Appendix 4: Loudspeaker Directivity and Distance Perception in Stereo Imaging
17.1
Distance Perception in
Real Life
Go to the middle of a snow-covered
frozen lake with a loudspeaker and a
friend. Sit there and close your eyes
and get your friend to place the
loudspeaker some distance from you.
Keep your eyes closed, play some
sounds out of the loudspeaker and try
to estimate how far away it is. You will
be wrong (unless you’re VERY lucky).
Why? It’s because, in real life with real
sources in real spaces, distance
information (in other words, the
information that tells you how far away
a sound source is) comes mainly from
the relationship between the direct
sound and the early reflections from
walls in your listening room. If you
don’t have any early reflections, then
you don’t have any distance
information. Add the early reflections
and you can very easily tell how far
away it is.
17.2
Distance Perception in a
Stereo Recording
Recording engineers have a basic trick
for controlling the apparent distance to
a sound source in a stereo recording
using the so-called “dry-to-wet” ratio
— in other words, the relative levels of
the direct sound and the reverberation.
To be honest, this is a bit of an
over-simplification, but it’s at the level
of knowledge one would typically have
if one were just starting out recording a
budding rock band in a garage.
Many classical recordings are made
with a pair of microphones. An
instrument that is on the left side of
the pair will produce a sound that is
slightly louder or slightly earlier in the
left microphone than in the right
microphone. This means that, when
you sit in the sweet spot and listen to
the stereo recording, you will hear that
source on the left side of the stereo
image. This effect is true not only for
the direct sound of the instruments
arriving at the microphone pair, but
also for the acoustic reflections off the
various surfaces in the recording
space. So, if the recording engineer
has been paying attention, the
distance information (the relationship
between the direct sound and the
reflections) has been captured in the
recording. This means that when you
listen to the recording, you not only
can tell where the instruments are
from left to right, but also their relative
distances.
17.3
Combining the Two
So, we know that early reflections tell
your brain how far away the sound
source is. Now think to a loudspeaker
in a listening room:
Case 1: If you have a listening room
that has no sidewalls, then there are
no early reflections, and, regardless of
how far away the loudspeakers are, a
sound source in the recording without
early reflections (e.g. a close-mic’ed
vocal) will sound closer to you than the
loudspeakers.
Case 2: If you have a listening room
with early reflections, and the
loudspeakers are less directional such
as BeoLab 90’s with their Beam Width
set to Wide or Omni, then the early
reflections from the side walls tell you
how far away the loudspeakers are.
Therefore, the close-mic’ed vocal track
from Case 1 cannot sound any closer
than the loudspeakers – your brain is
too smart to be told otherwise.
Case 3: If you have a listening room
with sidewalls and therefore early
reflections, but the loudspeakers are
directional such that there is no energy
being delivered to the side walls, then
the result is the same as in Case 1.
This time there are no early reflections
because of loudspeaker directivity
instead of wall absorption, but the
effect at the listening position is the
same. This is the case with BeoLab 90
when its Beam Width is set to Narrow.
The conclusion is that, in order to get
an accurate and precise representation
of the spatial properties in a stereo
recording, you should try to minimise
the levels of the early reflections from
the sidewalls in your listening room.
However, this means that you are
optimising the sound for the sweet
spot – on-axis to both loudspeakers.
When listening with friends, it may be
necessary to widen the loudspeakers’
Beam Widths.
1
This has been proven in various listening tests. For example, go check out “Psychoacoustic Evaluation of Synthetic Impulse Responses” by Per Rubak & Lars G. Johansen as a
starting point.
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