Traditional microphones capture tiny vibrations on the surfaces of objects caused by sound waves and turn them into audible signals. A microphone developed by researchers at the Beijing Institute of Technology operates differently: Rather than sound, the microphone listens with light. This light-enabled microphone is able to pick up sounds in situations where traditional microphones are ineffective, such as through a glass window. According to the researchers, the technique can work on everyday objects such as leaves and paper. “The new technology could potentially change the way we record and monitor sound, bringing new opportunities to many fields, such as environmental monitoring, security, and industrial diagnostics,” said research team leader Xu-Ri Yao. “For example, it could make it possible to talk to someone stuck in a closed-off space like a room or a vehicle.” Researchers have developed an inexpensive visual microphone detection system based on single-pixel imaging. DMD: Digital micro-mirror device. PDA: Photodiode array detector. Courtesy of Beijing Institute of Technology/Xu-Ri Yao. While this technology has seen previous exploration, its earlier iteration has required expensive optical equipment. The current project sought to simplify the process by using single-pixel imaging, which would make this technology more accessible. Single-pixel imaging captures images using just one light detector instead of a traditional camera sensor with millions of pixels. Rather than recording an image all at once, the scene's light is modulated using time-varying structured patterns by a spatial light modulator (SLM), and the single-pixel detector measures the amount of modulated light for each pattern. A computer then uses these measurements to reconstruct information about the object. The team used a high-speed SLM to encode light reflected from the vibrating surface. The sound-induced motion causes subtle changes in light intensity that were captured by the single-pixel detector and then decoded into audible sound. Team members then used Fourier-based localization methods to track object vibrations, which enabled efficient and precise measurement of minute variations. Single-pixel detectors record the information in a relatively small amount of data, which means the data can be transferred between devices quickly. The researchers tested the microphone, using it to reconstruct Chinese and English pronunciations of numbers as well as a segment from Beethoven’s Für Elise. They used a paper card and a leaf as vibration targets, placing them 0.5 m away from the objects while a nearby speaker played the audio. The system was able to reconstruct clear and intelligible audio, with the paper card producing better results than the leaf. Low-frequency sounds (1 kHz) showed slight distortion that improved when a signal processing filter was applied. Tests of the system's data rate showed it produced 4?MB/s, a rate sufficiently low to minimize storage demands and allow for long-term recording. Future plans with this technology are human pulse and heart rate detection, as well as a wider range for long-distance sound detection. This research was published in Optics Express (www.doi.org/10.1364/OE.565525).