Laser-Generated Sound: Direct Audio Transmission to a Listener Without Traditional Speakers

• Researchers use lasers to transmit sound at 60 decibels to a target person standing 2.5 meters away. 
• They develop a method that relies on the photoacoustic effect, which takes place when a material creates sound waves after absorbing light. 

The first laser was developed in 1960 through optical amplification by stimulated emission of electromagnetic radiation. Since then, we have come a long way. Now it is used in information processing (Blu-Ray), surgeries, bar code readers, holographic imaging, and material processing, such as cutting, marking, drilling and surface modification.

Recently, a research team at MIT demonstrated a very interesting phenomenon: they used a laser to transmit an audio message to a person’s ear without any kind of receiver device. This technology opens up numerous intriguing possibilities.

It can deliver information some distance away directly to someone’s ear. The lasers (in any settings) are completely safe for the skin and eyes. Thus, it can be used to communicate across halls or warn specific people of a dangerous situation.

How It Works?

To send audio messages, the system relies on the photoacoustic effect that takes place when a material creates sound waves after absorbing light. The intensity of light must vary to produce this effect.

In this study, authors used water vapor in the atmosphere to absorb light and generate sound. The technique works even in a dry environment because there’s always some water present in the atmosphere, especially around humans.

If you select a laser wavelength that can be easily absorbed by water, you do not need to have a large amount of water in the air. The higher the absorption, the greater the sound.

Reference: The Optical Society | doi:10.1364/OL.44.000622 | MIT

The technique is similar to dynamic photoacoustic spectroscopy that has been used for standoff detection of trace explosives. Researchers have shown that sweeping a laser beam (at a  wavelength absorbed by water) at speed of sound can be an efficient method of generating sound.

Credit: MIT’s Lincoln Laboratory

What’s really impressive in this laser sweeping method is that the audio message can only be heard at a specific distance from the sender. The signal could be sent to a particular person, instead of everyone who crosses the light beam.

Experiments

In experiments, researchers were able to transmit sound at 60 decibels to a target person standing 2.5 meters away, using commercially available equipment. The team also tested a conventional photoacoustic technique that does not require sweeping the laser. Instead, it modulates the power of the laser beam to encode the audio signal.

There is a tradeoff between these two methods. The laser sweeping yields sound with louder audio, whereas the conventional photoacoustics technique yields sound with higher fidelity.

Read: Existing Laser Technology Is Strong Enough To Attract Aliens 20,000 Light Years Away

The team believes that their method could be further improved to work outdoors at longer ranges. In fact, it could be used for super-targeted advertising, and military or spying purposes.