One of the most noticeable features of a hybrid car is its near-silence, at least when it's in electric mode.

But the quiet motor, much beloved by drivers, has struck up a controversy. Critics say the noiselessness is a hazard to pedestrians, especially to blind pedestrians, and Congress is considering a bill - introduced in April -- that would require safety standards, including an audible alert, for the vehicles (see Associated Press and The New York Times' Wheels blog).

Lotus Engineering, the consulting division of the Lotus sports car company, has come up with one possible solution. The company on Tuesday introduced a set of technologies to make electric and hybrid vehicles sound like regular cars.

One of the technologies is a loudspeaker system that synthesizes an engine sound - or "voice" -- using recordings from a "donor engine" and taking the speed and acceleration of the vehicle into account.

The system, which sits next to the radiator, projects sound only to the front of the vehicle, so that no sound would be heard once the vehicle has passed, the company said. (The announcement didn't say what would happen when the vehicle was backing up. Perhaps hybrids would need to be outfitted with the loud "beep-beep-beep" of delivery trucks.)

The loudspeaker only would kick in when hybrids are using the electric motor, turning off - and avoiding a double whammy of sound - when the internal-combustion engine starts.

It's an ironic technology to come from Lotus Engineering, which has spent years developing noise-cancellation technologies to make cars quieter. But those technologies also have a place in this approach.

Aside from the loudspeaker, Lotus Engineering is using two other systems would cancel out noise in the cabin so the driver hears "almost none" of the additional sound.

The technologies, called the Active Road Noise Cancellation and Engine Order Cancellation systems, use sensors and software to detect road and engine noise and then counter the noise using the car speakers. It's like noise-canceling headphones for the whole cabin.

The systems also are designed not to interfere with noises such as the radio or speech within the car, according to the company.

The company has installed its "Safe & Sound" technology on a standard Toyota Prius, the world's most popular hybrid, as a demonstration vehicle.

Not everyone loves the idea of making hybrids louder, however.

In comments on Green Car Congress and Treehugger, a number of environmentalists said the reduced noise of hybrids is one of the reasons they like the vehicles.

"It's a good thing such electronics weren't available when automobiles first hit the road, otherwise all of today's cars would still be making the sound of horse hooves striking cobblestones, in order to ‘help to identify vehicle distance and speed,' reads one Treehugger comment signed "Michael Long."