Edit. The countermeasure, according to an AI model is:
Avoid the direct beam of the LRAD by moving out of its narrow 15-degree coverage angle. Stepping off to the side drastically reduces exposure
Sound-dampening shields or barriers
Range style earmuffs and high-quality earplugs that attenuate sound significantly
* Double-bagging your ears (plugs + shooting range plugs) seems to help a lot.
* With a little more effort, you can build a handheld shield. Thin, hard layer on top (like plexiglass), which acts like a reflector, then a standard foam layer and cardboard to dampen whatever makes it through the reflector.
Besides that, it's not that much different than a water cannon truck or a phalanx shooting tear gas. Confronting both is a relatively high risk activity, and should be avoided by most protesters.