Signal Processing - November 2016 - 71

inside a vehicle is generally sparse in terms of the proposed
noise model. In fact, for the head-sized region of interest, it
was observed that only a single-noise mode was active at all
The input and output of the adaptive controller are theretimes. Therefore, it was predicted that the vehicle's built-in
fore spherical harmonic coefficients rather than the direct
two-channel audio system (the Ford Falerror sensor outputs or control speaker
con XR6 has four loudspeakers with steweights. As a result, the standard block
right now, the main
reo control) will be sufficient to attenuate
diagram for feedforward and feedback
drivers for cost are
the active noise mode. Figure 10 shows the
systems needs to be updated by addihardware components,
noise reduction observed over frequency
tional modal transformation blocks. In
for four different driving conditions,
[48], Spors et al. designed and simulated
particularly the extra
where it's observed that noise reduction is
a 2-D HOA-based massive feedforward
requirement for error/
relatively consistent with the attenuation
ANC system with 80 reference sensors,
reference microphones
levels varying between 35-15 dB across
80 error sensors, and 80 loudspeakers,
and control loudspeakers. 50-500 Hz. These results are quite prommainly for use in room noise cancelation.
ising in terms of the potential use of a
A 2-D feedback control system followvehicle's own audio system for effective ANC. A robustness
ing the HOA technique was recently proposed by Zhang
analysis of this system however is still to be carried out. The
et al. using 11 error sensors and 11 control sources [49].
theory of the aforementioned design was later extended to
This feedback system was later extended with a sparse
support multiple-region ANC control and tested in the same
FXLMS controller, particularly for the use in spatially
vehicle [51]. From this investigation, the authors concluded
sparse noise fields [50]. The main advantage of HOAthat a vehicle's integrated loudspeakers, when used as a stebased ANC as observed in both feedback and feedforward
reo system, are only capable of canceling the noise field up
systems mentioned above is the significant improvement of
to 200 Hz at the head positions of two seats simultaneously.
convergence time and the significant decrease of spatially
To achieve similar reductions over four headrest positions
averaged residual signal energy. However, due to the relationship N = ^kRh , the minimum requirement of sensors/
speakers to control a sizable enclosure is impractically
high, especially if the system is to be utilized for noise
reduction inside automobiles.
e nm (k) = d (k) nm + yl (k) nm .

(6)

Application of spatial sound field
control to car noise cancelation

Noise
Power Attenuation (dB)

In [16], Chen et al. investigated the applicability of spatial
regional control in ANC inside automobiles. The main purpose of the study was to derive the performance bounds of
a feedback system with the automobile's built-in speakers
utilized as control sources. Results were synthesized using
a fixed offline system based on transfer functions and noise
measurements done at the front-left headrest of a Ford Falcon XR6 (see Figure 9). The sensor array used was a commercially available 32-microphone spherical array (Eigenmike).
While the previously discussed spatial ANC systems
Figure 9. The hardware setup for regional and spatial feedback control.
preferred a spherical array of control
sources, the proposed system simplified this constraint to the vehicle's
-5
own audio system, based on a novel
-10
model for the primary noise field.
-15
This model was derived utilizing the
-20
spherical harmonic decomposition
-25
of the recorded noise field such that
-30
AC Only
Busy Road
it represents the primary noise field
-35
Engine Only
Freeway
in terms of an alternative set of basis
-40
functions. Based on a diverse set of
50
100
150
200
250
300
350
400
450
500
noise measurements obtained inside
Frequency (Hz)
the car (e.g., engine only, AC only,
road noise at specific speeds) the Figure 10. Performance bounds of spatial ANC: the noise power spectrum attenuation for four differauthors found out that the noise field ent driving conditions.
IEEE SIgnal ProcESSIng MagazInE

November 2016

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