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The best arrangement of the absorptive and nonabsorptive surfaces in the room can be seen in Figure 24. 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 that have parallel walls with little
or no absorption or diffusion. Sounds tend to bounce back and forth
between the parallel walls 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 (see Figure 25).
Walk slowly through the room, clapping your hands. No clear reflections
should be heard at any point in the room – especially not near the pri-
mary seating area. Listen for a “flutter echo” of the hand clap (a rapidly
repeating percussive sound, indicative of the sound bouncing between
two parallel walls). Again, the best home theaters are fairly “dead”
acoustically. This allows the program material and the playback system
to create the environment, rather than having the room’s native acoustic
signature color everything.
The solution for slap echoes is usually a combination of absorption and diffusion. Specifically, place absorptive mate-
rial behind the front speakers (heavy drapes, fiberglass, dense foam) and elements that will facilitate diffusion in the
rear of the room (bookcases, irregularly shaped furniture, etc.). This will effectively suppress the slap echoes while at
the same time providing a diffusive surface in the rear for the surround speakers. This enhances the enveloping char-
acteristic of the surrounds even further.
In those relatively rare cases in which you have the luxury of building the home theater room as new construction,
consider using nonparallel surfaces in the construction of the room. A difference of as little as 6 degrees will break
up the slap echoes very effectively. For example, “flaring” the side walls out from the front by approximately
S
urround speaker
S
creen speaker
ROOM ABSORPTION FOR HOME THEATER SYSTEMS
• “Dead” zone absorbs
front-speaker reflection
• “Live” zone provides
surround propagation
Figure 24.
Copyright 2000 Lucasfilm Ltd.
Loudspeaker
Listener
Figure 25.
FINE-TUNING YOUR AUDIO SYSTEM
S4VC/HC/Ai OM 2/28/06 4:55 PM Page 17