  Supplying a piezoelectric transducer with constant voltage or constant power
during a frequency sweep can lead to different results in the determination of
the acoustofluidic resonance frequencies, which are observed when studying the
acoustophoretic displacements and velocities of particles suspended in a
liquid-filled microchannel. In this work, three cases are considered: (1)
Constant input voltage into the power amplifier, (2) constant voltage across
the piezoelectric transducer, and (3) constant average power dissipation in the
transducer. For each case, the measured and the simulated responses are
compared, and good agreement is obtained. It is shown that Case 1, the simplest
and most frequently used approach, is largely affected by the impedance of the
used amplifier and wiring, so it is therefore not suitable for a reproducible
characterization of the intrinsic properties of the acoustofluidic device. Case
2 strongly favors resonances at frequencies yielding the lowest impedance of
the piezoelectric transducer, so small details in the acoustic response at
frequencies far from the transducer resonance can easily be missed. Case 3
provides the most reliable approach, revealing both the resonant frequency,
where the power-efficiency is the highest, as well as other secondary
resonances across the spectrum.
