When you turn an electric car on, it’s virtually silent. You might hear a faint whine of various components, but it’s nothing like the persistent growl of a petrol engine or the relentless clatter of a diesel engine.
What is vehicle noise made of?
The engine’s noise isn’t the only component of sound when you hear a vehicle drive by. There are three components which, together, are called roadway noise:
- Engine/transmission noise
- Aerodynamics and braking noise
- Tyre/road surface noise
At low speeds, engine noise in a vehicle with an internal combustion engine is by far the biggest component of the noise, with gearbox noise slightly less. If you want to hear extreme gearbox and transmission whine, check out this video
At slow speeds – less than 10km/h – the electric car has negligible noise, but the fuel car is up to 10dB louder, or twice as loud (dB means decibels and is a unit of measuring sound). The balance narrows as the speeds increase, though, because wind noise and tyre noise increase.
Wind noise is usually at low frequencies and therefore, while we don’t perceive it as loud, it actually is loud. Cyclists can experience noise amplitude (volume) of up to 80dB while cycling at moderate speeds, and the same amount of sound is created by wind being deflected around and under the vehicle. 80dB is the volume of shouting as loud as you can. Except that it is at low frequencies where we don’t perceive it as being so loud.
Tyre noise is the final component. The amount of noise depends on the road surface and the tyre compound. Tyre noise can be up to 10dB different between types – that’s twice as loud. Above 30km/h, tyre noise and aerodynamic noise become the predominant noise sources therefore unless the vehicle has a particularly loud engine, electric vehicles and fuel vehicles sound very similar.
When electric vehicles become more prevalent, areas of lower speed traffic will be quieter, but this will mean that pedestrians will have to be more careful as they won’t hear an electric vehicle coming while they are distracted on their phones or with their headphones on. The perceived noise benefit of an electric car may encourage manufacturers of petrol and diesel cars to innovate to make theirs even quieter, further reducing the difference. Because there is already a negligible difference at higher speeds, people living near motorways will not notice much difference until we get electric trucks (a much bigger component of truck noise is engine noise).
Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild
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