Automotive Engineering - June 2022 - 9

TECHNOLOGY REPORT
NVH
Expanding the 'bubble' of cabin acoustics
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.
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
AUTOMOTIVE ENGINEERING
Silentium wireframe showing ANC sensor positioning.
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.
Anthony Manias said the Personal Sound Bubble
is the future for in-cabin acoustic design.
Personal 'quiet bubbles'
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
June 2022 9
BOTH IMAGES: SILENTIUM
Automotive Engineering - June 2022

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