The technology was demonstrated in a that showed a standard Toyota Prius outfitted with Lotus Engineering's artificial engine noise technology.
Lotus said the cost of the technology is 'relatively modest' and similar in price to a 'conventional radio system.' 

When the car is operating on the electric motor only, a synthesized sound is projected through waterproof speakers mounted at the front of the vehicle. The system is automatic, and Lotus said the driver 'hears almost none of the additional sound.' Lotus engineers say the artificial engine noise 'sounds exactly like a conventional engine.... In order to generate the engine sound, recordings of a suitable donor engine were made,' the company said in a statement.

Hybrid and electric vehicles have been criticized because they are quiet when operating in electric mode. 'Quiet vehicles are highly problematic for blind pedestrians, who depend on the sound emitted by cars in order to travel safely and independently,' said the National Federation for the Blind on its Quiet Cars Web site.

Lotus Engineering said it can 'make any sound at any volume,' presumably making it easier for legislators to establish federal guidelines on how loud the sound coming from hybrid/electric vehicles should be.

Toyota has said it is studying the issue of hybrids that are too quiet. But the automaker has also noted that on the opposite side of the issue are advocates of reduced noise pollution.

No word yet from Lotus Engineering on when its artificial engine noise will be rolled out in a production vehicle.