The Pittsburgh Police used a new acoustic deterrent device as part of its actions against protesters during last week’s G-20 meeting.  The  Long Range Acoustic Device (LRAD), which emits a high-frequency sound similar to a smoke detector, is designed to disperse crowds using sound at or near the human pain threshold. Termed a “non-lethal weapon,” it was originally intended for use by ships trying to deter attack, and has also reportedly been used against crowds in various Middle Eastern conflicts in recent years.  With a source Sound Pressure Level of 146dB, its sound is above the threshold for pain (130dB) out to about 20 feet; at 50 feet (120 dB), it can cause some permanent hearing damage in 30 seconds.  At distances out to about 100 yards, it is louder than 106dB, which  can cause hearing damage in about 4 minutes. The video that follows after the break focuses more on the truck than on the responses of the crowd; some people have their fingers in their ears, but no one seems incapacitated at all, indicating that sound levels were loud but likely not generally painful.

From http://pittsburghpolice.net/2009/09/27/long-range-acoustic-device-lrad-g20-pittsburgh/