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Last year London's 8,000 or so buses traveled 279 million miles and completed 2.23 billion journeys – more than half of all bus trips completed in England. But they're not green enough. Yet.

According to mayor Sadiq Khan, London has Europe's biggest electric bus fleet with more than 200 currently in use. It's a small fraction of the total number of buses but there are already two fully electric bus routes: 43 (running between Muswell Hill and London Bridge) and 134 (North Finchley to Warren Street).


Now the capital's electric buses are set to get a new soundtrack. The purpose? To help protect people around them. As electric vehicles are almost silent at lower speeds, they're potentially dangerous to the pedestrians, who may not be paying attention or live with visual impairments. (Research from the US Department of Transportation has even suggested people are 40 per cent more likely to be hit by an electric or hybrid car than one with a noisy combustion engine).

From July this year, EU regulations stipulate that all new electric need to include an Acoustic Vehicle Alerting System (AVAS). This system must play a sound to alert people to the vehicle when it's moving. Cars moving at speeds of less than 20kph must have a minimum sound level of 56dB and older electric cars have to be retrofitted with a sound system by 2021.

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The EU regulations also apply to London's buses. In order to create a consistent sound across the city's buses, Transport for London (TfL), the transport regulator, has commissioned a sound to be used by all electric buses in the capital.

The sound itself – an electronic composition – is a short loop that will play constantly when a bus is in operation. When a bus is stationary a soft chord (technically, F#maj7) plays continuously with two rhythmic pulses added over the top. When the bus starts moving, a beacon sound (a single note of C sharp) is played every three beats over the rest of the track.


"It's meant to be super easy to listen to," says Matt Wilcock, of Zelig Sound, which created the short track for TfL. It's the first time the company has designed a sound for a moving vehicle, most of its work involves creating sound for TV commercials, films and games. "This approach of it being rhythmic and having a beacon sound has not been considered before. We wanted it to be nuanced enough that even people who are into music and sound are still going to appreciate it but also it has to be loopable, constantly."

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Wilcock started working with TfL on the sound early this year and says that aside from adding creative elements to the production, the focus has always been on safety. The company met charities, including those representing people with visual impairments and guide dogs, cycling groups, and environmental groups for feedback on the bus sound.

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Early iterations of the sound, which consisted of the ongoing chord and no rhythmic elements, faced criticism for not being distinct enough. "It wasn't going to stick out enough, it was too simple," Wilcock says. "That really focussed a set of rules." TfL and Zelig Sound have since conducted field trials around Tottenham, where the sound of the bus has been tested with pedestrians.


TfL says the new sound will be trialled from January for six months on the 100 bus route, running between St Paul's Cathedral and Shadwell. There will then be trials on the C10 route between Canada Water and Victoria from March, and the P5 route between Elephant and Castle and Battersea from May. The AVAS sound will be used across all of London's bus operators and the group says it "will be available to transport providers across the UK".

But why not just make electric buses sound like other combustion engine London buses? The option was never really a question, Wilcock says. "To me a diesel bus feels like a headache and it feels dirty," he says. "It's noisy and rattly. I generally don't get why you would want a diesel bus."

An independent analysis of the final track says it has characteristics that many people listening would recognise as "calmness, relaxation, and steadiness". However, the sound wasn't designed to be listened to continuously, Wilcock says. Instead it's meant to be heard whenever a bus approaches or is sat stationary for a few minutes at a bus stop – it's unlikely, even with London traffic, for a pedestrian to be walking alongside an electric bus for a long time.

The average speed of a London bus, according to TfL, is 9.28mph and it's at around this speed the sound will be at its optimum. The rules around AVAS say that as a vehicle increases its speed the pitch of a sound it emits has to increase, to alert those around it of the change. (Above 12mph the track fades out as the movement of electric buses becomes louder).

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"It feels like a piece of 20th century minimalism, but slightly electronic," Wilcock says. "We were saying when there are thousands of busses on the road, it's going to be like a giant minimalist sound installation."

But the track does face challenges when out in the real world. London buses don't have studio quality speakers or equipment. Instead they've been fitted with two speakers that Wilcock says are better designed for audio announcements than playing a multi-layered track. In future versions, the two speakers are going to be replaced by one new speaker that's better suited for the job.

As the AVAS rules have been in place since July, creators of electric vehicles are also designing their own sounds for cars. Jaguar Land Rover commissioned a separate track for its I-Pace vehicle and a concept car from Citroen has experimented with the idea of using a blend of male and female voices into a harmony.

While Citroen's concept is unlikely to ever reach the roads, sound designers will be experimenting with how the next generation of vehicles will sound. With their tests, the sounds of the urban environment around us is likely to change. "From a soundscape point of view it is going to change the city," Wilcock says. "Cities are going to get way more quiet but there's going to be other sounds out there."

Looking for a new car? Check out our guide to the best electric cars available.

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