Volvo Car Corp. runs one of the industry's most modern acoustic labs for this purpose. "The aim is to fine tune the noise level from the vehicle in order to create a seamless, pleasant sound experience," says Peter Mertens, Senior Vice President, Research & Development at the Swedish carmaker. The reason for the efforts is that electric cars typically unfold their horsepowers without much acoustic ado - in fact they are almost completely silent. But this silence is not always desired.

"The combustion engine sound is instinctively connected to our perception of driving a car. It works as an acoustic mat that blankets other sounds. When that mat is lifted off, you suddenly become aware of a number of other sounds," says Martin Spang at Volvo Car Corporation's Sound laboratory.

For instance, in a plug-in hybrid car, which combines conventional diesel power to the front axle with an electric motor driving the rear wheels, sounds such as the splashing of diesel in the fuel tank now penetrate the driver's consciousness when the car runs solely on electric power. There is the same heightened awareness of noise from the road surface, wind, pumps, fans and relays.When the diesel engine cuts in again, however, the car sounds once more like we are used to.

In order to find the right acoustic profile for the Volvo V60 Plug-in Hybrid, Volvo Car Corporation's experts are working on the creation of an ideal, uniform sound in their computers.

The challenge is to adjust all the various noise sources so that the car reproduces a uniform and recognizable sound irrespective of the propulsion mode currently being used. "What's important here is to strike the right balance between traditional and new sources of noise. Some sounds can be isolated and removed. In other cases it is up to our suppliers to develop quieter components together with us. What is more, customers will get used to the fact that electric cars sound somewhat different. This will become part of these cars' personality - their attraction and their trademark," says Spang.

How the "ideal" e-car sound would "look" like is yet unclear. Experts give the advice that it should be distinguishable from conventional cars with internal combustion engine. An overly futuristic sound is just as little desirable.