JBL SP MKII User Manual page 32

EXCESSIVE USE OF ABSORPTIVE MATERIALS
People are sometimes tempted to go overboard with absorptive material once they
discover how powerful its use can be. While the ideal home theatre should be consid-
erably "deader" acoustically than a typical living room, it still needs some reflectivity
and diffusion. In particular, the surround speakers depend on non-absorptive surfaces
for their operation, since they radiate virtually no sound directly at the listeners.
The best arrangement of the absorptive and non-absorptive surfaces in the room can
be seen in the diagram below. Most of the room surfaces are relatively absorptive, with
the notable exception of the rear wall and the highest portions of the other walls, which
should be diffusive.
"SLAP" ECHOES
"Slap" echoes are common in rooms which have parallel walls with little or no absorp-
tion or diffusion. Sounds tend to bounce back and forth between the parallel wall many
times before they die out, causing a characteristic bright, "zingy" sound and interfering
with the intended tonal balance and acoustic nature of the soundtrack.
Walk slowly through the room, clapping your hands. No clear reflections should be
heard at any point in the room—especially not near the primary seating area. Listen for
ROOM ABSORPTION FOR HOME THEATRE SYSTEMS
Surround speaker
Reflective "live" zone
Screen speaker
Absorptive "dead" zone
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• "Dead" zone absorbs
front speaker reflection.
• "Live" zone provides
surround propagation