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.

Beep, beep

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.

What sort of noise should electric-powered cars make? They could, perhaps, beep as some pedestrian crossings do, or buzz like a power tool. Having worked with blind subjects, Dr Rosenblum is convinced of a different answer: “People want cars to sound like cars.” The sound need not be very loud; just slightly enhancing the noise of an oncoming electric vehicle would be enough to engage the auditory mechanisms that the brain uses to locate approaching sounds, he adds.

Systems to do this 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.

The system Lotus uses was originally developed for a different reason: to cancel out intrusive noises inside a car. Sound-cancelling works by analysing any unwanted frequencies and then producing counteracting ones. The Lotus system was adapted so that it could also produce sounds that change with speed and use of the throttle, providing a familiar audible “feedback” to drivers of vehicles with a silent engine. Adding external speakers allows pedestrians to hear the noise too.

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.

Although some drivers might want to cruise in an electric car thundering to the sound of a mighty V8 engine, it is not necessary—and traffic police may have something to say about it. Synthesised engine noises could even help reduce noise pollution, says Mr Peachey. For instance, sound from the speakers at the front of an electric car (or the rear if reversing) is highly directional. This means it is more likely to be noticed by pedestrians in front or behind the vehicle. The noise from an internal combustion engine, however, radiates in many directions—including upwards into offices and bedrooms.

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.