We have seen methods for reproduction of
sound that can place sources as being fixed in space. The listener is now free
to move and turn without sweet spot restriction.
The ability to accurately place multiple
reproduced sound sources at points that remain fixed in space when observed
from anywhere in a defined listening region raises the issue of possible format
restrictions.
Each array needs to cover a target
listening area with rendered sound based on the listening area requirements.
Three formats arise. In all of the three cases, the listener has a defined
listening region but cannot stray from this region without destroying the
perception of sound objects fixed in space. In particular, the listener cannot
pass the line of the first loudspeakers of the arrays or the wavefront creation
technique collapses.
Voyeur format
The distance and direction information remains correct in a
region in space provided the listener is in a target listening area in front of
the arrays. This allows for render of sound objects from well in front of the
array, back to the vanishing point or acoustic horizon behind the array. This
would suit 2D and 3D screen-based cinema for example.
Bubble format
The listeners can be seated or walk completely around the
region of space where the arrays are located, but cannot enter the region of
the arrays. The listeners will perceive sound objects from well forward of the
array “bubble”, back to the vanishing point well behind the bubble, and for
each listener location. This requires the use of multiple or sectored array
structures to present the different perspectives as the listeners position
themselves in different locations or move around the arrays. This format would
suit conference call systems, “concerts in the round” and point of sale
apparatus for example.
Immersive or Virtual Reality format
This is the
complement to the bubble format. The listener is now inside a region surrounded
by the arrays and perceives the correct sound source distance, direction and horizon
or vanishing point, but cannot leave the region. This would suit “cinema in the
sphere” and immersive virtual reality applications, for example.
The use of these separate formats will assist in balancing
array design requirements against scope of audience placement in the intended
application. But it does not allow for the situation where a person wants to be
able to walk around, up to, through and away from each or any sound source
created in space.
We will now consider how this could be done. The first
obvious requirement is that the loudspeaker sources creating the sound fields
will need to be truly omni-directional in nature as total room coverage will be
required. One easy method of achieving this is to use half sources and take
advantage of the boundary image to complete the spherical source.
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