There are many types of sound generation and thus many types of speakers. An interesting new development in the field does not yet have a name, so in this article it is simply called laser speaker. The laser speaker was developed at the Massachusetts Institute of Technology by Ryan Sullenberger, Sumanth Kaushik and Charles Wynn. It should someday enable using a laser to produce sounds at a certain point somewhere in a room. No special receaving device should be necessary for this purpose, only the air (in particular the water contained) should be enough for the technology to work.
At first this sounds impossible To achive this the photoakoustic effect is used. t is a way to create sound from light. And this is how it works: A medium (in this case the air) is irradiated with laser light and part of the energy is absorbed by the water in the air. The water is thereby heated and changes its volume. It displaces the air and thus creates a pressure wave. By deliberately flashing with laser light (amplitude modulated), it is now possible to shape the pressure waves so that we perceive a sound when the pressure waves reach the ear.
How the photoacoustic effect of laser loudspeaker works.
This sound is still very quiet and not limitet to one point (it can be heared along the entire laser beam). For this reason, the research team has come up with a way to amplify it. The laser is moved back and forth using a rotating mirror. At the point where the sound is to be generated (at the ear of the receiver), the speed of movement is the speed of the sound. As a result, the many small pressure waves add up to a large one, which then reaches the ear at a higher volume.
Using a laser, the effect of the laser speaker is enhanced.
At the moment there are difficulties to producing more than one frequency. To achieve this, it would be necessary to use several lasers or to change the movement speed of the laser beam which would lower the volume. Because it is necessary to limit the laser to harmless energies (because it should be usable near human beings), future will show if this developement will get to a usable speaker. But this will need further research. It is perhaps interesting that especially low notes are easier to produce than higher notes, which is totaly different to other speakers.
Should the laser speaker become usable, there are many different applications. For example, the researchers thought about communication across loud rooms or whispering message without being perceived by others. That qualities can be achieved, which make it possible to transfer music is currently not expected. Nevertheless, applications could certainly be found in the field of music, for example as an invisible text sheet for a singer or as a local bass amplifier in combination with other speakers.
Sullenberger, Ryan M.; Kaushik, Sumanth; Wynn, Charles M., Optics Letters Vol. 44, No. 3. Photoacoustic communications: delivering audible signals via absorption of light by atmospheric H2O. 2019.