The New Scientist has this story:
Sound Neutralzing Technology ‘Confuses’ Abusive/Racist Chants at Stadiums
During tests of the prototype system, volunteers were surrounded by loudspeakers simulating the sound of a chanting crowd and were asked join in. But one speaker replayed the crowds chant with a short delay. When the delay was greater than 200 milliseconds the volunteers found it too difficult to chant coherently.
My 4-year-olds can do kind of the same thing. It’s really annoying.
The downside of the technology, of course, is that its use won’t be restricted to sports crowds. Expect repressive political regimes to be first in line; this thing would be great at breaking up political demonstrations.