TORONTO - The risk of hearing loss posed by so-called sound cannons was on trial Wednesday as civil liberty groups made a last-minute pitch to prevent police from using the devices on unruly G20 protesters.

Lawyers for police and the Canadian Civil Liberties Association argued back and forth on just how dangerous the cannons are just 48 hours before the summit was set to begin in Toronto.

Police have bought four sound cannons, which can emit a loud beeping noise that sounds like a smoke detector.

Officers say they will use them as megaphones to broadcast pre-recorded voice messages to crowded demonstrations.

But lawyer Paul Cavalluzzo, who was hired by the civil liberties association and the Canadian Labour Cognress, said police should be prevented from using the devices because they can cause permanent hearing loss.

Cavalluzzo said Toronto residents should not be used as "guinea pigs" to test whether the sound cannons are safe.

He also accused police of relying on manufacturer studies about safety, rather than independent research.

Darrel Smith, lawyer for Toronto police, argued the sound cannons are needed to communicate with a crowd so noisy it may drown out the sound of a traditional megaphone.

He compared the maximum decibel level to that of an ambulance siren or leaf blower.

"They present no greater danger of hearing loss than a weed wacker," said Smith.

Cavalluzzo said police officers are not properly trained to use the devices and criticized the test used in training as too easy.

He said police changed their sound cannon procedures as recently as Tuesday night.

"What we have here are very vague protocols that change on a daily basis and it's totally unreasonable to expect the officer on the ground to understand what he or she is supposed to do," he said.

"We've seen what happens with Tasers, which are not regulated."

Smith said the two devices cannot be compared because Tasers are designed to cause bodily harm.

"It is not a Taser. It's fear mongering to make that analogy," he said.

Smith said police would move at least 75 metres away from a crowd before changing the settings to full blast.

He said that when the cannons are used to project a voice, sound levels reach the mid-90 decibel range.

That is about the same level as the sound of a subway train or lawnmower.

Ontario Superior Court judge David Brown will give his decision on Friday morning.

World leaders are descending on the central Ontario community of Huntsville on Friday for the G8. The G20 meets Saturday and Sunday in Toronto.

Protesters rallied in Toronto for the third-straight day Wednesday. The latest demonstration was the largest and most aggressive yet, with about 350 people protesting world environmental policies going face-to-face with about 100 police officers.

The demonstration ended without incident, but not before anti-police slogans were hurled as police formed a human shield to keep protesters at bay.

Ontario Superior Court Judge David Brown will give his decision on Friday morning.

World leaders are descending on the central Ontario community of Huntsville on Friday for the G8. The G20 meets Saturday and Sunday in Toronto.

Protesters rallied in Toronto for the third-straight day Wednesday. The latest demonstration was the largest and most aggressive yet, with about 350 people protesting world environmental policies going face-to-face with about 100 police officers.

The demonstration ended without incident, but not before anti-police slogans were hurled as police formed a human shield to keep protesters at bay.