Forget the megaphones. Police will have a much more high-tech — and louder — option to make themselves heard over the din of Manhattan traffic and noisy protesters outside the Republican National Convention (search).

It's called the Long Range Acoustic Device (search), developed for the military and capable of blasting warnings, orders or anything else at an ear-splitting 150 decibels.

Authorities on Thursday unveiled a mini-arsenal of devices and counterterrorism equipment they're getting ready for the convention, which opens a week from Monday. At a training site on Thursday, police practiced disarming a truck bomb at a checkpoint. Scores of officers also made mock arrests of police academy cadets who posed as protesters.

Chanting "no justice, no peace," the cadets surrounded a bus full of "delegates" before officers in riot gear raced in, slapped on plastic "flex cuffs" and led them away to vans.

The demonstration was intended to show how the nation's largest police department hopes "to put a comprehensive security net over Madison Square Garden and the rest of the city," said Police Commissioner Raymond Kelly.

"I think you'll see we're prepared."

The department recently bought two of the 45-pound acoustic sound machines for $35,000 apiece, and plans to mount them on Humvees posted outside Madison Square Garden (search). It would mark the first time the instrument — which can beam sounds for 300 yards or more — has been used by a civilian force.

"We believe we'd be able to use them in a number of scenarios," said Paul Browne, the police department's chief spokesman.

Two possible uses cited by Browne: directing crowds to safety following a terrorist attack or other calamity, and reminding protesters where they're allowed to march and rally.

The military, which has used the machines in Iraq (search), bills them as a "non-lethal weapon" designed to disperse hostile crowds or ward off potential foreign combatants by delivering prerecorded warnings in several languages and, if needed, an earsplitting screeching noise. But police insist the latter feature won't be used at the convention.

"It's only to communicate in large crowds," Inspector Thomas Graham of the department's crowd control unit said Thursday.

Graham said police had tried out the device in Times Square, and found it delivered clear, even sound over four blocks. Decibel readers will be used to keep the volume at a safe level, he added.

Still, Bill Dobbs of United for Peace and Justice (search), which has planned a massive anti-war demonstration on the eve of the convention, called the sound system "a potential Big Brother nightmare."

Police "are trying to use technology and machinery to control every aspect of life on the street, rather than relax a little and let a part of democratic society unfold," he said.

Mobile metal barriers — a variation of those installed outside government buildings, courthouses and embassies — will form a series of checkpoints around the arena. Once a bus, truck or car is secured between two barriers, it will be screened for bombs or other contraband by cameras that provide real-time video images from underneath.

The department also will deploy a new fleet of motor scooters to cut through gridlock should trouble arise. Hand-held radiation detection devices will help officers patrolling the streets and subways to guard against a "dirty bomb."