In many test-drive reports, the electric car is described as being totally silent. From the acoustic expert’s perspective, however, the electric car’s silence is a bit of a misnomer.
"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 Spång at Volvo Car Corporation’s Sound laboratory.

Altered acoustic environment
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 this uniform 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 Martin Spång.

One of the automotive industry’s most modern acoustic labs Volvo Car Corporation’s new acoustic lab for cars, inaugurated in 2009, is one of the most modern in the automotive industry. Here, in its 2500 square metre concrete chamber, the silence is so oppressive that it is almost audible.
Sound-absorbing wedges coating the walls and ceiling are designed to dampen reflecting sound. The lab is entirely insulated from the rest of the building to prevent any background sound from seeping through.
"When we inaugurated the lab we recorded an interior noise level of 17 decibels. This is such a quiet background noise level that you’d be able to hear someone’s tummy rumble from the other side of the chamber," says Martin Spång.
By way of comparison, the volume of a normal conversation is 60-70 dB.

Simulating various surfaces
The floor is hard and smooth so as to resemble and reflect sound in the same way as a road surface does. Test cars can be driven on rollers with various surface treatments to simulate different road types.
"The vehicle’s sound level is closely connected to the luxury car experience. Having access to a state-of-the-art acoustic lab is extra valuable now when we explore how electric power influences this crucial sound experience," says Peter Mertens.