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Electric cars and noise

The sound of silence

May 7th 2009
From The Economist print edition

Sound generators will make electric and hybrid cars safer

WHEN cars run on electric power they not only save fuel and cut emissions but also run more quietly. Ordinarily, people might welcome quieter cars on the roads. However, as the use of hybrid and electric vehicles grows, a new concern is growing too: pedestrians and cyclists find it hard to hear them coming, especially when the cars are moving slowly through a busy town or manoeuvring in a car park. Some drivers say that when their cars are in electric mode people are more likely to step out in front of them. The solution, many now believe, is to fit electric and hybrid cars with external sound systems.

A bill going through the American Congress wants to establish a minimum level of sound for vehicles that are not using an internal-combustion engine, so that blind people and other pedestrians can hear them coming. The bill’s proponents also want that audible alert to be one that will help people judge the direction and speed of the vehicle. A similar idea is being explored by the European Commission.

Researchers made sophisticated recordings of Toyota Prius hybrids running on electric power and petrol-engined cars approaching at 5mph from different directions. These were played to a group of subjects wearing headphones. The subjects were asked to press one of two buttons to identify which way the vehicle was coming from as quickly and accurately as possible. As expected, they could determine the direction of the petrol-engined cars much faster. When natural background sounds, like the engine of a parked car, were added, the hybrids’ direction sometimes could not be detected until they were perilously close. Both sighted and blind subjects gave similar results.

Dr Rosenblum and his colleagues recently repeated the experiment outside in a car park. This time blindfolded subjects stood three metres away from the point where the vehicles passed. The researchers found that the hybrid vehicles had to be around 65% closer to someone than a car with a petrol engine before the person could judge the direction correctly.

Systems to make noises are already being developed. Lotus Engineering, the consultancy of a British sportscar-maker, recently signed an agreement with Harman Becker, a producer of audio systems, to commercialise one. Lotus has worked on a number of hybrid and electric vehicles and it was while these were moving around its factory that the engineers thought they would be safer if they made a noise.

It is possible to create a different sound within a car from the one that is heard outside, says Colin Peachey, a chief engineer with Lotus. Manufacturers could create their own sounds according to how they perceive their models. Carmakers already take engine noises seriously enough to use acoustic engineers to tune exhaust pipes, especially for high-performance cars. Drivers of electric cars might in future even be able to select different engine sounds, and maybe download them like ringtones.

Unique engine noises would still be possible. A sound-generator will be fitted to the Fisker Karma, a luxury plug-in electric hybrid which goes into production later this year. It will both alert pedestrians and enhance the “driver experience”, says Russell Datz of Fisker, based in California. As the Karma uses new technology it is fitting that its sound should also be new, he adds. But Fisker still has to decide what a luxury electric car should sound like.