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Hydrophone calibration with an extended frequency range

A new kind of measurement setup enables the simultaneous calibration of amplitude and phase in a frequency range up to 100 MHz

09.10.2017

Hydrophones are used to characterize the acoustical pressure of ultrasonic signals in water. They are commonly used by medical equipment manufacturers to characterize the acoustical fields of diagnostic and therapeutic medical equipment. Especially for measurements that are mandatory by law – e.g. the Medizinproduktegesetz for the protection of the patient – or defined in international standards, calibrated hydrophones must be used. The use of those hydrophones provides traceability of the measurement to the SI system. For this task PTB, operates a primary standard and performs hydrophone calibrations for international customers – manufacturers of medical products and measurement systems, laboratories, regulators and research institutes.

In the past few years, a new measurement setup (shown in figure 1) for the primary calibration of hydrophones was developed. The main part of this setup is a high frequency vibrometer that measures the displacement of the water surface introduced by the ultrasonic wave. The ultrasonic pressure inside the water is then calculated from the displacement. The hydrophone under test is exposed to the same ultrasonic field and the hydrophone signal is measured. By comparison, the frequency-dependent calibration factor between the hydrophone voltage and the acoustical pressure is determined.

Figure 1: Measurement setup for the hydrophone calibration consisting of a vibrometer (1), a water container (2), an ultrasonic transducer (3) and a 3-axis positioning platform (4). After the characterization of the ultrasonic pressure, the hydrophone is submerged into the water container.

 

The ultrasonic signal is generated by a piezoelectric transducer. The excitation is performed using a short electric pulse and by exploiting the nonlinear wave propagation in water, which generates spectral components in a typical frequency range from 1 MHz to 100 MHz simultaneously. The frequency-dependent calibration factor is evaluated in amplitude and phase by using Fourier transformation. Figure 2 shows the results for the calibration of a membrane hydrophone and compares them with the results of the previous method. The previous method uses mono-frequent sinusoidal burst excitation and a homodyne interferometer, and during primary calibration only the amplitude response is characterized. The characterization of the phase response is currently performed by a secondary method, where a broadband hydrophone with a flat frequency response is used as a reference.

 

 

Figure 2: The left-hand diagram shows the amplitude response of a membrane hydrophone. The calibration using the vibrometer (black line) was compared to the result of the current primary standard (blue dots). The uncertainty intervals (grey region and error bars) are presented for an expansion factor of k=2. The right-hand diagram shows the phase response of the hydrophone. It was compared to the result of a secondary method (blue). In both cases, there is an excellent agreement of the different measurements.

 

The uncertainty budget of the new measurement system was investigated based on the GUM and its extension to multivariant quantities to handle complex values. Apart from the significantly shorter duration of the measurement and the possibility to calibrate customer hydrophones directly with the primary standard, the new technique offers a quality of calibration data that is internationally unique. Especially for the increasingly used signal deconvolution approaches, customers of PTB can refer to a considerably improved metrological basis.

 

Literature:

[1] WEBER, M. & WILKENS, V.: Using a heterodyne vibrometer in combination with pulse excitation for primary calibration of ultrasonic hydrophones in amplitude and phase. In: Metrologia 54 (2017), Nr. 4, pp. 432-444 Opens external link in new windowdoi: 10.1088/1681-7575/aa72ba

[2] EICHSTÄDT, S. & WILKENS, V.: GUM2DFT—a software tool for uncertainty evaluation of transient signals in the frequency domain. In: Measurement Science and Technology 27 (2016), Nr. 5, p. 055001 doi: Opens external link in new window10.1088/0957-0233/27/5/055001

[3] EICHSTÄDT, S. & WILKENS, V.: Evaluation of uncertainty for regularized deconvolution: A case study in hydrophone measurements. In: The Journal of the Acoustical Society of America 141 (2017), Nr. 6, pp. 4155-4167Opens external link in new window doi: 10.1121/1.4983827

 

Contact person:

Martin Weber, AG 1.62, Opens window for sending emailmartin.weber(at)ptb.de