Expanding the ‘Bubble’ of Cabin Acoustics

Active noise cancellation and related technologies are changing the interior experience for EVs and AVs. Experts from Silentium explain.

Silentium wireframe showing ANC sensor positioning. (Silentium)

Noise cancellation has been a viable technology for many years, used on board turboprop aircraft to counter engine noise and familiar to consumers when applied to headphones to reduce background noise, among other applications. Like many technologies, advances in computer processing speeds and computer chip design, as well as loudspeaker and microphone design has enabled more effective noise cancellation systems to be developed.

Advanced active noise cancellation techniques significantly alter and tailor in-vehicle sound profiles. (Silentium)

Israel-based Silentium has been active in this field for many years with automotive customers including Jaguar Land Rover and others. “The objectives of applying our active road noise technology solutions to any car is to make the cabin quieter,” explains Anthony Manias, head of automotive at Silentium, “You give more of a premium feel, particularly under some use conditions where, in the case of road noise, due to the contact between the tire and the road, this would produce unwanted noises inside the cabin.”

The ANC solution

The traditional route to contain noise has been to use passive sound deadening techniques including sound absorbing materials. Silentium’s solution, called Active Noise Cancellation (ANC), “enables more freedom for design and sometimes cost saving,” Manias said. The ANC technology uses the vehicle’s audio system to control, significantly reduce, and eliminate unwanted noises in the cabin.

With input from the vehicle OEM, Silentium’s technique is to measure actual noise at the ear of the vehicle occupants under different use conditions, at different speeds, on different roads and surfaces, with windows up and windows down and with different levels of occupancy in the car. Company engineers use a vast amount of data and proprietary tools to analyze and predict ANC performance under those conditions.

Silentium can then propose a range of solutions to the customer. This might involve using the loudspeakers already fitted to the vehicle, or it could involve incorporating more loudspeakers if the customer wants higher performance, or alternatively improving some loudspeakers if the objective is higher performance up and down the audible frequency range.

The company also carries out a noise marking exercise, which Manias describes as “characterizing each vehicle with an acoustic signature, a transfer path for the noises, particularly airborne noises coming into the cabin. Engineers analyze and measure the vehicle to find out the optimum number of sensors required within the cost criteria. They then position and orient the sensors to detect the unwanted noise and therefore create the anti-noise signals, “ideally to coincide at the ear drums at the same time as the noise and therefore cancel it.”

Although all vehicles are different, they are similar in some respects when it comes to noise. “Really, what you find out is that tire rumble, cavity noise permeates the cabin in the region of between 100 and 300 hertz, typically, about 120 to 150 hertz,” he explained.

Personal 'quiet bubbles'

Anthony Manias noted the Personal Sound Bubble is the future for in-cabin acoustic design. (Silentium)
Occupant-monitoring cameras offer new opportunities for creating new vehicle cabin environments, said Silentium’s Tzvika Fridman. (Silentium)

Silentium has been using headrest speakers in its quest to improve ANC performance. According to Manias, these provide “surety to cancel higher frequencies” where the wavelengths are very small. Because of this, positioning of the anti-noise signal is critical and the node that cancels the noise signal needs to be very accurate. Having proximity speakers near the occupants’ ear drums helps to reach the higher frequencies.

Silentium has been working on ANC systems for electric vehicles, which generate lower noise levels than those powered by ICEs. But when the noise is mitigated, “the user can experience the other noises that were masked by the internal combustion engine more and more,” explained Silentium CTO Tzvika Fridman. The old noises that were always there now become more dominant, including wind noise and HVAC noise.

“There is a new challenge, a high frequency whining noise that is introduced in electric vehicles, a tonal component that adds noise to the cabin but it’s above 1,000 hertz, sometimes above 1,500 hertz,” Fridman said, adding that Silentium’s active acoustic software package can mitigate this. Developing technologies such as driver- and occupant-monitoring cameras in vehicles will bring further advantages, Fridman noted.

“You can use the cameras to locate the head, or the ears and once you have the pre-knowledge of where they’re located, you can make the frequency range much wider”, he said. With this technology, fewer loudspeakers may be needed to achieve the same performance once it is possible to precisely locate the position of occupants’ ears in the cabin. That wider frequency range should make it easier to control both the low frequency noises and deal with the EV-related high frequency sounds, he said.

Being able to provide more effective ANC control also opens up a further possibility. Manias refers to this as personal sound: “Not only do you have a “quiet bubble”, a personal quiet bubble, but you cut out unwanted noise for the occupant. Typically, you want to cut out that noise for all occupants”, he explained.

Vehicle occupants now have the opportunity, especially with headrest or proximity speakers, to also have their individual sound. “You can be listening next to me, I’m driving the car, you can be listening to your radio, or favorite song and I could be listening to mine. You won’t hear mine and I won’t hear yours,” he asserted. “We can both hear the passengers behind, or the ambulance on the outside, or other noise, so the system can basically tailor to your personal sounds.”

This is a technology that Silentium is at an advanced stage of developing. The company refers to it as a Personal Sound Bubble. Manias noted that it is the future for in-cabin sound design. “Personalized individual sound zones for each occupant including autonomous vehicles and hopefully later in mass transportation, where you can have not just a quiet bubble; you can also have a noisy bubble with the noises that you want, without disturbing your adjacent passengers,” he said.