An array of electret-biased frequency-selective resonant microelectromechanical system (MEMS) acoustic sensors was proposed to perform analysis of stress pulses created during an impact between two materials. This analysis allowed classification of the stiffness of the materials involved in the impact without applying post-impact signal processing. Arrays of resonant MEMS sensors provided filtering of the incident stress pulse and subsequent binning of time-domain waveforms into frequency-based spectra. Results indicated that different impact conditions and materials yielded different spectral characteristics. These characteristics, as well as the resulting sensor array responses, are discussed and applied to impact classification. Each individual sensor element in the array was biased by an in situ charged electret film. A microplasma discharge apparatus embedded within the microsensor allowed charging of the electret film after all device fabrication was complete. This enabled electret film integration using high-temperature surface micromachining processes that would typically lead to discharge of traditionally formed electret materials. This also eliminated the traditional wafer-bonding and post-fabrication assembly processes required in conventional electret integration approaches. The microplasma discharge process and resulting electret performance are discussed within the context of the MEMS acoustic sensor array.