The sound of silence

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.
Although there is little data on accidents, the latest research suggests there is cause for concern. Vehicles operating in electric mode can be particularly hard to hear below 20mph (32kph), according to experiments by Lawrence Rosenblum and his colleagues at the University of California, Riverside. Above that speed the sound of the tyres and of air flowing over the vehicle start to make it more audible.
The 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 tickover 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.