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Researchers have created a microphone that listens with light instead of sound. Unlike traditional microphones, this visual microphone captures tiny vibrations on the surfaces of objects caused by sound waves and turns them into audible signals.

"Our method simplifies and reduces the cost of using light to capture sound while also enabling applications in scenarios where traditional microphones are ineffective, such as conversing through a glass window," said research team leader Xu-Ri Yao from Beijing Institute of Technology in China. "As long as there is a way for light to pass through, sound transmission isn't necessary."

In the journal Optics Express, the researchers the new approach, which applies single-pixel imaging to sound detection for the first time. Using an optical setup without any expensive components, they demonstrate that the technique can recover sound by using the vibrations on the surfaces of everyday objects such as leaves and pieces of 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 Yao. "For example, it could make it possible to talk to someone stuck in a closed-off space like a room or a vehicle."

Simplifying the setup

Although various methods have been used to detect sound with light, they require sophisticated optical equipment such as lasers or high-speed cameras. In the new work, the researchers set out to use a computational imaging approach known as single-pixel imaging to develop a simpler and less expensive approach that would make optical sound-detection technology more accessible.

Single-pixel imaging captures images using just one light detector—or pixel—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, 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.

To apply single-pixel imaging to sound detection, Yao's team used a high-speed spatial light modulator 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 decoded into audible sound. They used Fourier-based localization methods to track object vibrations, which enabled efficient and precise measurement of minute variations.

"Combining single-pixel imaging with Fourier-based localization methods allowed us to achieve high-quality sound detection using simpler equipment and at a lower cost," said Yao. "Our system enables sound detection using everyday items like paper cards and leaves, under natural lighting conditions, and doesn't require the vibrating surface to reflect light in a certain way."

Another advantage of using a single-pixel detector to record light intensity information is that it generates a relatively small volume of data. This means that data can be easily downloaded to storage devices or uploaded to the internet in real time, enabling long-duration or even continuous sound recording.

Capturing sound

To demonstrate the new visual microphone, the researchers tested its ability 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 meters away from the objects while a nearby speaker played the audio.

The system was able to successfully reconstruct clear and intelligible audio, with the paper card producing better results than the leaf. Low-frequency sounds (<1 kHz) were accurately recovered, while high-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.

"Currently, this technology still only exists in the laboratory and can be used in special scenarios where traditional microphones fail to work," said Yao. "We aim to expand the system into other vibration measurement applications, including human pulse and heart rate detection, leveraging its multifunctional information sensing capabilities."

They are also working to improve the system's sensitivity and accuracy, while also making it portable enough for convenient everyday use. Another key goal is to extend its effective range to enable reliable long-distance sound detection.