1.5 Motivation of Higher-Order Ambisonics
Diffuseness, spaciousness, depth? Diffuse sound fields are typically characterized
by sound arriving randomly from evenly distributed directions at evenly distributed
delays. It is practical knowledge that the impression of diffuseness and spaciousness requires benefits from decorrelated signals, which is typically achieved by large
distances between the microphones rather than by coincident microphones.
Due to the evenness of diffuse sound fields, one would still hope that a low spatial
resolution is sufficient to map diffuseness and spatial depth of a room, using coin-
cident microphones or first-order Ambisonics. Nevertheless, high directional corre-
lation during playback destroys this hope and in fact yields a perceptually impeded
playback of diffuseness, spaciousness, and depth.
The technical advantages in interactivity and VR as well as the known short-
comings of first-order coincident recording techniques offer enough motivation to
increase the directional resolution and go to higher-order Ambisonics, as presented
in the subsequent chapters. For professional productions, it is often not sufficient to
only rely on first-order coincident microphone recordings. By contrast, higher-order
Ambisonics is able to drastically improve the mapping of diffuseness, spaciousness,
and depth, as shown in the upcoming chapter about psychoacoustical properties of
many-loudspeaker systems.
Recording with a higher-order main microphone array increases the required
technological complexity. Nevertheless, digital signal processing and the theory pre-
sented in the later chapters is powerful nowadays to achieve this goal.
After all, it seems that delay-based stereophonic recording, such as AB, or
equivalence-based recording, such as ORTF, INA5, etc., is often required and well-
known in its mapping properties for spaciousness and diffuseness, correspondingly.
What is nice about higher-order Ambisonics: it can make use of these benefits by
embedding such recordings appropriately, see Fig. 1.20.
Facts about higher orders: Ambisonics extended to higher orders permits a refine-
ment of the directional resolution and hereby improves the mapping of uncorrelated
sounds in playback. Figure 1.21a shows the correlation introduced in two neighbor-
ing loudspeaker signals when using Ambisonics, given their spacing of 60◦. Given
the just noticeable difference (JND) of the inter-aural cross correlation, the figure
indicates that an Ambisonic order of ≥3 might be necessary to perceptually preserve
decorrelation.
For this reason, the perception of spatial depth strongly improves when increasing
the Ambisonic order from 1 up to 3, Fig. 1.21b. However, this is only the case when
seated at the central listening position. Outside this sweet spot, higher orders than
3, e.g., 5, additionally improve the mapping of depth [19]. Therefore, higher-order
Ambisonics is important for preserving spatial impressions and when supplying a
large audience.
Figure 1.22 shows that the sweet area of perceptually plausible playback increases
with the Ambisonic order [20]. With fifth-order Ambisonics, nearly all the area
spanned by the horizontal loudspeakers at the IEM CUBE, the 12 × 10 m concert
space at our lab, becomes a valid listening area.
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