Ppb-Level Photoacoustic Detection of Chloroform Using Four-Microphone Array

Abstract

The photoacoustic spectroscopy (PAS) system commonly enhances the efficiency of optical-acoustic-electrical energy conversion by increasing the laser power, optimizing the resonance characteristics of the photoacoustic cell (PAC), and improving the sensitivity of acoustic sensors. However, conventional systems using a single-microphone or a dual-microphone differential setup for point sampling of the photoacoustic signal fail to account for its spatial distribution, leading to a loss of spatial gain. Drawing on microphone array theory derived from sonar technology, this study, for the first time, presents a PAS sensing system based on a four-microphone array, which is applied to detect chloroform gas. The microphones are positioned at 90° intervals around the PAC resonance chamber wall, enhancing the spatial sampling rate of the signals. A digital phase-locked algorithm demodulates the combined signals from the four microphones into the concentration data. Experimental results show that, compared to a single-microphone system, the four-microphone array system increases sensitivity by a factor of 4, doubles the signal-to-noise ratio, and achieves a minimum detection limit of 69 ppb, demonstrating a significant improvement in sensitivity by capturing the spatial distribution of PA signals.