Definitely scary shit.
There are methods that can be used to alleviate the effects of audible offensive weapons, from placards used in protests to some solid shielding otherwise (makeshift riot shield), but for inaudible subsonic weapons, I only have theories based on the way RF works. In short, aluminum foil *should *work to protect from inaudible weapons like shown in the video. If the shield were to be made concave and faced towards its origin, it could end up focusing it back to where it came from as well. “Space blankets” should act as a shield as well without reflecting it back (instead dispersing it).
Isn’t there a problem with subsonic audio in that it’s really hard to determine the direction it’s coming from? Although with a crowd big enough I suppose you could probably get a visual indicator of where the source is based off of people’s reaction to it.
Edit: After watching the video I don’t really think a person holding a shield would be able to reflect something that was able to clear a space as wide as a street. I’d also wager that the people welding such a weapon likely have the appropriate hearing protection to keep the system from effecting their selves.