Polyvinylidene Fluoride (PVDF) Piezoelectric Microphones and Their Application to Sound Source Localization

Abstract

In this study, we prototyped and characterized novel piezoelectric microphones that utilize the piezoelectric effect of Polyvinylidene fluoride (PVDF), and successfully achieved sound source localization using an array of microphones built from PVDF. The high flexibility and durability of these PVDF microphones makes them ideal for acoustic sensing in large-area electronics, but their high variability and low sensitivity create significant challenges. Through our experiments characterizing the PVDF microphones, we found that their frequency response is not flat, but rather is dominated by resonant peaks. We were able to develop models to successfully predict the frequency of a microphone’s resonant peaks based on the geometry and tension of its PVDF film. However, we were unable to model microphone sensitivity at resonance due to significant sensitivity variation, even among mechanically-similar microphones with nearly identical resonant frequencies. This variation was likely due to material differences between PVDF strips, which could have affected the strain patterns of the PVDF film or the mechanical-to-electrical transduction properties (e.g. piezoelectric coefficients). Because of this high variation in sensitivity, our experiments were unable to demonstrate any strong correlation between film tension and overall sensitivity of the PVDF microphones. We also tested the linearity of the PVDF microphones, and found significant harmonic distortion and nonlinearities at non-peak frequencies. Although the mi-crophones exhibited reasonable distortion levels at resonant frequencies, their high distortion at non-peak frequencies makes them ill-suited for applications requiring high-quality sound recordings. However, PVDF microphones are still desirable for many applications that do not require such high-quality recording, such as sound source localization (SSL); their flexibility offers significant advantages over other solutions. We implemented an SSL technique called SRP-PHAT to determine the directionality of incoming sound approaching a microphone array. Although the initial SSL results were inaccurate using the entire PVDF microphone array, we developed a strategy to exclude certain microphones from the algorithm based on their signal-to-noise ratios. This strategy significantly improved SSL performance, and allowed us to accurately detect directionality of sound coming from four different locations to within 2◦ of error.