Called a long-range acoustic device (LRAD) it can either blast out a beam of ear-piercing sound or be a more focused version of a bullhorn. The volume can reach about 150 decibels at a three feet away, and be heard clearly at distances of nearly two miles. It emits sound in a focused beam, which means that a lot of the sound won't be heard on either side of it.
The LRAD also made an appearance at the Occupy Wall Street protests last fall, although in that case, it was used as a bullhorn not for crowd control. It has also been used to defend cruise ships from pirates and as part of psychological operations in Iraq. They were first developed in the wake of the attack on the USS Cole in 2000.
Unlike conventional speakers, which use a single diaphragm to make the sounds, LRAD uses an array of smaller transducers that convert the electrical energy to vibrations -- sound.
San Diego-based LRAD Corporation, which makes the device, told the BBC that it isn't supposed to be a weapon, but a way to defuse tense situations by making instructions from law enforcement intelligible because the sound quality is better than a bullhorn.
A Ministry of Defense spokesman told the BBC: "As part of the military contribution to the police-led security effort to ensure a safe and secure games, a broad range of assets and equipment is being used by our armed forces. This includes the LRAD (Long Range Acoustic Device) which will be deployed during the Olympic Games primarily to be used in the loud hailer mode as part of the measures to achieve a maritime stop on the Thames."