The future sound of our cities is already being decided.(Getty Images: Andrew Merry)
The dull hum of the internal combustion engine, our global urban soundtrack, will one day be replaced by its silent successor: the electric car.
Vehicle manufacturers have long tinkered with the noise made by exhaust pipes and tyres. They spend "millions of dollars a day on just how the car sounds", according to acoustician Luke Zoontjens.
BMW even had a designer create the precise click of the car door.
But at slow urban speeds, a battery-powered vehicle is a sonic blank slate — and car companies are already attempting to fill it.
Atlanta-based electronic musician Richard Devine was recently hired by Jaguar to help develop the "entire audio language" of its new I-Pace electric car.
There's no escaping that the hum of a petrol-powered car engine comes weighted with personal and social histories.
"When you're dealing with an electric vehicle, it can quickly sound like you're driving a tiny scooter if you're not careful," he warned.
That new car sound
On the road, sound has always been both functional and aesthetic.
We use it to recognise the danger of a truck reversing, explained Darrin Verhagen, a sound designer at RMIT University, but also to send messages about sophistication and importance. Think of the roar of a sportscar.
In the case of Mr Devine, the company gave him visual and audio references to build on — "pleasing, elegant, smooth, electrical and futuristic" — but he also nodded to pop culture. Think podracers in Star Wars and those light cycle bikes from Tron.
But however free-ranging the brief, sound designers' first instinct may be to reflect the intimate car memories of their customers.
"If you're trying to appeal to someone with high disposable income, who may be over 40 or 50 … that may well be by delivering to them the sonic language of the vehicular experience they grew up with," Dr Verhagen said.
Mr Devine recorded the sound of Jaguar's traditional engine-driven line to find its audio signature: what a marketer might call its "purr".
He worked to remodel some of the harmonics from the original engine, so there would be a recognisable character to the new experience, while adding additional layers to create a "hybrid sound".
In the future, however, people may become less emotionally attached to the traditional auditory experience of a Harley Davidson or Lamborghini — giving sound designers virtually limitless scope.
Iain Suffield, an acoustics technical specialist with Jaguar, said the physical shape of the car and the location of the sound also played an important role.
"If we wanted a very powerful sound, we might bias it low frequency and we might bias it quite low in the car," he explained.
"If we wanted a sharper, shriller, edgier sound, we might bias it higher and forwards in the car, so it sounds like the car is pulling you forwards."
The sound of safety
Electric cars will also need to have exterior safety sounds.
At slow speeds, they are much quieter than vehicles with internal combustion engines and thus pose a danger to pedestrians.
At higher speeds they make a similar racket to old-fashioned cars, thanks to tyre and wind noise.
The United Nations recently introduced a regulation requiring electric vehicles to install an Acoustic Vehicle Alerting System; the Australian Government is considering the case for a similar requirement.
Mr Suffield said the challenge was to design a sound that is "psycho-acoustically useful" — in other words, easily understood as a warning by passers-by.
He described the car's safety sound as a low, whirring hum.
The electric car is a sonic blank slate. (Getty Images: Carolyn Hebbard)
In fact, vehicle safety sounds have already evolved. Mr Zoontjens, an acoustics principal at SLR Consulting, explained that a truck reversing on a worksite, for instance, will often use a "broadband sound" these days rather than a shrill, tonal "beep beep beep".
A broadband sound uses a wide range of frequencies at once. Distant traffic, ocean surf or the whirr of a fan are generally considered broadband.
It could also be a solution for the exterior of electric cars, he suggested, as broadband sound can be heard easily nearby but is "less noticeable and less annoying at further distances".
The hum of the city
But while much of the discussion around out-of-car sounds has focused on safety, there's always the possibility it could be another marketing opportunity.
In the future, each electric car might have its own signature, in an attempt to confer a sense of quality on the brand every time the car is heard.
If so, what will happen when traffic gets heavy? Will we be overwhelmed by a cacophony of safety sounds and uniquely branded hums and buzzes?
The roar of our cities is already growing. While many cars types have gotten quieter, there are now simply more of them.
In New South Wales, for instance, the general level of road traffic noise is thought to have increasedsignificantly in recent decades.
The sound of traffic could change with more electric cars on the road. (Unsplash: Connor Williams)
It's a balance, however. Meisha Stevens, an acoustic engineer at Wood & Grieve Engineers, said that while low frequency sounds like traffic can be annoying, they can also have a masking effect.
Without these sounds, we may become "hyper-aware" of other, more irregular noises that are not being muffled by the steady din of engines.
"Things that are constant like traffic going by — they do play a role in our soundscape," she said.
For now, the future sound of our cities rests in the hands of a few musicians and sound designers.
Unless the rule-makers step in, it's likely that every available murmur, crunch or wheeze will exploited as a sonic billboard. It's valuable sensory real estate.
"Absolutely there will be a battle playing out for the hearts and minds of car consumers, and sound will have a fundamental part to play in that," Dr Verhagen said.