Civil liberty groups made a last-minute pitch to prevent police officers from using sound cannons on unruly crowds just days before G20 leaders make their weekend descent on Toronto. Lawyers for police and the Canadian Civil Liberties Association argued back and forth on just how dangerous the cannons are and what the risk of hearing loss is. Police have bought four sound cannons, which emit a loud beeping noise similar to a smoke detector. Police will also use the sound cannons to broadcast pre-recorded voice messages to crowded demonstrations. Lawyer Paul Cavalluzzo, who was hired by the civil liberties association and the Canadian Labour Congress, said Torontonians should not be used as “guinea pigs” to test the sound cannons. He argued that the devices can cause permanent hearing loss and said police are not properly trained to use the devices. He also accused police of relying on manufacturer studies about safety, rather than independent research. Police lawyer Darrel Smith argued that sound cannons are needed to communicate with crowds so noisy they may drown out a traditional megaphone. Smith compared the maximum decibel level to that of an ambulance siren or leaf blower, and said police would move at least 75 metres away from a crowd before changing the settings to full blast. Although Cavalluzzo compared the use of sound cannons to unregulated Tasers, Smith said the two devices are not comparable because Tasers are designed to cause bodily harm. Ontario Superior Court judge David Brown will give his decision on Friday morning.