When EVs are “Too Quiet,” Active Noise-Canceling Sensors Can Mute Road Noise

January 28, 2021 by Jake Hertz

When traditional combustion engines aren't drowning out typical road sounds, outside noise pollution can become a nuissance—or even dangerous—for EV occupants. ANC sensors, commonly used in noise-canceling headphones, can be a useful solution.

Those who have been in EVs before often report that the engines make virtually no noise—especially compared to conventional automobiles. While this feature may seem to be a plus for the user experience, occupants may alternatively hear noises from the road that are normally drowned out by combustion engines. 

According to automotive electronics manufacturer Molex, these noises often come in the form of a humming, low-frequency broadband sound transmitted from road surfaces into the vehicle through the tires, suspension, and body components. 

At best, incessant road sound can be an annoyance; at worst, it can cause drowsiness in drivers. As such, active-noise cancelation—and specifically road-noise cancelation in EVs—has become a focus for electronics designers striving to improve ADAS systems and the overall automotive user experience. 


The basic principle behind road-noise cancelation

The basic principle behind road-noise cancelation. Image used courtesy of Molex

Active-Noise Cancelation: From Headphones to EVs

Electronic designers tasked with eliminating road noise are now turning to active-noise cancelation (ANC) technology. ANC is not a new technology by any means. Back in 1994, IEEE contributors defined ANC as a secondary noise source that creates an acoustic field to eliminate unwanted noise.

This system often involves two sensors: one to measure the noise field and one to measure the attenuated (or canceled) noise. This two-sensor ANC system will often include an input sensor, error sensor, adaptive filter, and canceling source. 


Block diagram of a typical ANC system

Block diagram of a typical ANC system. Image used courtesy of IEEE

 ANC is commonly associated with noise-canceling headphones, which involves adaptive controllers and cancelation paths. Now, car manufacturers are adapting ANC systems as a small and efficient alternative to large and expensive sound-absorbing materials that would typically insulate car interiors from road sounds.


A Road Noise-Canceling System

This week, Molex announced its solution to unwanted road noise—a family of road noise-canceling (RNC) accelerometer and microphone sensors that leverage active noise canceling (ANC) technologies


Molex RNC sensor

Molex RNC sensor. Image used courtesy of Molex


To deal with the harsh environments of an automobile, the new sensor utilizes Analog Devices’ Automotive Audio Bus (A2B) technology to transmit noise signals to the processing unit in fewer than two milliseconds. Once processed, the system produces a destructive waveform to cancel out the noise.

Together, Molex's RNC sensors and ADI's A2B technology converts the EV's chassis vibration into a signal that produces a cancelation soundwave. 

The new sensors are also compatible with “Active Acoustics” software from Silentium, an ANC software specialist. Combined with this software, Molex says its sensors can eliminate "up to 90% of unwanted noises across a broadband of frequencies (from 20 Hz to 1 kHz)."  


Teaming With ADI's A2B Network Technology

Timing is key for canceling the unwanted soundwave. Molex intends for four to eight of its new sensors to be installed on the chassis frame to capture vibration energy at the earliest opportunity. Further, the A2B network technology shortens the time it takes for the sensor to receive the vibration signal.

Using the A2B technology, the sensors can be daisy-chained as opposed to a star-pattern, which is conventionally used. This significantly reduces cabling weight and associated failures. 


Daisy-chaining with the new Molex sensors

Daisy-chaining with the new Molex sensors. Image used courtesy of Molex


Further, Molex claims that this routing technique minimizes signal travel time to the processor, allowing for faster responses and more efficient noise canceling.


RNC Eliminates "Highway Hypnosis"

While one of the virtues of EVs is its silent propulsion, the tradeoff—from a design and user perspective—is issues with road noise. Molex says its new sensors alongside ADI's network technology can help improve RNC by speeding soundwave cancelation time and saving cabling weights. 

RNC sensors, A2B technology, and Active Acoustics software also yield valuable safety benefits by eliminating road-noise, which can have a soporific effect on drivers. “Working with Molex accelerates our ability to usher in a new era of safer, more enjoyable driving experiences,” explains Yoel Naor, CEO of Silentium. “Seamless integration of Molex’s sensors with our software also reduces automotive design cycles and engineering costs.”