Crowd Control: How the 'Sonic Cannon' Works

Police in Pittsburgh showed off the latest in crowd control Thursday as they reportedly used "sound cannons" to blast the ears of protesters near the Group of 20 meeting of world economic leaders.

City officials said it was the first time such sound blasters, sometimes called sound weapons, were used publicly. But what exactly are they?

"There was an array of sound amplifiers used during the demonstration," Lavonnie Bickerstaff of the Pittsburgh Bureau of Police, told LiveScience, adding, "The Poconos police brought the long-range acoustic device with them, but I don't know whether it was used."

The long range acoustic device (LRAD) is designed for long-range communication and "unmistakable warning," according to the American Technology Corporation, which develops the instruments.

"The LRAD basically is the ability to communicate clearly from 300 meters to 3 kilometers" (nearly 2 miles), said Robert Putnam of American Technology's media and investor relations. "It's a focused output. What distinguishes it from other communications tools out there is its ability to be heard clearly and intelligibly at a distance, unlike bullhorns."

Its shrill warning tones can be heard at least 1,600 feet (500 meters) away and depending on the model of LRAD it can blast a maximum sound of 145 to 151 decibels — equal to a gunshot — within a 3-foot (one meter) range, according to American Technology. But there is a volume knob, so its output can be less than max, Putnam noted.

On the decibel scale, an increase of 10 (say, from 70 to 80) means that a sound is 10 times more intense. Normal traffic noise can reach 85 decibels.

For comparison, a jet engine sends out an ear-splitting 140 to 180 decibels of sound. Human conversation hovers at about 60 decibels. Permanent hearing loss can result from sounds at about 110 to 120 decibels in short bursts or even just 75 decibels if exposure lasts for long periods, according to the National Institutes of Health (NIH) and other sources.

Anything over 120 decibels is liable to be noticeably painful for some individuals, and 150 decibels would hurt anyone's ears. Such sounds damage small hair cells in the inner ear that convert sound energy into electrical signals that travel to the brain. "Once damaged, our hair cells cannot grow back," the NIH states.

But Putnam said under normal circumstances the LRAD is not harmful. "There's no way it can hurt you unless you have the ability to stand in front of it closely for several minutes," Putnam said in a telephone interview.

If you did stand there at length, "It's like having a rock concert in three hours given to you in a half-hour," he added.

The instrument's volume, along with its high-pitched tone, make for "painfully loud sound frequencies that are concentrated in a narrow beam and easily direct them at a target, not unlike using a spotlight," according to Gizmag.

Putnam said the frequency of LRAD ranges from 2,800 Hertz to 3,000 Hertz. That's similar to the pitch of human speech, which is between 500 Hz and 3,000 Hz, the NIH states.

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