Sonic weapons were, in fact, used against G20 protesters in Pittsburgh, PA. As reported in the New York Times article “Protesters are met by tear gas at G20 conference”:
“The police fired a sound cannon that emitted shrill beeps, causing demonstrators to cover their ears and back up… City officials said they believed it was the first time the sound cannon had been used publicly. ‘Other law enforcement agencies will be watching to see how it was used,’ said Nate Harper, the Pittsburgh police bureau chief. ‘It served its purpose well.'”
The debate over the use and impact of sound will surely be fueled by this event. Stay tuned.
In this video you can hear the sound cannon, however it is unclear how far away it is and the decibel level.