A new technology, developed by the MIT Lincoln Laboratory, uses laser light to excite moisture in the air around the ear of a target, whispering a personal message from several feet away.
"Our system can be used from a distance to transmit information directly to someone else's ear," says MIT team leader and physicist Charles M. Wynn.
You probably do not need to count potential applications for such applications a device that ranges from military applications to targeted advertising.
Almost everywhere ambient noise is a problem – or earphones are inappropriate – a laser whisper whispered could do the job.
If you are worried about the internet "Laser in the ear", the developers promise it is harmless.
"It's the first system to use lasers that are completely safe for the eyes and skin to locate an auditory signal for a specific person in any environment." says Wynn.
The technology that directs sound into a narrow field has been around for decades.
Most use speakers to amplify sound waves or channel ultrasound in a small area. While useful to keep the sound relatively narrow, they require the listener to be within a relatively short distance of the source.
Laser-based optical technologies have the potential advantage of spreading over a greater distance, and higher frequencies allow much greater range ̵
At the heart of MIT's new technology is a 1.9 micron thulium laser. Thanks to a principle known as the photoacoustic effect, water vapor in the air absorbs the emission of the laser and vibrates at audible frequency.
"This can work well in relatively dry conditions, as there is almost always some water in the water, especially in the human environment," says Wynn.
"We found that we do not need a lot of water when we use a laser wavelength that is very strongly absorbed by water, which was key because the stronger absorption leads to more sound."
The team tested two methods of sound transmission. The first simply varied the amplitude of the laser through a modulator in a continuous wave, producing vibrations that could be picked up by a microphone from 2.5 meters away.
A second process triggered the modulator and used a mirror to sweep the laser at dynamic sound velocity in so-called dynamic photoacoustic spectroscopy, effectively generating loud waves by shaking more water particles.
"There are trade-offs between the two techniques," says optomechanical engineer Ryan M. Sullenberger.
"The traditional photoacoustic method delivers a sound with higher sound quality, while the laser sweeping tone produces louder sound."
A potential disadvantage of the laser wiggle method is that it operates at a very specific distance from the transmitter. As you get closer, the laser beam is set too high. Everything continues, and the frequency falls below the audibility.
Although a volume of 60 decibels (the level of a normal conversation) was only tested for a few meters, researchers believe that they can now increase both distance and volume. They have a proof-of-concept.
"We hope that eventually a commercial technology will develop," says Sullenberger.
This means that somebody in the future may be able to carry whispers from long distances to your ear, which can be both disturbing and exciting at the same time.
This study was published in OSA Publishing .