaken using the primary standard method of three-transducer spherical wave reciprocity [1, 2]. This technique has the advantage that it can be implemented with high accuracy and does not require a previously calibrated reference transducer. However, it requires a minimum of three measurement arrangements and is relatively time consuming. This also represents a logistical problem in the APV due to device mounting requirements.
An alternative approach is to rely on a relative calibration method such as comparison or substitution. Although not as accurate, it enables calibrations to be performed more swiftly. However, for this technique, the reference transducers must be calibrated over the full range of environmental conditions of the device under test. In addition, the reference transducers should not vary greatly in performance with temperature and depth as this can lead to large corrections being applied to the reference sensitivities. This would introduce potentially large uncertainties into the calibration process.
The reference transducers used to gain the results shown here had previously been calibrated by NPL using the primary standard method of free-field reciprocity in the NPL Open Tank Facility (OTF) at ambient pressure and at 18 °C. The coefficients used to correct the reference sensitivities for any small variations in temperature and pressure were obtained by conducting further calibrations at various simulated ocean conditions.
Measurement Techniques
Electrical Admittance
The electrical impedance or admittance is an important parameter describing the behaviour of underwater electroacoustic transducers. The variation of the electrical impedance of a device with pressure and temperature may be a useful indicator as to its performance as a projector or hydrophone when subjected to those same changes in pressure and temperature. The impedance analyser used for these measurements (a Hewlett Packard HP4294A) uses continuous wave sinusoidal signals, and so, in some cases, it was necessary to position the hydrophone centrally in the vessel to take full advantage of the performance of the acoustic wedges [1, 2, 3, 4, 5, 6, 7, 8, 9].
Receive Sensitivity
To determine the receive sensitivity for the hydrophone data shown below, one of three techniques were employed. Reference transducers were used to either perform a relative calibration by substitution using a calibrated hydrophone, or by placing the hydrophone under test in the field of a calibrated projector. Any small variations in sensitivity of the reference transducers were corrected automatically in the calibration software. The primary standard method of three-transducer spherical wave reciprocity was also used [1, 2, 3, 4, 5, 6, 7, 8, 9].
Hydrophone List
- Brüel and Kjær 8103
- Brüel and Kjær 8104
- Brüel and Kjær 8105
- ITC1001
- ITC1032
- ITC1042
- ITC1089C
- ITC1089D
- Reson TC 4014
- Reson TC 4033
- Reson TC 4034
- USRD F30
- USRD H52
Conclusion
It can be seen that even hydrophones of relatively simple design can exhibit significant variations in response with hydrostatic pressure and temperature. Of course, such variation does not mean that the hydrophone is a 'bad' hydrophone. The influence of the variations shown here are likely to be negligible if the device is only to be used in a laboratory tank at modest depths and over a limited range of temperatures. However, if a device is to be used over the wide range of conditions that may exist in the ocean or open-water environment, then it is desirable that the transducer be characterised to determine the variation in its response so that this variation may be accounted for in any measurements taken in the field.
One conclusion that may be made from the results shown here, is that a measurement of electrical impedance may be used as a rapid check on the potential for variation in sensitivity, as the results of measurements indicate that changes in the electrical impedance are in some cases a good indicator of changes in the absolute sensitivity when a device is subjected to variations in the measurement environment.
Brüel and Kjær 8103 - Temperature
Brüel and Kjær 8104 – Temperature
Brüel and Kjær 8105 – Temperature
ITC1001 – Temperature
ITC1032 – Temperature
ITC1042 – Temperature
ITC1089C – Temperature
ITC1089D – Temperature
Reson TC 4014 – Temperature
Reson TC 4033 – Temperature
Reson TC 4034 – Temperature
USRD F30 – Temperature
USRD H52 – Temperature
References
[1] ROBINSON, S P and DORÉ, G R. Uncertainties in the calibration of hydrophone at NPL by the three-transducer spherical-wave reciprocity method in the frequency range 10 kHz to 500 kHz. NPL Report RSA(EXT)054, National Physical Laboratory, UK, 1994.
[2] IEC 60565: 1977. The calibration of hydrophones. International Electrotechnical Commission, Geneva.
[3] BOBBER, R J. Underwater Electroacoustic Measurements. USA, Peninsula Press, (2nd edition), 1989.
[4] BEAMISS, G A, HAYMAN, G and ROBINSON, S P. The provision of standards for underwater acoustics at simulated ocean conditions by use of the NPL acoustic pressure vessel. NPL Report CMAM 76. National Physical Laboratory, UK, 2001.
[5] BEAMISS, G A, HAYMAN, G and ROBINSON, S P. Variation in free-field response characteristics of hydrophones with temperature and depth. Proceedings of the Institute of Acoustics, volume 24, part 2, 2002.
[6] BEAMISS, G A, ROBINSON, S P, HAYMAN, G and ESWARD, T J. Determination of the variation in free-field hydrophone response with temperature and depth. Proceedings of the Sixth European Conference on Underwater Acoustics, ECUA2002, Gdansk, p 635-640, June 2002.
[7] BEAMISS, G A, ROBINSON, S P, HAYMAN, G and ESWARD, T J. Determination of the variation in free-field hydrophone response with temperature and depth. Acta Acustica – Acustica, 88, 799-802, October 2002.
[8] ABLITT, J, BEAMISS, G A, ROBINSON, S P and HAYMAN, G. Hydrophone performance variation with water temperature and depth, Proceedings of Undersea Defence Technology 2006, Hamburg, Germany, June 2006.
[9] BEAMISS, G A, HAYMAN, G, ROBINSON, S P and THOMPSON, A D. Improvements in the provision of standards for underwater acoustics at simulated ocean conditions. NPL Report DQL-AC 010, December 2004.