Signal Processing - November 2016 - 66
Error
Microphone
Noise
Source
Loudspeaker
Primary
Noise
e (n )
y (n )
Feedback
ANC
Figure 5. A SISO adaptive feedback ANC system.
and a single error microphone. Based on experimental results,
the optimum positioning for the aforementioned devices were
chosen, and during driving tests on rough asphalt and curvy
roads at 60 km/hour, a reduction of 6-dB A-weighted sound
pressure level (A-weighting accounts for the relative loudness
perceived by the human ear) in the road booming noise was
achieved. Due to the high cost of accelerometers, feedforward
ANC is generally regarded as unsuitable for mass production, however, with the recent introduction of low-cost MEMS
accelerometers, it is expected to change.
Feedback control systems
control), particularly surrounding the driver's/passenger's
head. Regional or local control of noise reduces the volume
over which the noise energy has to be minimized and therefore
reduces the constraints on the control system and increases the
control bandwidth. In [6], the authors investigated a regional
narrowband feedforward system over two regions, one rectangular region across the front seats and a second rectangular
region across the rear seats. The performance of the system
was investigated through simulations and synthesis based on
transfer functions measured in a rectangular car cabin mockup. The control system comprised four secondary sources
positioned at the standard car audio loudspeaker positions,
and eight error sensors positioned at the four head rest positions (two sensors on each headrest). The acoustic potential
energy within the control regions were shown to be significantly
reduced at frequencies up to 370 Hz. This is around twice the
control bandwidth of global feedforward control using a similar
system. The authors mention a potential issue with the regional
feedforward control strategy, i.e., the system is said to be susceptible to unobservable modes that result in enhancements in
the regional acoustic potential energy. However, it has also been
shown that these effects can be limited by using control effort
weighting parameters.
In addition to engine noise cancelation, feedforward systems are also applied in road noise cancelation. In [17], Oh
et al. presented a leaky constraint MIMO feedforward ANC
system for road booming noise control in a midsize passenger
vehicle using two accelerometers, two control loudspeakers,
This section discusses the adaptive feedback control systems
used for broadband ANC. Unlike feedforward systems, feedback systems directly employ the signal(s) from error sensor(s)
to drive the secondary source(s) via a controller. Since the
error sensor signal is fed back to the secondary source, the
system cannot be optimized on a frequency-by-frequency
basis as in feedforward control, and, therefore, the whole frequency response (broadband) must be considered at all times.
The performance of feedback control systems are limited by
their stability, which is largely dependent on the system delay.
Therefore, the control bandwidth of feedback control systems
are inversely proportional to the spacing in between the error
sensor(s) and secondary source(s).
Feedback control algorithms
SISO feedback control
A single-channel adaptive feedback ANC system, as shown
in Figure 5, was first proposed in [18]. Based on the internal
model control (IMC) architecture, an adaptive feedback system can be viewed as an adaptive feedforward system, which
synthesizes or regenerates its own reference signal using the
error signal and the adaptive filter output. The basic concept
of this model is to estimate the primary noise d (n) present at
the error sensor and use it as a reference signal x (n) for the
adaptive filter. If the secondary path transfer function S (z) is
known, the primary noise signal d (n) can be synthesized using
t (z) = E (z) + St (z) Y (z),
X (z) / D
d (n )
Σ
S (z )
x ′(n )
d ′(n )
y (n )
S (z )
y ′(n )
"
W (z )
"
x (n )
e (n )
S (z )
LMS
Σ
Figure 6. A SISO adaptive feedback ANC system using the IMC architecture.
66
(4)
where the notation " ^ " represents an estimated value. Therefore, the reference signal synthesis technique filters the secondary source signal y (n) using the secondary path estimate
St (z) and combines it with e (n) to regenerate the primary
noise. Figure 6 shows a complete SISO feedback ANC system
using the FXLMS algorithm with secondary path cancelation
as discussed in the section "Broadband Feedforward ANC."
When applying a feedback control system, the overall control
system stability is very important next to the noise reduction
level. This is generally analyzed by checking the Nyquist stability criterion, which states that the polar plot of the openloop response must not enclose the Nyquist point (-1, 0) as
~ increases from - 3 to + 3 [19]. In a practical system, since
the open-loop response often varies with time, it is typical to
IEEE SIgnal ProcESSIng MagazInE
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November 2016
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Table of Contents for the Digital Edition of Signal Processing - November 2016
Signal Processing - November 2016 - Cover1
Signal Processing - November 2016 - Cover2
Signal Processing - November 2016 - 1
Signal Processing - November 2016 - 2
Signal Processing - November 2016 - 3
Signal Processing - November 2016 - 4
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Signal Processing - November 2016 - 148
Signal Processing - November 2016 - Cover3
Signal Processing - November 2016 - Cover4
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