Road noise can be quite disturbing for people who live close to a road. It can wake them up at night and generally adds to a background noise which reduces a person’s peaceful enjoyment. Are electric cars the answer to this problem? To answer that, we need to know what roadway noise consists of.
Roadway noise is a compound of:
- Tyres rotating on the tarmac (tyre noise)
- Air being deflected around the car (aerodynamic noise)
- The engine producing power for the wheels (engine noise)
- The car’s transmission and other mechanical noises (transmission noise)
- The brakes on the brake discs (brake noise)
When a car is stationary, there is no tyre, aerodynamic, transmission or brake noise in a car with an internal combustion engine (ICE – i.e. petrol- or diesel-powered). In an electric car, or EV, there’s no engine noise either, although there may be a faint background hum of some kind of motor working, such as an air conditioning fan.
Score 1 to the EV: when traffic is stationary, the EV will be all but silent (unless the manufacturers have programmed a synthetic sound to alert pedestrians).
Once traffic starts moving, the noise characteristics change. Acceleration in a fuel-powered engine is noisy, depending on how hard the vehicle accelerates. Tyre noise will increase – it’s the noise created by the treads of the tyre deforming as they come into contact with the rough road surface. There will be minimal aerodynamic noise while the speeds are low – under 15mph. There will be some transmission noise, too.
Electric vehicle motors make very little sound regardless of whether the acceleration is abrupt or slow. They also have much more simplified transmission systems – no gearbox, for example. But, the EV still has to roll on tyres, so noise will increase. The EV is still ahead, though.
Once the EV and the ICE reach 50mph, the majority of the noise is tyre noise. Here, the type of tyre makes much more of a difference than any other factor. Some tyres can be 10dB louder (that’s twice as loud) as others. Aerodynamic noise is also significant, especially at low frequencies. Engine and transmission noise is now a minor component in the car’s overall noise.
This levels the playing field between the two types of car: at motorway and open road speeds, an EV will be only slightly quieter than an ICE car (less than 3dB, which is the threshold for human perception of a change in volume). Where they will make a difference is at busy urban intersections where idling and accelerating engines create much more noise than an electric vehicle.