What are 'sound cannons' and why are police using them in Ferguson?

The police response to the protests that have erupted in Ferguson, Missouri, has involved the use of military-grade equipment including "sound cannons" and stun grenades in a sign of a growing militarisation in the US police, according to critics.

Swat teams have reportedly been using the acoustic devices, also known as "sound bombs", as well as tear gas and rubber bullets, to disperse angry crowds.

"I'm a soldier, I'm a military officer and I know when there's a need for such a thing, but I don't think in a small town of 22,000 people you need up-armor vehicles," Cristian Balan, a communications officer in the US army told the Guardian when protests first erupted following the death of unarmed black teenager Michael Brown.

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Police say the weapons are being used as a deterrent to safeguard the community and property, but campaigners say such actions only serve to escalate levels of violence.

"American neighbourhoods aren't war zones and police officers should not be treating us like wartime enemies," says Kara Dansky, senior counsel with the Center for Justice.

But what exactly have St Louis police been using against crowds in Ferguson and why?

What are sound cannons?

Sound cannons, also known as Long Range Acoustic Devices (LRAD) or sound bombs, are acoustic weapons often used as a crowd control tactic. The controversial devices have been used across the world from Gaza to Australia and are now being used on the streets of Ferguson according to numerous reports.

They were originally developed in response to the bombing of the USS Cole missile destroyer in 2000, according to Slate. They were then developed further as a sound-based weapon to deter pirates at sea.

The devices emit high frequency sound waves that can reach 149 decibels – well above the 130-decibel threshold where permanent hearing loss occurs. As well as deafness, the sound is also enough to cause severe headaches and loss of balance.

What else is being used?

Heavily armoured tanks known as Mine Resistant Ambush Protection Vehicles (MRAPs) have been widely used throughout the protests in Ferguson, according to Vox. Many have questioned why the tanks, which are designed to withstand landmine explosions in warzones, have been used on the streets of a small town in the US.

Ferguson police chief Tom Jackson justified their use because "people are using bombs now." However, there have been no reports of bombs being used by protesters. "Unless you live in downtown Kabul, there is no practical need for anything like that in your town," said broadcaster John Oliver.

Police and Swat teams have also been pictured wearing more protective gear than soldiers would in live combat. "Keeping the peace at a demonstration essentially means having police officers in standard everyday uniforms, not military garb," said former Seattle police chief Norman Stamper. Otherwise, they can come to "view the community as the enemy".