Signal Processing - November 2016 - 69

capable of creating custom-designed engine sounds, thereby
EHE and RMT systems are both ASC solutions that synthehelping to retain an OEM-specific sound DNA for the car
size artificial sound to improve the driving experience. The
[42].
EHE technology provides desirable linear (or sporty) sounds
by masking sound anomalies that occur during acceleration
[40] while the RMT technology provides additional control
Practical limitations of current ANC systems
parameters to synthesize a seamless sound experience during
In this section, we discuss the main limitations related to mass
variable powertrain and cabin modes (e.g., cylinder deacproduction of ANC systems inside automobiles. As reviewed
tivation/reactivation, hybrid operation) [38]. Bose ASM
in [44], these include
was previously available only for vehicles with Bose sound
■ effects due to constrained number of microphones/speakers
system hardware however, since 2013, it was released as a
■ stability issues in feedback control systems
software solution integrated in a chip for global auto man■ system latency
ufacturers. Currently, Bose ASM is integrated in vehicles
■ uncertainties
manufactured by GM, Nissan, Audi [41], Porsche etc., par■ ANC system integration issues
ticularly in their luxury divisions.
■ system production tuning issues.
Introduced recently in 2015, HALOsonic is a another
A brief discussion on each of the aforementioned concommercially available noise management solution providstraints is as follows. Due to cost restrictions, the numbers
ed by Harman International and Lotus Cars, which comof speakers and microphones employed in ANC systems are
prises a suite of four technologies to enhance the in-car
limited. Currently, a typical production set-up consists of four
audible environment and improve pedestrian safety [42],
microphones and four-five speakers, which can only achieve
[43]. The two technologies directly relatglobal noise control over 30-250 Hz (the
in practice, the extension
ed to ANC are the Road Noise Cancellower limit is determined by the speaker
ation (RNC) system and the Engine Order
of a SiSO feedback system characteristics, whereas the higher limit is
Cancelation (EOC) system. The RNC sysdetermined by vehicle interior and comto a MiMO feedback
tem is a broadband feedforward control
ponent placement). Control up to at least
system is somewhat
system with accelerometers as reference
200 Hz is often desired because 200 Hz is
complex due to the
sensors. It is based on road noise cancelequivalent to the firing frequency of an I4
need to calculate the
ation solution originally presented in [28].
engine at 6,000 r/minute, which is the domeigenvalues of the openThe HALOsonic EOC system, is a cominant cause of booming noise. The active
bined system with feedforward control to
number of engine orders of a typical autoloop response to assess
reduce noise due to engine rotations and a
mobile is however much higher, and with
the controller stability.
feedback controller to reduce noise due to
new technologies like cylinder deactivation,
internal combustion engine and exhaust components. Note
there exist extra noise components that need to be addressed.
that the design and specifications of the above systems may
Therefore, improved ANC performance requires increased
vary based on the size, shape, and cost of the vehicle model
numbers of microphones/speakers with increased computaof interest. The remaining two technologies of HALOsontional requirements, memory, and higher tuning efforts due to
ic focus on ASC or electric sound synthesis in quiet cars
the increased complexity.
(e.g., electric cars, hybrid cars). This is done in two areas;
Another practical constraint that effects the the perfor1) internally, to improve the passenger experience and 2)
mance of ANC systems, particularly employing feedback
externally, to improve the safety of pedestrians. The intercontrol loops, is stability. To minimize side effects due to
nal (iESS) system helps synthesize an exhilarating engine
stability, it is important to carefully calculate the sensitivsound that adds a very emotional element to the overall
ity function of the closed loop. At present, there exist
driving experience. It helps reinstate original engine sound
advance modeling techniques to understand the system
in case of sound loss due to original equipment manufacbehavior through computer-aided technology [41], which is
turer (OEM) features such as downsized engines and the use
expected to make significant progress in the coming years.
of turbochargers. iESS also offers multiple engine sound
Within the working frequency range of an ANC system,
modes (e.g., normal, moderate, and sporty engine sound) for
another practical limitation that arises is system latency.
the same car, thereby enhancing car occupants' emotional
This is the signal latency added by processes like analog-toexperience. The external (eESS) system mainly provides
digital and digital-to-analog conversion, and digital signal
safety to quieter cars, where the active system is optimized
processing (DSP) latency. It is a difficult task to pinpoint
to operate in urban environments with the greatest risk of a
the exact instance when system latency starts to deteriorate
collision with pedestrians, especially high-risk groups such
the ANC performance. In practice, an ANC system latency
as the elderly, children, cyclists, and particularly the blind
(latency in the microphone input-ANC processing-speaker
and their guide dogs. eESS helps automakers comply with
output loop) of 2 ms is considered to be acceptable while
governmental safety regulations. The sound of a car engine
3 ms is the upper limit to avoid significant degradation [44].
is an integral part of the experience behind the wheel and
ANC performance in realistic automobile cabins is often
plays a crucial role in defining the DNA of the car. eESS is
affected by uncertainties such as the number and placement
IEEE SIgnal ProcESSIng MagazInE

November 2016

Table of Contents for the Digital Edition of Signal Processing - November 2016