14
A
COUSTICAL
P
ROBLEMS IN
L
ISTENING
R
OOMS
A home THX audio system, such as the Synthesis Four system, addresses many of the problems common to high-quality
reproduction of music or soundtracks in a home environment. For example, the dispersion pattern of the front LCR speakers
minimizes the effects of floor and ceiling reflections. Still, the single most important variable in any sound system is the
listening environment. Room reflections create spurious false images and “comb filter” interference effects which alter
the tonality of the system while degrading the localization of specific sounds. Larger rooms sustain echoes that degrade
dialogue intelligibility and detail. All rooms have standing waves that emphasize certain frequencies at the expense of
others, based on the dimensions of the room.
Other concerns include environmental noise, which is often greater than people realize. Although they might become
accustomed to its presence and “tune it out,” it still reduces the perceived low-level resolution of the system. In
addition, the profound bass capabilities of a home THX audio system can create distracting rattles that lesser
systems might never evoke.
This section of the manual contains a variety of suggestions for addressing some of the peculiarities of the listening
environment to improve the accuracy of your Synthesis Four system’s sound reproduction.
R
OOM
R
EFLECTIONS
The most troublesome room reflections are usually
the early reflections of the front LCR speakers off the
floor, ceiling and side walls. These reflections reach
the listener’s ears after a delay, with respect to direct
sounds, and blur the perceived image. They can also
degrade dialogue intelligibility through the same
mechanisms (see Figures 21 and 22).
The design of the Synthesis Four speakers minimizes
the floor and ceiling reflections. As an extra
enhancement, it is often a good idea to place a thick,
absorptive carpet between the front speakers and
the listening position, just to further reduce this pri-
mary reflection from floors with hard surfaces. A rug
made of wool will generally have more uniform
absorption characteristics than one made from syn-
thetic fibers.
JBL Synthesis Four speakers have broad dispersion in
the horizontal plane in order to ensure a wide usable
listening area. This design choice can induce reflec-
tions off the side walls, especially in installations
where they are relatively close to the front speakers.
With conventional speakers, these reflections can be
reduced simply by angling the left and right speakers
inward somewhat.
F
INE
-T
UNING
Y
OUR
A
UDIO
S
YSTEM
t
1
t
2
Direct
Reflected
Result: (+6dB)
Direct
Reflected
Result: (–
∞
dB)
Copyright 2000 Lucasfilm Ltd.
t
1
t
2
An Additive
Frequency
A Canceling
Frequency
Copyright 2000 Lucasfilm Ltd.
+6dB
Sound Level
Peak and dips at various frequencies
Resultant Frequency Response From Reflection
Frequency
0dB
Figure 21.
Figure 22.
