[akfish]

The principle seems simple. It's too bad that 1.9 μm laser is neither easy nor cheap to obtain. Otherwise it could be doable for DIYers.

I imagine you can certainly drive people crazy with this thing. For some one who doesn't know this kind of technology's existence, it's pure magic.

[pmx]

There is an alternative method to achieve similar results using very high frequency (40khz in the case of the video below) audio as a carrier for audio that is within the human range. It modulates the amplitude of the carrier signal (in the same way that AM radio does). The high frequency audio interacts with surfaces it hits and kind 'self decodes' into human-perceivable audio. It's doable by DIYers with a little electronics knowledge, too!

https://www.youtube.com/watch?v=aBdVfUnS-pM