JBL SP MKII User Manual page 30

lesser systems might never evoke.
It can be tempting to try to solve all of these problems with the indiscriminate use of
sound-absorbing products, but even this technique has its pitfalls.
All of these common acoustical problems will be addressed in this section. Once
again, these techniques are not necessary for a successful Home THX Audio System
installation. Rather, they are provided to solve occasional problems and to provide fur-
ther enhancement possibilities.
ROOM REFLECTIONS
The most troublesome room reflections are usually the early reflections of the LCR
speakers off the floor, ceiling and side walls. These reflections reach the listener's ears
delayed with respect to direct sounds and blur the perceived image. They can also
degrade dialog intelligibility, through the same mechanisms.
The design of the THX LCR 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 primary reflection
from floors with hard surfaces. A rug made from wool will have more uniform absorption
characteristics than one made from synthetic fibers.
The THX LCR speakers have broad dispersion in the horizontal plane in order to ensure
a wide usable listening area. This design choice can induce reflections off of the side
walls, especially in installations where they are relatively close to the front speakers.
These reflections can be reduced simply by angling the left and right speakers inward
somewhat.
If giving the speakers some "toe-in" is not enough, the next step is the strategic place-
ment of absorptive materials on the side walls. These range from commercially available
fiberglass and dense foam to heavy draperies and even large, overstuffed furniture. The
optimal position for these materials can be found with a small hand mirror and an assis-
tant. Sit at the primary listening position and have the assistant slowly slide the mirror
along the wall. When you can see any of the front speakers reflected in the mirror, mark
the wall at the mirror for later placement of absorptive material.
A variation of this method is especially helpful in rooms which are already fairly "dead"
acoustically. Rather than using absorptive material in rooms like these, try using diffu-
sion instead. Commercially built diffusers are available but large bookcases and irregu-
larly shaped furniture will also serve the same purpose. They reflect sounds in a highly
randomized way which effectively "scatters" the sound in all directions. Place the dif-
fuser where you would otherwise place the absorptive material (using the "mirror trick"),
to break up the first early reflections and scatter them randomly throughout the room.
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