Listening Room Acoustics; Loudspeakers; Source Devices - Bang & Olufsen BeoLab 90 Technical Sound Manual

Figure 14.6: An example of a worst-case
placement of loudspeakers with respect
to the listening room. No two distances
between a loudspeaker and an adjacent
wall match each other.
It should be noted that the primary
casualty of poor loudspeaker
placement in a listening room will be
the spatial representation of your
recordings. The precision and accuracy
of the stereo imaging, as well as the
sensation of envelopment from the
recording will be adversely a ected by
early reflection patterns that are not
matched for the two loudspeakers.
This problem is minimised by using
BeoLab 90's narrow beam width,
however, even this mode can benefit
from correct loudspeaker placement in
the room.
Finally, it is recommendable (but
certainly not required) that the
loudspeakers be positioned a minimum
of 1 m from the closest walls. The
Active Room Compensation algorithm
will compensate for changes in the
BeoLab 90's timbral response caused
by adjacent boundaries. However,
placing the loudspeakers slightly
distant from reflective surfaces will
reduce these boundary e ects, and
therefore also reduce the amount of
compensation that is required by the
ARC filters.
14.2 Listening Room
Acoustics
The BeoLab 90 has two features that
can overcome some detrimental
e ects of the listening room's
acoustical behaviour (Beam Width
Control and Active Room
1
See "Room dimensions for small listening rooms" by Dr. Trevor Cox for a good introduction to this topic.
Compensation). However, the sound of
any loudspeaker can be optimised by
improving the room's acoustics.
One of the main acoustical problems in
listening rooms is that of room modes
or resonances. These occur because
the room behaves very much like an
organ pipe, naturally "singing" at
specific frequencies that are
determined by the dimensions of the
room. Without correct acoustical
treatment, these resonances are
almost unavoidable. It is preferred to
ensure that the resonances in the
room's three dimensions (length,
width, and height) do not overlap each
other. This means that the better
listening rooms have complex
relationships between these three
dimensions. For example, a "worst
case" for a listening room would be a
cube, where all three dimensions are
identical, thus all resonances have the
same frequencies. A next-worst case is
one where a dimension is a multiple of
another, for example, a room that is
9m x 6m x 3m. In a best case, the
ratios of the room's dimensions would
have non-simple values (e.g. 1 : 2.16 :
2.96 – so, as an example, 3m x 6.48m
1
x 8.88m).
A second issue in many listening rooms
is that of hard, reflective surfaces –
particularly in locations where the
sound from the loudspeaker is directly
reflected to the listening position.
There are two ways to alleviate this
problem: absorption and di usion. In
order to absorb a sound wave so that it
does not reflect o a surface, an
absorptive material such as fibreglass
insulation or acoustical foam must be
placed on the surface, or in the path
taken by the reflection. A reflection can
be di used by making the reflective
surface irregular. For example, placing
a bookcase at the point of reflection
will help as a di usor if the books are
arranged in random heights and
depths.
Finally, it is wise to absorb the sound
waves that would be reflected o the
floor (e.g. with carpet or a rug) and
ceiling (using absorptive ceiling tiles).
47
This will also help to reduce the overall
reverberation time of the room.

14.3 Loudspeakers

For "critical" or "serious" listening
sessions, it is recommended that the
upper fabric frame be removed from
the loudspeaker's high speaker section.

14.4 Source Devices

When connecting an audio source to
the BeoLab 90, there are some basic,
general rules that should be followed in
order to get the optimal performance
from your system. Note that these are
general rules – so there are exceptions.
If possible, the source should be
connected to the BeoLab 90
using a digital audio connection.
If the source device has a volume
control it should be disabled and
the BeoLab 90's volume control
should be used instead
If the source has two analogue
outputs: one volume-regulated
and the other at a fixed level, the
fixed-level output should be used
If you are connecting a source
using a line-level analogue input
(RCA or XLR), check the source
device's datasheet to find its
maximum output level and set
the value appropriately on the
BeoLab 90 (See Maximum Input
Voltage). If the maximum output
of your device is greater than the
BeoLab 90's maximum possible
setting (6.5 V RMS) then it is
recommendable that the source
device's output level is reduced if
possible, either within its own
settings or using an external
attenuator. Table 14.1
14.7 show the necessary
attenuation to reduce various
voltage levels to 6.5 V RMS.
and Figure