This microphone picks up sounds by watching them

Sound travels in waves of vibrating air molecules that bump into each other. Microphones work by picking up these vibrations. Scientists had wondered if they could make microphones that see those vibrations rather than hear them. Now researchers in China have built such a device.

The new gadget photographs tiny disturbances made as sound waves subtly quake the surface of an object. A computer then turns those images back into sound. This means the microphone can work even when it isn’t able to “hear.” For instance, when a glass barrier dampens or blocks sound waves.

“This is a promising technology to detect faint audio signals,” says Varun Raghunathan. He’s an optics engineer at the Indian Institute of Science in Bengaluru. He didn’t work on the new tech. But it could someday be used in environmental, industrial and security-monitoring applications, he says.

Listening without hearing

This is hardly the first attempt to make a microphone that works with light. Alexander Graham Bell, the inventor of the telephone, built one back in 1880. He called it a photophone. It carried sound on a beam of light.

Sound travels as air transfers vibrations from one molecule to the next — until they reach our eardrums. When sound waves hit objects, such as a leaf or sheet of glass, they vibrate those surfaces, too. The movements are too tiny for us to see. Still, they can be measured.

That’s what the photophone did. It directed a speaker’s voice onto the back of a flexible mirror. As someone spoke, the sounds disturbed the air. Those disturbances, in turn, made the mirror flex back and forth. This alternately transformed the mirror’s surface into concave and convex shapes. And that flexing changed how light reflected off it.

At the listener’s location, the light pulsated with each spoken word. The pulsing light was beamed onto a set of metallic gratings that turned the light back into sound.

At first, the device could transmit sound across a room. In a later test, the photophone transmitted sound from one building to another some 200 meters (660 feet) away.

Today’s telephones use the same principle as the photophone. They rely on electricity instead of light.

Catching sound one pixel at a time

More recently, efforts to make optical microphones have depended on high-speed cameras or a laser’s precisely controlled beam of a single color (wavelength) of light. These setups tend to be both complex and costly.

A team of researchers in China has now taken a different approach: single-pixel imaging.

“An image is composed of many dots of different colors and intensities,” explains Xu-Ri Yao. A physicist, he led this work at the Beijing Institute of Technology. “When we use a camera to capture an image,” Yao says, “numerous photoelectric sensors record the intensity of each dot. Such photoelectric sensors are called pixels.”

Cameras usually have a lot of these sensors arranged on a flat surface. The light intensity at those sensors gets combined and displayed as an image.

Single-pixel imaging, in contrast, uses just one sensor. It collects data for an image by making measurements as it scans a scene. Then computers assemble all these data to reconstruct an image.

In this way, Yao’s team could pick up how sound waves had subtly shaken a paper card or a leaf. A computer then decoded these data into audible sound.

The device successfully captured Chinese and English pronunciations of numbers. It also decoded an excerpt from a famous short piece of music by Beethoven: Für Elise.

“This method enables sound detection using everyday items, like paper cards and leaves, under natural lighting,” Yao says. What’s more, it creates a relatively small amount of data. That makes it easy to download for storage or to upload to the internet.

Looking at the future

The device can pick up vibrations from faint audio. But right now this only works in a laboratory.

Yao’s group is also working to make its device more sensitive and accurate. So far, it can capture sound from about a half meter (1.6 feet) away. The scientists hope to increase that distance. They also aim to make the microphone smaller and more portable — “a device that ordinary people can carry around conveniently.”

The researchers suspect one application could be listening for a human pulse and heart rate. The team is fine-tuning the microphone’s ability to measure a pulse, Yao says.

Detecting sound in environments that are noisy or have a lot of movement remains a key challenge, Yao says. That will take more advanced computer algorithms, along with improved hardware and extensive field testing. But if they succeed, he says, this tech could prove “stable, practical and widely applicable in real-world scenarios.”