This article addresses acoustic wave propagation in a piezoelectric ink jet printer. Acoustic resonances limit the operating frequency of ink jet devices and influence the timings of the electrical drive signals. In this study, the resonant frequencies in a multichannel printhead are determined through feedback from the fluid to the piezoelectric structure using an electrical impedance analyzer. We also analyze the influence of channel length on resonant frequency. In addition, the effect of different boundary conditions on the acoustic resonance of the channels was observed. Because the channels walls are compliant, the propagation of acoustic pressure waves in them is slower than the speed of sound in the fluid, which is a fluid property. The electrical impedance measurements allow the determination of the effective speed of sound in the channel and the optimal timing for the driving electrical signal. During printhead operation, the drop velocity can be modified by changing the duration of the electrical pulse sent to the piezoelectric actuator. The timing that produces the maximum drop velocity can be also related to the effective speed of sound in the channel. Comparison of the two data sets show that a printhead channel has an acoustical behavior closer to an open-open pipe.