Published on June 5th, 2016 |
by Michael Barnard
June 5th, 2016 by Michael Barnard
There’s a lot of discussion and publication about electric cars being quieter. Some of it is focused on low-speed pedestrian and cyclist safety. Some of it is focused on reductions in traffic noise as it pertains to the health of people living near roads. But how much quieter will traffic really be due to electrification of transportation? The answer is interesting and complex, as almost everything related to noise and perceptions of noise is.
Basic traffic noise is a combination of engine noise, tire noise, road noise (different surfaces have different noise characteristics), and wind passage noise. The faster vehicles are traveling, the more road and tire noise dominate, and the more wind passage noise contributes.
Electric cars almost entirely eliminate engine noise, and the relatively high-pitched noise electric motors do emit doesn’t propagate as far. They currently tend to have lower coefficients of drag, reducing wind passage noise at higher speeds, although this is likely to diminish as an advantage as batteries get cheaper and more power and it’s easier just to push air out of the way for stylistic “advantages.” However, these contributions are enough lower than road and tire noise that the impacts will likely not be noticeable to most people most of the time. Electric car tires currently tend to be quieter than other tires but that too will likely diminish as an advantage, and tires overall have become much quieter in the past few decades. There is hope, however, so keep reading.
What’s important is the breakdown of contribution of different elements of noise from vehicles. Most publications and analyses focus on one aspect or another, but not comparisons. That said, there are a couple of sources available for consideration.
This graph from a mid-2000s publication out of Purdue and the University of Central Florida is the most coherent resource I have been able to find on the subject.
For contrast, this Netherlands report on efforts related to diminishing traffic noise is also useful to look at.
Note that, in one case, tire and road noise are separated, and in the other, they are merged. This is a result of different methodologies and approaches. There are lower and higher noise tires and lower and higher noise roads.
In general, these are broadband, dBA (weighted to human hearing) noise levels. As a result, they tend to be additive. However, the decibel scale is logarithmic, which means the apparently high level of 60 dBA at 50 kph in the first chart is only about half as loud apparently as the ~68 dBA for tire noise.
Further confusing things, humans can only perceive a difference in noise of 3–5 dBA. The variance between the tire noise contribution and the overall noise is under 3 decibels per the Purdue source. That means that, even at 110 kph (slow highway speed), eliminating the contribution of both engine and wind passage noise might not be a discernible difference to someone standing by the road.
However, there are four other factors at play which will make a difference to greater and lesser extents.
The discussion so far pertains to basic noise with well-tuned mufflers. It doesn’t account for the spiking noise of broken mufflers or intentionally noisy internal combustion vehicles such as Harley Davidsons, sports cars, and coal-rolling pickup trucks. These spike sounds can be very disruptive and much more annoying than basic traffic sounds. Electric vehicles don’t have mufflers, so there’s nothing to break there. And electric vehicles are rapidly taking over as performance vehicles due to their excellent torque characteristics. As electric vehicles begin to dominate, broken or intentionally noisy mufflers will substantially diminish. Noise pollution will be about as socially acceptable as diesel smoke emissions in the future, and about as necessary.
Squealing and chirping of tires will diminish. Electric motors are so much more controllable at all RPMs that traction control is much better. And torque jolts with shifting don’t exist in single-gear sports cars, so occasional chirps shifting to second or third disappear. Watch any “Tesla vs embarrassed sports car” drag race and you’ll notice the complete lack of smoke and tire spinning from the Teslas. They just roll up to the line and disappear into the distance without drama. This won’t help with painted pavement in parking garages of course, but that’s a relatively narrow problem.
Internal combustion engine noise and wind passage noise have strong low-frequency sound components (as does tire/road noise). Low-frequency sound travels further than higher frequency sounds with less attenuation as lower frequencies are absorbed less by atmospheric humidity (the most relevant factor for traffic noise). Eliminating the engine noise contribution to low-frequency sounds and diminishing the wind passage sound might very well shift the balance of road noise to reduce the distance it propagates somewhat. This is speculation on my part, however, based on my knowledge of acoustics, and relatively irrelevant compared to road and tire contributions regardless.
Tire and road noise have been strong focus areas for decades in terms of engineering reductions to traffic noise, for the simple reason that they are the largest contributors. As a result, there are much-less-noisy road surfaces mandated in many areas and tires have reached a bit of a plateau of quietness on the average vehicle. Basically, almost all traffic has become much quieter with the exception of intentionally noisy or otherwise broken vehicles in the past decades.
By some estimates, improvements in materials, construction, contours and, above all, tread patterns, have made today’s passenger cars less than 20% as noisy as those produced 30 years ago.
Commercial trucks have improved even more dramatically, producing less than 10% of the noise of their 1980s predecessors.
The references to 10% and 20% trigger the question as to whether that is absolute-sound-pressure level or perceived-sound-pressure level. It’s likely absolute, and given the logarithmic nature of decibel scales and how people perceive noise, that likely means that tires sound about half as noisy, not one-tenth as noisy.
So, what does this net out to? Well, basic traffic noise from highways won’t get perceptibly quieter due to electric cars. Urban road traffic noise below 40 kph likely will, as engine noise starts to dominate at lower speeds. And we can all look forward to fewer and fewer noisy motorcycles and trucks, and no economy cars with broken mufflers.
Here are some related references for those wanting to go deeper:
- The Fight Against Tire Noise – Tire Review Magazine
- Perception Vs. Reality: What Our Ears Hear
- Tire Noise
- Page on various aspects of noise
- Highway Traffic and Construction Noise – Problem and Response
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