The Future of Cars is Electric! How will this change our designs? (Part 2 - Acoustics)

Recently it seems that the technology and the public opinion surrounding electric vehicles have advanced enough that they look likely to form a significant part of our environment in the relatively near future.

Jeremy Climas has published a blog post describing some of the aspects of the changes this might bring for the Mechanical and Electrical engineers here at Max Fordham. In the Acoustics team, we have also been thinking about how this might make noise produced in the future different to what we can go out and measure today.

Road noise is the largest single contributor to most people's exposure to sound throughout the day. It seems common sense that if a significant portion of cars are replaced by electric models, noise levels produced by roads would be greatly reduced. After all, the first time many people notice electric cars is seeing one sit silently at traffic lights before pulling away with barely a sound.

However, before we get excited and tell you that in 10 years’ time you’ll be able to sleep with your bedroom window open (even if your house overlooks a motorway), it’s important that we think about why cars are noisy in the first place.

If you ask most people what noise a car makes, they are likely to think about the noise of the engine, and perhaps make some sort of raspberry sound. This engine noise, along with any of the other mechanical parts of the car, are collectively called the “propulsion noise”.

This isn’t the only source of noise, though. The tyres against the ground and the car moving through the air both make up a significant part of the noise produced by modern cars. These sources make up what is called the “rolling noise”.

Several sets of measurements have found the relationships between the two sources for cars travelling at different speeds. These are shown in the graphs below.

The propulsion noise increases with speed in a linear way, getting louder as the speed of the car increases. Rolling noise is lower than propulsion noise at low speeds. However, at around 20mph, the rolling noise becomes the main source of noise for modern light vehicles.

The way that sources of noise add together means that, at high speeds, the propulsion noise has no significant effect on the total noise of the car.
 

Replacing the engine of a car with an electric motor reduces the sound power of the propulsion noise by an average of around 10dB (a difference that typically feels about half as loud), but does nothing to reduce the rolling noise.

The result of this, which has been found to be the case in real-world tests, is that electric cars are only noticeably quieter than cars with an internal combustion engine at speeds of less than about 20mph.

In addition, safety concerns based on the low noise levels at low speeds have led the EU to introduce laws requiring all new electric vehicles to be fitted with an Acoustic Vehicle Alerting System (AVAS) by 2019 and all existing vehicles to be retrofitted with one by 2021.

These systems are designed to replicate engine noise to some degree, so that pedestrians, especially those that are partially sighted, are made aware of the approach of a vehicle. While there is no requirement for the AVAS to be as loud as the engine of a conventional car, at least some of the noise eliminated by electrifying the propulsion will be added back in, in the interests of safety.

At this point, you may conclude that electric vehicles will have a negligible impact on the noise produced by most roads in the future. However, cars aren’t the only sources of sound on the roads.

Compared with cars, heavy vehicles (buses, lorries, construction vehicles, and so on) produce a very different character of noise. Their propulsion noise is much louder, relative to their rolling noise, and continues to be so up to speeds of around 50mph.

While there has not been as much attention on the electrification of heavy vehicles as there has been on cars, thousands of electric buses are being introduced around the country, and BMW, Volvo and Tesla have all announced the production of electric lorries.

Measurements of a few early models of electric heavy vehicles suggest that engine noise could be reduced by between 10dB and 15dB. Assuming this to be the case, we could see a significant reduction of noise from heavy vehicles, even up to speeds of 60mph.

In order to try and predict how these differences might change the noise made by real roads, we cross-referenced the data shown in the graphs above with real traffic data (including the number of vehicles, the average speed, and the percentage of heavy vehicles) for a patch of East London that includes a number of roads of different sizes and types.

In the graph below, it is assumed that the traffic flows stay the same as they are now. Three sets of noise levels are shown; the noise as it is estimated to be now, the predicted noise if 100% of cars became electric, and the predicted noise if 100% of vehicles, including heavy vehicles, were electrified.

Looking at the results, it seems that electrifying all cars would only have a small effect on overall noise levels, with levels reducing by no more than half a decibel.

Electrifying all heavy vehicles as well would reduce the noise levels produced by all the roads modelled here by at least 3dB. This isn’t a huge reduction, but it could be enough to make sleeping with the windows open a little more comfortable.

With all things considered, it’s too early to start building new homes assuming the noise coming from the road outside won’t bother people in a few years’ time. Even if all vehicles, including heavy vehicles, become electric, we still aren’t expecting a significant change.

As global temperatures rise, people living in older buildings may be forced to open their windows more often to keep cool. If electric heavy vehicles don't become common relatively quickly, it seems likely that more people, rather than less, will be disturbed by noise in the future.

Josh Rodell is an Acoustic Engineer in our London office and can be contacted on 020 7267 5161 and by email at j.rodell@maxfordham.com.