Acoustical Crosstalk in CMUT Arrays
One of our recent research topics is acoustic crosstalk in 1-D and 2-D CMUT arrays, both in immersed operation. Acoustic crosstalk is the coupling of energy between cells and therefore also between elements of an ultrasonic transducer array. An optimized design in terms of low acoustic crosstalk performance is required for many applications of CMUT arrays, such as medical imaging and therapeutics. Thus, besides the characterization of CMUT arrays concerning acoustic crosstalk, our research also is focussed on modelling of wave propagation phenomena in CMUT arrays and on various solutions of how to minimize acoustic cross-talk.
By using optical displacement measurements the main crosstalk mechanism in a state-of-the-art CMUT array was identified experimentally. The array used for these first experiments is shown in Fig. 1.
FIGURE 1. Three different views of a 1-D CMUT array used for cross-talk measurements with specifications.
This array was used in the setup shown in Fig. 2. The main advantage of this setup is that both the displacement of CMUT cells and the electrical signal of the CMUT elements can be measured simultaneously. The measurement is done along the whole CMUT array, i.e. a linescan is performed. The animation (link to x-talk animation-avi) illustrates how the measurement is done. Concerning the voltage curves shown in this animation, which correspond to the acoustic velocity, it is important to note that the electrical signal is always the contribution of all cells of one element. The displacement curve, however, only shows the movement of a single cell in the center of the same element.
FIGURE 2. Experimental setup for crosstalk measurements.
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Animation of the Acoustic Cross-Talk Measurement Procedure
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If all these curves, shown in this animation, are put together in the spatial-time representation, one can see that different crosstalk waves travelling at different phase velocities are present (Fig. 3), i.e. acoustic crosstalk in a CMUT array is a multimode wave propagation phenomena. Therefore, a 2D FFT can be performed to calculate the frequency-wavenumber domain representation. Figure 4 shows an example of this transformation including the labels of the identified propagation modes. The crosstalk wave consists of three distinct components: S0 Lamb wave mode, A0 Lamb wave mode, and a dispersive guided mode.
FIGURE 3. Visualization of crosstalk signals in the spatial-time domain. Displacement measurements and electrical measurements are compared.
FIGURE 4. Wavenumber frequency domain representation of the measured time-spatial signals (displacement along a 1D CMUT array) obtained by a 2D FFT. The scale is in dB normalized to the maximum transmitter displacement.
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Visualization of the Acoustic Cross Talk in a 2D CMUT Array
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More details about this work can be found in [1, 2].
[1] B. Bayram, M. Kupnik, G. Yaralioglu, Ö. Oralkan, D. Lin, X. Zhuang, A. Ergun, A. Sarioglu, S. Wong, and B. T. Khuri-Yakub, “Characterization of Cross-Coupling in Capacitive Micromachined Ultrasonic Transducers,” in Proc. IEEE Ultrason. Symp., 2005, pp. 601–604.
[2] B. Bayram, M. Kupnik, G. Yaralioglu, Ö. Oralkan, D. Lin, A. Ergun, S. Wong, and B. T. Khuri-Yakub, “Finite element modeling and experimental characterization of crosstalk in 1-D CMUT arrays,” submitted to IEEE Trans. Ultrason., Ferroelect., Freq. Contr., 2006.
