Acoustic holography

It's been a while since I posted anything new on this blog and I apologize. Yes, I got distracted.

Also, I've been putting off discussing one of my most interesting "great ideas" because -- well, because it's somewhat disturbing, as it could be used for surreptitious surveillance. Recent events, however, have forced my hand, so to speak, encouraging me, finally, to publicly speculate regarding this potentially controversial idea. More on this later.


We're all familiar, of course, with visual holography, where a "light field" rather than an image is recorded. To quote the Wikipedia article on holography, a hologram is:

. . . an encoding of the light field as an interference pattern of seemingly random variations in the opacity, density, or surface profile of the photographic medium. When suitably lit, the interference pattern diffracts the light into a reproduction of the original light field and the objects that were in it appear to still be there, exhibiting visual depth cues such as parallax and perspective that change realistically with any change in the relative position of the observer.

Central to the process is the production of a "reference beam," i.e., a beam of pure, coherent, light produced by a laser generator. Since I'm not qualified to explain the process in any detail, I'll refer you to the Wikipedia article referenced above.

It occurred to me some time ago that it might be possible to produce something very similar in the acoustic realm. I speculated that an ultrasonic sine wave might function in a manner equivalent to a laser beam, since both carry a single frequency. It might be possible, therefore, to construct a system in which an entire acoustic environment could be recorded and played back in a manner directly analogous to what happens in a visual hologram.

Picture, for example, a complex environment, say the entire stage of an orchestral performance. Instead of a direct recording of the audio information of the performance via microphones, as is now the case, the entire acoustic "ambiance" could be recorded as a set of interference patterns, based on the ultrasonic "reference beam." When played back along with the original pitch of the reference beam, the sounds would be "resolved" into a purely acoustic environment, so detailed that one could actually walk through it. In other words, while the acoustic "hologram" is being played one could move through the acoustic space at will, concentrating at one point on, say, the woodwind section and at another, the viola section, etc. Or, simply by means of an appropriately designed smart phone app, one could move through the audio environment virtually, letting the sounds move around you while you remain seated -- or as you dance.

What's disturbing about this idea is it's potential for surveillance. Picture a restaurant, for example, or even an environment as complex as Grand Central Station. An acoustic hologram would have the potential to record the entire environment, which, when reproduced, would enable the listener to pick out any conversation simply by either moving through the space or "scrolling" through it virtually.

I have no idea whether or not a technology of this sort is possible. But if it is, I'd be surprised if someone hadn't already thought of it -- especially since it would be so useful to government agencies as a means of gathering intelligence. Which brings me to the recent development alluded to above, i.e., the very odd story involving US and Canadian diplomats suffering from hearing loss. From NBC news:

The U.S. government is consulting with outside experts to determine what happened to American diplomats in the Havana embassy who returned home with damaged hearing, officials told NBC News on Thursday. What caused the Americans in Cuba to suddenly begin losing their hearing remains a mystery, intelligence officials said. A range of possibilities are being considered, based on the reported symptoms, and it’s not clear whether ultrasonic sound waves were to blame.

"Ultrasonic sound waves" indeed! What are they up to, these Cubans, I wonder? Hmmm . . .