That's silly.
British idiot authorities have
announced in advance that they're going to use a loud noise weapon to disperse possibly unruly crowds at the Olympic event.
Defending yourself is dead easy.
Good earplugs are all you need.
Flents "Quiet Please" brand is widely available and highly effective. Practice in advance to get the
insertion right.... there's more to it than simply stuffing the plugs in. When it's right you'll know it: lots of discomfort and
zero sound. (The E-A-R brand is marketed as 'professional'; more expensive but no better than Flents.)