In a paper describing their system, the researchers used two different laser-based methods to transmit various tones, music, and recorded speech at a conversational volume. Such photoacoustic communication, say the researchers, could be used to send highly targeted audio signals through the air across noisy rooms or to warn individuals of a dangerous situation such as an active shooter.
"Our system can be used from some distance away to beam information directly to someone's ear," says research team leader Charles M. Wynn. "It is the first system that uses lasers that are fully safe for the eyes and skin to localize an audible signal to a particular person in any setting."
The researchers based their approaches on the photoacoustic effect, which occurs when a material forms sound waves after absorbing light. In this case, the researchers used water vapor in the air to absorb light and create sound.
"This can work even in relatively dry conditions because there is almost always a little water in the air, especially around people," says Wynn. "We found that we don't need a lot of water if we use a laser wavelength that is very strongly absorbed by water. This was key because the stronger absorption leads to more sound."
One of the new sound transmission methods was based on dynamic photoacoustic spectroscopy (DPAS), which the researchers had previously developed for chemical detection. Using DPAS, they had found that scanning, or sweeping, a laser beam at the speed of sound could improve chemical detection.
"The speed of sound is a very special speed at which to work," says Ryan M. Sullenberger, first author of the paper. "In this new paper, we show that sweeping a laser beam at the speed of sound at a wavelength absorbed by water can be used as an efficient way to create sound."
In the DPAS-based photoacoustic communication approach, the researchers changed the length of the laser sweeps to