Bang & Olufsen BeoLab 90 Technical Sound Manual page 53

0
−5
−10
−15
−20
10 100 1,000
Frequency (Hz)
Figure 16.3: Distance to loudspeaker =
2 m. Distance to wall = 3 m. Wall is per-
fectly reflective and the loudspeaker is
perfectly omnidirectional. The red line
is the magnitude response of the direct
sound. The blue line is the magnitude
response of the reflected sound. The
black line is the magnitude response of
the combination.
Conversely, if we move the wall closer,
we do the opposite (the problem gets
worse, but at a higher frequency), as
can be seen in Figure 16.4.
0
−5
−10
−15
−20
10 100 1,000
Frequency (Hz)
Figure 16.4: Distance to loudspeaker =
2 m. Distance to wall = 0.25 m. Wall is
perfectly reflective and the loudspeaker
is perfectly omnidirectional.
line is the magnitude response of the
direct sound. The blue line is the mag-
nitude response of the reflected sound.
The black line is the magnitude re-
sponse of the combination.
So, if you have a room with only one
wall which is perfectly reflective, and
you have a perfectly omnidirectional
loudspeaker, then you can see that
your best option is to either put the
loudspeaker (and yourself) very far or
very close to the wall. That way the
artefacts caused by the reflection are
either too quiet to do any damage, or
have an e ect that starts at too high a
frequency for you to care. Then again,
most room have more than one wall,
the walls are not perfectly reflective,
and the loudspeaker is not perfectly
omnidirectional.
So, what happens in the case where
the loudspeaker is more directional or
you have some absorption (better
known as "fuzzy stu ") on your walls?
Well, either of these cases will have
basically the same e ect in most cases
since loudspeakers are typically more
directional at high frequencies – so you
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get less high end directed towards the
wall. Alternatively, fuzzy stu tends to
soak up high frequencies. So, in either
of these two cases, you'll get less high
end in the reflection. Let's simulate
this by putting a low pass filter on the
reflection, as shown in Figure 16.5,
16.6 and 16.7
distances as the simulations in Figures
16.2, 16.3, and
0
−5
−10
−15
−20
10
Figure 16.5: Distance to loudspeaker =
2 m. Distance to wall = 1 m. Wall is ab-
sorptive and/or the loudspeaker is direc-
tional at high frequencies. The red line
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is the magnitude response of the direct
sound. The blue line is the magnitude
response of the reflected sound. The
black line is the magnitude response of
the combination.
The red
0
−5
−10
−15
−20
10
Figure 16.6: Distance to loudspeaker =
2 m. Distance to wall = 3 m. Wall
is absorptive and/or the loudspeakers
is directional at high frequencies. The
red line is the magnitude response of
the direct sound. The blue line is the
magnitude response of the reflected
sound. The black line is the magnitude
response of the combination.
which have identical
16.4 – for comparison.
100 1,000 10,000
Frequency (Hz)
100 1,000 10,000
Frequency (Hz)
53
0
−5
−10
−15
−20
10 100 1,000
Frequency (Hz)
Figure 16.7: Distance to loudspeaker =
2 m. Distance to wall = 0.25 m. Wall
is absorptive and/or the loudspeaker is
directional at high frequencies. The red
line is the magnitude response of the di-
rect sound. The blue line is the mag-
nitude response of the reflected sound.
The black line is the magnitude re-
sponse of the combination.
What you can see in all three of the
previous plots is that, as the high
frequency content of the reflection
disappears, there is less and less e ect
on the total. The bottom plot is
basically a proof of the age-old rule of
thumb that says that, if you put a
loudspeaker next to a wall, you'll get
more bass than if it's farther from the
wall. Since there is not much high
frequency energy radiated from the
rear of most loudspeakers, Figure
is a pretty good general representation
of what happens when a loudspeaker is
placed close to a wall. Of course, the
exact behaviour of the directivity of the
loudspeaker will be di erent – but the
general shape of the total curve will be
pretty similar to what you see there.
So, the end conclusion of all of this is
that, in order to reduce undesirable
artefacts caused by a wall reflection,
you can do any combination of the
following:
move the loudspeaker very close
to the wall
move the loudspeaker farther
front the wall
sit very close to the wall
sit farther away from the wall
put absorption on the wall
However, there is one interesting
e ect that sits on top of all of this –
that is the fact that what you'll see in a
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16.7