But... brain damage?

The level of energy at work, required to cause harm like that would have to operate on the level of two sonar pings from hell, fortunitously converging to form a node with appropriate geometry to harm a watermelon-sized organ.

We know that blasts from high explosives can transmit brain damaging pressure waves through durable vehicular armor, and there aren’t many other sources of energy known to send opportunistic signals through mediums, quite like that.

If I needed to surreptitiously send an accoustic message to an audience positioned at a distance, I might need to use a very powerful source, with a locally inaudible signal that only incidentally becomes audible when crossing other signals, but...

Such a transmission would probably only be effective up to the horizon, and require line-of-sight reception. Provided that the distance for line-of-sight access up to the horizon is about 25-ish miles, depending on topography, and the loudness required for that kind of reach, generating two beams (as powerful as noises produced by attenuated explosive blast waves) seems like it would require some pretty heavy equipment. Like room-sized equipment.

Think about what it takes to push a sound wave several miles, like a ripple across a pond, and what that sounds like up close. Race tracks, rock concerts, passenger jets. These things all readily occupy a three meter cube, at least. Not to mention that the fidelity of such loud noised is very low, and blurred by the intervening air, the further away you get, meaning any digital packet-based transmission would require really coarse, slow bandwidth.

That rules out a lot of sneaky carry-on luggage sized equipment, for generating such signals. Which points, maybe, to no less than two off-site phased-array tone generators forming crossed noise beams for receipt by someone.

Still a lot of mystery to suss out, and a lot of gray areas to speculate upon. And I’m tossing a lot of assumptions into the mix, like whether there was deliberate intent at all, and whether side effects were emergent or not. And while there’d be limits and ceilings based on line of sight, that doesn’t rule out scale of equipment or range of activity.

The only takeaway is that brain damage at a distance requires a lot of intense energy, usually explosive, although deleterious explosive energy and associated noise can scale from sizes relative to something as small as fire crackers and .22L rounds on up.