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17 September 2015

What will cities sound like when they're filled with electric vehicles?

A MOVING MOTOR VEHICLE unavoidably makes noise in three main ways. First, the sound of the engine and exhaust. Second, the swish of air over the vehicle. Third, the noise of the tyres on the road surface. There are also the possibilities of sounding the horn and playing music with a one-kilowatt subwoofer in the back, but these are optional extras.

The second and third factors are significant for vehicles moving at high speed. Tyres make a noise by simultaneously compressing and releasing innumerable tiny air pockets trapped between the tarmac and the rubber, as if the vehicle were rolling along a strip of bubblewrap. Smoother road surfaces are less noisy but offer less grip in the wet.

But traffic doesn’t move particularly fast in cities. Transport for London figures for 2012 show that average speeds on major roads across London were slightly under 20 mph, and around just 9 mph on major roads in central London. This leaves the noise of engines and exhausts as the major factor in urban traffic noise, excepting countries where high accident rates provoke drivers to sound their horns a lot.

Urban traffic produces a lot of rumbling noise from around 200 Hz right down to the infrasonic frequencies which we can’t hear and which most mics are only weakly responsive to. You can see this in a spectrogram showing the two channels of a stereo recording I made beside west London’s busy Cromwell Road:

The traffic was pretty loud, but there’s nowhere outdoors in London where you can’t hear traffic rumble. It’s like the sound equivalent of atmospheric haze and it’s there because low-frequency sounds are much better at travelling long distances than high-frequency ones.

A pure tone of a bassy 100 Hz has a wavelength of nearly 3.5 metres which can bend around obstacles like walls and buildings. One of 2000 Hz has a wavelength of just 17 cm, small enough for the human head to start creating the acoustic shadow which helps our sense of hearing locate where sounds are coming from. Higher-frequency sounds can also be attenuated outdoors by the atmosphere itself, depending on its temperature and humidity.

The motors of electric vehicles don’t make much noise at all and you might expect cities to become a lot quieter once they’re a popular way to get from Acton to Balham. It’s curious to think how, within the lifetimes of many people alive today, petrol and diesel vehicles might end up largely confined to museums or doing laps at nostalgic ‘internal combustion fairs’. You can imagine earnest talk of how much more soulful the petrol engine was and how the world has turned into one big padded cell devoid of all danger and excitement.

Marta Santambrogio’s Fuzzy Logic Project looks forward to a future where electric cars compensate for their relative silence by playing music, turning traffic into a ‘moving orchestra’:

The Fuzzy Logic Project notes optimistically that A recent European law states that new models of electric and hybrid vehicles will have to make a noise by 2019: a great design opportunity! Manufacturers have already begun fitting noise-making apparatus, such as the Nissan Leaf’s ‘vehicle sound for pedestrians’ and this exciting jet-like swoosh from Harley Davidson:

Unfortunately, a close reading of the European Union directive mentioned by the Fuzzy Logic Project suggests tight constraints which won’t leave much room for cars playing tunes like ice cream vans. Article 8 of Regulation (EU) No 540/2014 requires that manufacturers start fitting an ‘Acoustic Vehicle Alerting System’ or AVAS to all hybrid and electric vehicles from 2019. The kind of sound is further defined in Annex VIII:

The sound to be generated by the AVAS shall be a continuous sound that provides information to the pedestrians and other road users of a vehicle in operation. The sound should be easily indicative of vehicle behaviour and should sound similar to the sound of a vehicle of the same category equipped with an internal combustion engine.

The reasoning behind this is obvious enough. Electric vehicles need to make a noise otherwise pedestrians will be at greater risk of being run over by them. Trams already deal with this by ringing bells frequently along their routes (although residents in Edinburgh’s posh West End district successfully lobbied last year to have them silenced). Many people crossing the street already seem to have quite poor situational awareness, either through the miracle of alcohol or the irresistible Insect-o-cutor glow of a smartphone screen, so they’re going need all the help they can get. This rules out the use of novel sounds.

The dismaying conclusion is that the London traffic of the future may sound like the present, only more so, given that the city’s population will continue to grow. The best that can be hoped for is that manufacturers will reproduce fake internal combustion engine noises minus the lowest frequencies which carry the furthest, although this may clash with the widespread perception of bass tones as somehow giving you more for your money and, particularly for men, conveying an impression of potency.

More on the possible future sounds of London in another blog post soon.

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