A sensor previously used for military operations can now be tuned to secretly locate and record any single conversation on a busy street

Mind where you sit (Image: Pierre C./Plainpicture)

EVERYONE knows that to have a private chat in the NSA era, you go outdoors. Phones, the internet, email and your office can all be compromised with ease. But soon even that whispered conversation in the park may no longer be safe from prying ears.

Carrying out covert audio surveillance along a city street or a wooded path, say, currently requires parabolic microphones, which look like large, clear salad bowls and need a direct, unobstructed view of the subject. Hardly 007 territory.

Now, a Dutch acoustics firm, Microflown Technologies, has developed a matchstick-sized sensor that can pinpoint and record a target’s conversations from a distance.


Known as an acoustic vector sensor, Microflown’s sensor measures the movement of air, disturbed by sound waves, to almost instantly locate where a sound originated. It can then identify the noise and, if required, transmit it live to waiting ears.

Conventional microphones work when sound waves make a diaphragm move, creating an electrical signal. Microflown’s sensor has no moving parts. It consists of two parallel platinum strips, each just 200 nanometres deep, that are heated to 200 °C. Air molecules flowing across the strips cause temperature differences between the pair. Microflown’s software counts the air molecules that pass through the gap between the strips to gauge sound intensity: the more air molecules in a sound wave, the louder the sound. At the same time, it analyses the temperature change in the strips to work out the movement of the air and calculate the coordinates of whatever generated the sound.

Until now, the military has been using an early version of the sensor to pinpoint enemy planes and rockets. A single sensor can track and identify multiple distant jets, mortar rounds and sniper rifles in any environment.

Earlier this year, Microflown’s researchers discovered by chance that the device can hear, record or stream an ordinary conversation from as far away as 20 metres, says Hans-Elias de Bree, the firm’s co-founder. Signal-processing software filters out unwanted noise like wind or traffic commotion. Work is now underway to increase the range.

Given a battery and a tiny antenna, the sensor could be attached to traffic lights, a shrub or park bench. Such systems can be teamed with surveillance cameras. Detecting a shout or a gunshot, the sensor can direct the camera to the precise location of trouble, the way our ears work with our eyes. It can then start recording everything that is being said in that location.

Given a battery and antenna, the sensor could be attached to traffic lights or a park bench

A number of countries are now testing the matchstick sensor attached to drones and crewed vehicles, says de Bree. He foresees governments placing them on small dirigibles that tail suspects or hover over political rallies.

“Not only could this work, it has worked,” says Ron Barrett-Gonzalez at the University of Kansas. He has helped boost the sensor’s range by 28 per cent to more than 25 metres. It will be possible to record a parade of people on a busy sidewalk all day using a camera and acoustic sensor, and tune into each conversation or voice, live or via stored files, he says.

Security technologist Bruce Schneier says this new capability is unwelcome – particularly given the recent claims about the NSA’s success at tapping into our private lives. “It’s not just this one technology that’s the problem,” Schneier says. “It’s the mic plus the drones, plus the signal processing, plus voice recognition.”

Listening to the skies A tiny sensor that can eavesdrop on private conversations is not just useful to big brother (see main story). Ecuador is using sensors that measure airflow for something other than spycraft. The government is putting sensors in, around and near airports to form an acoustic air-traffic control system. The sensors pinpoint a plane’s direction by analysing the air movement. Software can tell if a plane is climbing, descending or straining with cargo. While geographical features such as mountains can play havoc with radar returns, the comparative simplicity of passive listening can make Microflown’s sensors less easy to fool. They are also much cheaper than radar equipment.

This article appeared in print under the headline “The tiny spy”