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pieces of felt can be affixed to the back of a painting (in the bottom corners) to prevent
audible rattles against the wall. Likewise, strips of felt can be wedged into a loose win-
dow rattling in its frame. Recessed lighting fixtures can be tightened up. A piece of
cloth can be placed under offending knick-knacks.
Every Home THX Audio System should be subjected to the rattle test at least once—
the difference in low level resolution and in freedom from pseudo-distortion is some-
times large, and the effort involved is quite small.
BACKGROUND NOISE
The effects of background noise on system performance is dramatic, yet often over-
looked. Most people might think of it merely as a minor inconvenience, yet it has a pro-
found effect on the way we perceive sound.
The presence of more-or-less constant background noise alters the way we perceive
volume, since subjective loudness is a relative measure. In a quiet room, even a 70 dB
SPL sound can seem fairly loud. In a noisy convention center, the same volume would
be barely audible. Since there is a practical upper limit to both the volume to which we
should expose ourselves and to the volume a given system can reproduce, having a
relatively noisy environment effectively limits the perceived dynamic range of the pro-
gram material. This, in turn, limits the dramatic effect which might have been intended
by the director (or the performer, if listening to music).
Constant background noise also obscures, or masks, low-level signals which are fre-
quently important in films. Many scenes use subtle ambient noises to set the mood
prior to an important event—without the full perception of the whispered secret or the
barely-heard creaking of a door, the impact of the following scene is diminished.
It has been demonstrated that even a relatively narrow-bandwidth noise can effectively
reduce our hearing acuity over a broad range of frequencies, far greater than the noise
itself. When you add up all the various sources of noise from electric motors, noisy
heating/cooling systems, outside noises, plus noises that even audio and video com-
ponents can introduce such as noisy transformers, motors in laser players, or projector
fan noise, our ability to discern the low-level information in the soundtrack is greatly
compromised—and the director's intention along with it.
BACKGROUND NOISE SOLUTIONS
Many sources of noise in a home environment can be addressed simply. Locating the
home theater in the basement often removes it from many household noises as well as
isolating it from the other family members. Taking care to completely seal windows and
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