Signal Processing - May 2017 - 46

Second-Order Ambisonics

36

36

28

28

20

20

Mean Response Angle (°)

Mean Response Angle (°)

Intensity

12
4
−4
−12
−20

100

12
4
−4
−12
−20
−28

−28
−36

PSR (Second Order)

−28 −20 −12 −4
4
12
Stimulus Angle (°)
(a)

20

−36

28

−28 −20 −12 −4
4
12
Stimulus Angle (°)
(b)

20

28

I am Certain
I Have a
Slight Doubt
I Have a
Doubt
I am Really
Not Sure

80
60
40
20

I Have No Idea
0

Middle
On-Center Position
(c)

Right

Left

Middle
Off-Center Position
(d)

Right

FIGURE 8. The results of a subjective listening experiment with the second-order Ambisonics, PSR, and intensity methods: (a) and (b) indicate localization performance, while (c) and (d) indicate locatedness performance. (a) and (c) are for a listener in the center of the listening area facing the midline
between two loudspeakers (see Figure 5) and facing a loudspeaker, respectively. (b) and (d) are for an off-center listener-more precisely, 20 cm behind
and to the left with respect to the on-center position. (Figure used courtesy of [29]).

The issue of intensity versus time-intensity techniques
is a matter of debate among audio engineers and recording
artists, and the widely held view is that although time-intensity stereophony provides more natural-sounding sources,
intensity stereophony provides more stable imaging. This
result provides a new insight into the issue and demonstrates
that time-intensity techniques, if designed with a careful
consideration of the underlying psychoacoustical requirements, are capable of actually providing a stable auditory
perspective. The development of techniques for higher-order
differential microphone arrays [55], [56] enabled the design of
more sophisticated directivity patterns than those achievable
by commonly used first-order microphones. This allowed the
implementation of different panning laws and psychoacoustical panning functions in the multichannel microphone array
design process.
46

Enlarging the optimal listening area
When a listener moves away from the center of the sweet spot,
the auditory event shifts in the direction of the closest loudspeaker. This is due to the fact that the signal from the closest
loudspeaker arrives earlier when compared with what is
observed at the sweet spot. Position-independent stereo [57],
[58] aims to alleviate this problem by designing loudspeaker
directivity patterns in a manner that compensates for the
incongruent time delay via appropriate intensity differences.
The design method proposed for this purpose by Rodenas
et al. [59] consists of two separate optimization procedures. The
first procedure involves finding a common directivity pattern for
the left and right loudspeakers of a stereophonic setup to provide
the level differences needed to compensate for the incongruent
time differences over a desired listening area. Such directivity patterns can be obtained by beamforming using an array of

IEEE Signal Processing Magazine

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May 2017

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Table of Contents for the Digital Edition of Signal Processing - May 2017
