Appendix 3: The Influence Of Listening Room Acoustics On Loudspeakers; Early Reflections - Bang & Olufsen BeoLab 90 Technical Sound Manual

Appendix 3: The Influence of Listening Room Acoustics on Loudspeakers
A room comprised of large flat
reflective surfaces with little acoustical
absorption has a very di erent
acoustical behaviour from a recording
or mastering studio where the final
decisions about various aspects of a
recording are made. Consequently,
this must have an e ect on a listener's
perception of a recording played
through a pair (assuming stereo
reproduction) of loudspeakers in that
room. The initial question to be asked
is "what, exactly, are the expected
e ects of the room's acoustical
behaviour in such a case?" The second
is "if the room has too much of an
e ect, how can I improve the situation
(e.g. by adding absorption or changing
the physical configuration of the
system in the room)?" The third, and
possibly final question is "how can a
loudspeaker compensate (or at least
account) for these e ects?"
The e ect a room's acoustical
behaviour has on a loudspeaker's
sound can, at a simple level, be
considered under three general
headings:
Early Reflections
Room Modes
Reverberation
16.1 Early Reflections
Early reflections, from sidewalls and
the floor and ceiling, have an influence
on both the timbre (tone colour) and
the spatial characteristics of a stereo
reproduction system. We will only
discuss the timbral e ects in this
article.
Figure 16.1: The sound arriving at a lis-
tener from a loudspeaker in a room with
only one wall. Note that the sound ar-
rives from two directions – the first is
directly from the loudspeaker (in red).
The second is a "first reflection" o the
wall (in blue).
Let's start by assuming that you have
a loudspeaker that has a magnitude
response that is perfectly flat – at least
from 20 Hz to 20 kHz. We will also
assume that it has that response
regardless of which direction you
measure it in – in other words, it's a
perfectly omnidirectional loudspeaker.
The question is, "what e ect does the
wall reflection have on the measured
response of the loudspeaker?"
Very generally speaking, the answer is
that you will get a higher level at some
frequencies (because the direct sound
and the reflection add constructively
and reinforce each other) and you will
get a lower level at other frequencies
(because the direct sound and the
reflection work against each other and
"cancel each other out"). What is
potentially interesting is that the
frequencies that add and the
frequencies that cancel alternate as
you go up the frequency range. So the
total result looks like a comb (as in a
comb that you use to comb your hair,
if, unlike me, you have hair to comb).
For example, take a look at Figure 16.2.
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0
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−20
10 100 1,000
Frequency (Hz)
Figure 16.2: Distance to loudspeaker =
2 m. Distance to wall = 1 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.
You can see that, at the very low end,
the reflection boosts the level of the
loudspeaker by a approximately 5 dB
(or almost two times the level) at the
listening position. However, as you go
up in frequency, the total level drops to
about 15 dB less before it starts rising
again. As you go up in frequency, the
level goes up and down. This
alternation actually happens at a
regular frequency spacing (e.g. a notch
at multiples of 200 Hz) but it doesn't
look regular because the X-axis of the
plot is logarithmic (which better
represents how we hear di erences in
frequency).
What happens if we move the wall
further away? Well, two things will
happen. The first is that the reflection
will be quieter, so the peaks and
notches won't be as pronounced. The
second is that the spacing of the peaks
and notches in frequency will get
closer together. In other words, the
e ect starts at a lower frequency.
For example, take a look at Figure 16.3.
10,000