Diaphragm Design for an Electret Microphone Stethoscope

This article investigates the design elements of a digital stethoscope and its signal fidelity in the presence of ambient noise. While the acoustic impedance matching literature demonstrates that signal pickup can improve when the diaphragm's acoustic impedance closely matches that of the auscultated surface, this approach considers only one interface between two materials. Acoustic impedance matching with two materials may not explain the full picture for electret microphones, which introduce an air gap in their design and therefore have two interfaces between three materials. Introducing a hole in the electret stethoscope diaphragm reduces the three material impedance matching problem into a two material problem. Our empirical results with a 3-D printed electret stethoscope show that a hole in the diaphragm improves signal quality in a variety of tested settings. We additionally propose signal fidelity and noise leakage statistics, a signal quality score, and an amplitude spectrum, all based on the empirical distance correlation. We utilize these statistics to evaluate how all combinations of four diaphragm materials, six diaphragm thicknesses, and presence or absence of a hole affect signal quality of lung sounds in different ambient noise conditions on an acoustic phantom. Moreover, a result of this work is the fabrication of a high-quality and low-cost (U.S. $5) digital stethoscope that can be fabricated with a 3-D printer, soldering iron, electret microphone, and readily available materials.