JBL SP MKII User Manual page 33

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 theatres 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. You can also use the hand claps in chapters 17 and 18 of WOW!
The solution for slap echoes is usually a combination of absorption and diffusion.
Specifically, placing absorptive material behind the front speakers (heavy drapes,
fiberglass, dense foam) and diffusion in the rear of the room (bookcases, irregularly-
shaped furniture, etc.) will deliver the greatest benefits. 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 characteristic of the surrounds even
further.
In those relatively rare cases where you have the luxury of building the home theatre
room from scratch, consider using non-parallel surfaces in the construction of the
room. A difference of as little as 6° will break up the slap echoes very effectively. For
example, "flaring" the side walls out from the front by approximately 6° and having the
ceiling rise toward the rear of the room at a comparable rate will do wonders for the
room's acoustics, if the wall design is solid and the angles are clearly intentional from
the outset.
RATTLES
Rattles in the room are structural resonances (as opposed to standing waves, which
are airborne resonances) which the system may stimulate due to its broad frequency
response and wide dynamic range. They are particularly prominent for sounds in the
lower frequencies, and can sound like distortion. Sources of rattles include: furniture,
loose window frames, walls, lighting, fixtures, ventilation systems, and even knick-
knacks on various shelves around the room. The simplest way of identifying these rat-
tles is by using the Rattle Test found on WOW! (Chapter 16). This is an extremely slow
low frequency sweep from 20 Hz to 500 Hz, recorded at reference level. 10dB of out-
put level increase over standard level might be necessary to allow hearing all the room
rattles. Be careful with this test, as it is also a severe test of associated amplifiers and
speakers.
As the sweep makes its way up the frequency range, you will probably find a surpris-
ing number of rattles in your room. All of these rattles will occur at one time or another
during music or movies, but are usually perceived as background noise or distortion in
the system.
Once identified, eliminating the rattles is usually straightforward. As an example, small
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