Should electric cars be made to go 'vroom'?
With whisper-quiet electric cars set to proliferate, the motor industry is under pressure to give them an artificial noise for safety purposes, but should they sound like traditional petrol vehicles?
It is an unsettling experience watching a car drive around, hearing little more than the whisper of the wind it generates as it glides down the road.
There would have been little time to get out of its way had it gone unseen.
Such a moment is the essence of the debate over how electric and hydrogen fuel cars should sound in the future.
The answer could determine how different cities could sound in 10 or 20 years. The rise of the electric car presents a rare opportunity to tackle the persistent roar of traffic that many city dwellers are used to.
Electric and hydrogen fuel vehicles are inherently quiet. The sound of the tyres on the road is noisier than the engine and this could prove lethal at slow speeds for pedestrians and cyclists.
Motor manufacturers acknowledge there is a problem. Some, like Nissan, are already fitting speakers under the bonnets of these vehicles to emit sounds for the safety of other road users.
Others are planning on doing the same.
And with the UK government predicting 8,600 electric cars will be sold by the end of this year, sound engineers say there is a pressing need to come up with some ground rules as to what sounds to pick.
So why not just stick with the current sound of a conventional combustion engine?
"That would be losing a huge opportunity," says Prof Paul Jennings from Warwick University who leads a project looking into alternative vehicle noises.
"We all know traffic noise can be annoying and the levels are significantly high. We do not want to lose the benefit we could get from using new reduced sounds."
He says: "The challenge is to create sounds that are as safe as possible but also ones that are much more pleasing for the urban environment."
As part of the engineering project, an electric vehicle called Elvin (Electric Vehicle with Interactive Noise), is being driven around the university campus demonstrating a small sample of made-up sounds.
Chief among them are a deep sounding buzz, a high pitched hum and one that sounds like a piece of cardboard stuck in the spokes of a spinning bicycle wheel.
And then there is something that sounds like a 1950s UFO movie.
"It's important that the sound is associated with transport and a lot of how people interpret sound is based on their experiences," says Prof Jennings.
"For electric vehicles and futuristic vehicles a lot of those experiences are from the media, from games and from science fiction films."
There are certain noises that people associate with transport and they alert us that a vehicle is heading in our direction.
Whatever car manufacturers choose will only be effective if it does the same.
But balancing the needs for safety and improving the environment are not the only requirements. Clearly the manufacturers will have a big say in what happens.
If, for example, Ferrari were to ever make an electric car they would want it to sound like a Ferrari does now.
"Manufacturers know what the right sound is for their product," says Roger Williams, whose company Novisim provides computer simulators to help design car sounds.
"What they are keen on is a sound that matches the image of their vehicle, whether it's a sporty one or a luxury one."
Other road users will want to have their say too. In the next couple of months a group of blind and partially sighted people will be invited to Warwick University to give their verdict on what they hear.
The synthetic "engine noises" will be played to them through one of Novisim's computer programmes, which can simulate how a whole town of full of electric cars would sound all at once.
"Guide dogs are obviously trained to respond to the noises of vehicles," says Andrea Cooper, a student at the university who has been blind since birth.
"So obviously I think it's incredibly important for the sounds of the engines to reflect traditional engines sounds that we are used to."
The more you delve into the issue the more apparent the complications become.
How loud they should these vehicles be? Do they need to make a noise when they are stopped at the traffic lights? How would hundreds of unmatched vehicle noises sound altogether on our streets?
Researchers at Warwick University intend to make the results of their research available to the EU, which is planning to legislate on electric and hydrogen car sounds sometime in 2011.
The conclusions will determine the noise you hear when you open your window 20 years from now.