In the distance, you hear the rumble of a high-performance engine approaching. Your experienced ear identifies it as a flat-six Porsche. Sure enough, a few seconds later a Boxster growls past, and you smile to yourself that once again your finely honed, petrol-head senses haven't let you down.
Or have they? This is no ordinary Porsche, it's a Boxster E. The flat-six has been replaced with a liquid-cooled lithium-ion battery, driving an electric motor in the rear axle. That might sound like heresy to a brand purist, but it's not surprising in a market where almost every manufacturer is developing or preparing to launch an electric model. The surprise here is that, despite its electric motor, it sounds exactly like its petrol-driven brothers.
Instead of being emitted by twin exhausts, the notes are played from a loudspeaker, rising and falling in pitch and volume with the engine speed. The driver, incidentally, has another speaker inside the car so he or she can share in the illusion.
Is this the result of a Porsche engineer with a sense of humour and too much time on his hands? Not a bit of it. The Boxster E may only be a development model, but it represents Porsche's solution to a problem that the motor industry, countless pressure groups, and legislators worldwide are currently grappling with.
Engineers have been working for decades towards the holy grail of zero-emission motoring. Counting the lack of engine noise as a welcome bonus, no one gave much thought to the fact that it warned other road users, particularly pedestrians, of an approaching vehicle.
Pedestrians rely as much on their ears as their eyes when deciding whether it's safe to cross the road. Pressure groups in the US representing a variety of road users, including the blind, successfully campaigned for a solution to the problem, resulting in the well-intentioned Pedestrian Safety Enhancement Act.
Similar legislation will surely follow in Britain and the rest of the EU, with sources for the latter confirming: “We are closely and actively following developments at an international level on this issue.”
Existing legislation states, in effect, that motorised vehicles must emit a noise to warn other road users of their presence. What it does not dictate, however, is what that noise can or cannot be, nor how loud it should be. Those details are, at the moment, being left to the makers.
History of is littered with examples of where lawmakers got it wrong, but they pale beside the number of catastrophic decisions made by an unregulated marketplace. Left to their own devices, what noises are the manufacturers planning to impose upon electric car buyers?
Porsche's fellow German manufacturers are approaching the issue with customary thoroughness. Audi and Mercedes are keen to formulate a sound that will have some form of identity, but they appear to be treading a different route from Porsche. Mercedes talks of “a naturally produced sound that does not mimic an internal combustion engine”.
Dr Ralf Kunkel, head of acoustics at Audi, says: “One obvious approach for an electric car sound would be to work on the basis of the familiar sound of a combustion engine. But we're studying several other approaches at present.
“Some sounds, such as the rustling of leaves or the twittering of birds, are not viable options for Audi. However, the sounds used for spaceships in films are reminiscent of car sounds, yet also very different, making this a rather interesting approach.”
Nissan is far more explicit with its Leaf, with a turbine-like hum at frequencies ranging from 600Hz to 2.5kHz depending on the speed of the car. The seven-person development team, headed by Tsuyoshi Kanuma, consulted extensively with classical music composers before arriving at the final sounds. The noise needed to carry well without being too loud, and a breakthrough came after examining how opera singers make themselves heard above the sound of the orchestra.
Before his current role, Kanuma had spent 30 years as a vibration and noise engineer, trying to make conventional Nissan cars as quiet as possible. The irony in this change of focus certainly doesn't escape him.
In Britain, Lotus Engineering has developed the Halosonic sound system, which offers a choice between V8 and V12 engines or, like Audi, a spaceship. These and similar firms represent the more responsible end of the market.
Bran-Rae, which produces and distributes electric vehicle sounds, is a somewhat less established firm. Its EVtones website offers a seemingly unlimited range of noises for electric vehicles, which are simply downloaded onto MP3 players in the same manner as mobile phone ringtones, then played through the vehicle's external speaker. An indication of how serious they are is their suggestion that you “pull up next to that Harley, switch to chopper sound, and have fun as they listen in disbelief”.
It also claims that every sound has a tone embedded in it at a frequency that only dogs can hear, in order to “to alert seeing-eye dogs”.
Before we dismiss this as “only in California”, reflect for a moment that the same 18-year-old who currently fits an exhaust the diameter of a baked-bean can to his Opel Corsa will be able to download a custom tone and be playing it on the main road moments later. Beauty is in the ear of the beholder.
Hope of establishing some sensible boundaries can be found at Warwick University, where Professor Paul Jennings is leading research into alternative vehicle noises, based on the premise that sticking with the sound of conventional internal combustion would be a wasted opportunity, since high levels of traffic create a din that is intrusive and annoying for many people.
Jenning's team is looking for sounds that are more pleasant but still as safe as possible and which people will still associate with approaching traffic. As part of the project, the team has been driving an electric vehicle around the university campus, demonstrating and testing a range of specially created sounds.
Current favourites include a deep buzzing, a high-pitched hum and one that sounds like a piece of cardboard stuck in the spokes of a spinning bicycle wheel. Groups of blind and partially sighted people have been invited to the university to give their verdict. These demonstrations also involve synthesising how a whole town of electric cars would sound. Only by doing so can you hope to make any meaningful comparisons between what we have now and what we might have in 10 years' time.
There are still only about 2500 electric cars in the UK. To push that figure up will take a combination of economic necessity, environmental awareness and style - and that includes a distinctive, stylish sound. - The Independent