How quiet is too quiet for an electric vehicle?

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ATTI’s resident columnist believes it is only a matter of time before someone files a legal claim that an electric vehicle is too quiet.

There’s a reassuring narrative that many of us tell ourselves on airplanes. For example, you hear a loud ‘barking dog’ sound on an A320 but you don’t know it’s just the hydraulic pump. You’ll probably tell yourself some version of, “Wow that’s a strange sound, but I’m sure it’s normal for this equipment.” Truth be told, most of us would rather not know what the noise is, as long as it’s meant to be there. For those less able to self-soothe, there are threads on Q&A sites like Quora offering reassurance. 

What if the equipment isn’t an airplane but your new electric vehicle? Automotive manufacturers are acutely aware that the EV era presents a whole litany of NVH challenges, not least around noise. Unlike aircraft passengers, a large portion of electric car owners not only find these whirring and whining noises unwelcome, but they want to know what they are and they want them gone. 

What every acoustics engineer knows, but few consumers do, is that EVs are in fact not silent. In traditional internal combustion engine vehicles, the biggest noise contributor is the powertrain, usually followed by road noise and wind. There is ancillary system noise, but it cannot be heard. Other hidden sounds include electric motor generators, PCU switching, power-split systems and planetary gearing components, even down to sounds from the steering rack, the AC and the wiper motors. 

What masks all this clatter is the ICE; remove that and a variety of noises are unveiled. Solutions have
had to be developed for all of them, including sound packaging, alterations in the tone of the source motors and injection of masking frequencies.

It’s predominantly a consumer issue rather than a regulatory one, but that’s not to say that the argument surrounding how an EV should sound has no legal teeth. Almost every electric vehicle attribute has its
own consumer class action risk. Actions have been taken over many things, from false advertising – particularly around range issues and alleged safety risks of autopilot systems – to incident-based cases such as the battery fires. It’s only a matter of time before we see the plaintiff’s bar round up class actions for what they will call excessive noise. There’s plenty of chatter about wind noise, road noise, AC motor noise, etc. It just needs some momentum and a couple of lemon law cases and there will be a class action. 

On the flip side of the noise challenge, there is the issue of EVs being too quiet. This was identified
as a safety compliance issue some time ago and was therefore regulated. The earliest interventions included a 2019 European law called the Regulation on the Sound Level of Motor Vehicles. That made it a legal requirement for all new electric cars to make a sound at speeds below 20km/h to warn other road users of an approaching EV. The automotive industry saw it coming and, as a response, manufacturers developed acoustic vehicle alert systems. 

Noise reduction remains a primary target for auto makers when engineering their EVs. If they allow the driver to hear the transmission or auxiliary systems, which the ICE would have masked previously, they run the risk of complaints – not to mention consumer class actions. But to ensure the car doesn’t fall foul of the noise regulations, manufacturers must also develop a warning sound below 20km/h.

As ever, technology has the answer. To help engineers understand where those unwelcome noises are coming from, the industry has forged newfangled tools to localize noise sources and even subsources. Acoustics visualization, for example, has become a popular choice of methodology to depict noise sources.
I applaud those NVH wizards who put their heart and soul into crafting that perfect sound signature.

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