A New York appeals court on Wednesday upheld a lower court's ruling that two police officers' use of a "sound cannon" was an unconstitutional abuse of force.

The New York police officers used a sound cannon, officially known as a Long Range Acoustic Devise (LRAD), in December 2014 to disperse people protesting the decision not to indict a policeman over the death of a black man during an attempted arrest.

Six protesters filed a complaint, saying they suffered from migraines, vertigo, sinus pain and hearing problems as a result of the incident.

"The problem posed by protesters in the street did not justify the use of force, much less force capable of causing serious injury, such as hearing loss," the court said in its ruling.

It upheld a lower court's ruling that the devices were likely to be a source of police violence if they were misused.

Developed in the 2000s primarily for military needs, the LRAD emits a high-pitched sound in a specific direction that is unpleasant enough to cause crowds to disperse.

The devices can also serve as powerful megaphones to instruct a crowd.

"The NYPD should overhaul its policies and practices regarding LRAD uses to reflect the reality that LRADs are potentially deadly tools, requiring meaningful training and supervision to use safely," Gideon Oliver, an attorney for the plaintiffs, said in a statement.

According to its manufacturer's website, the LRAD is in use in more than 250 cities, counties and US states, and "is not a weapon."

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