What are audible enclaves? – The Hindu

When sound waves are emitted from a source, like a speaker, they diverge as they pass through the air in a phenomenon called diffraction.
| Photo Credit: Desmond Lian/Unsplash

Sound consists of waves moving through a medium, moving the medium’s particles back and forth. How fast this back-and-forth motion happens determines the sound’s frequency. The faster they move, the higher the frequency of the sound.

When sound waves are emitted from a source, like a speaker, they diverge as they pass through the air in a phenomenon called diffraction. Higher frequency sounds diverge more. Devices called parametric array loudspeakers are still able to create focused ‘beams’ of sound. They emit high-frequency waves modulated with an audio signal. As the waves travel through the air, they self-demodulate to produce a sound wave confined in a narrow beam, audible only to those in the beam’s path.

Audible enclaves go a step further. In a study published on March 17 in Proceedings of the National Academy of Science, researchers in the US reported doing this by using two high-frequency waves of different frequencies. They are inaudible in this form. But when they intersect, non-linear interactions cause them to produce a sound wave at that spot, audible only to people nearby.