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doors can also make a significant difference in reducing outside noise.
Heating and cooling systems are more challenging. Sometimes, the answer may be as
simple as using a "whistle-free" diffusion grille rather than one which creates undue
noise from turbulence. In cases of new construction, using larger-diameter air ducts for
lower air velocity is very beneficial. You can go further by using ductwork which is lined
with acoustically-absorptive material. Where possible, longer ducts which have several
turns further reduce the sound of the airflow, by eliminating the straight path from the
heating/cooling system to the room.
Some of the construction techniques used to minimize the transmission of external
sounds into the listening environment include:
• Double or triple layers of sheet rock (gypsum board)
• Double wall construction, meaning two complete sets of studs (preferably stuffed with
fiberglass insulation)
• Double wall construction with staggered studs (minimizes transmission of vibrations
from one set of studs to the next)
• Floating floor construction (again, preferably stuffed with fiberglass; this also can
enhance the perceived bass, since the subwoofers may cause structural vibrations
through the false floor which then get transmitted up through furniture)
• Seal all windows, doors, vents
• Seal and caulk all apertures in the wall (electrical outlets, through-wall plumbing, etc.)
Finally, transient noises (traffic on the street, dripping faucets, etc.) distract your atten-
tion away from the program material, and remind you that you are in your home
theatre/living room rather than a participant in the action of a movie.
STANDING WAVES
A "standing wave" is what causes a pipe of a particular length in a large pipe organ to
have its characteristic pitch. The pipe literally amplifies certain frequencies, based on
its length and the wavelength of the frequency.
A typical rectangular room has three characteristic "lengths," and thus three fundamen-
tal standing wave frequencies. In addition, multiples of these frequencies are also
amplified. These frequencies are often referred to as "room resonances" or "room
modes"—that is, the frequencies at which the room tends to vibrate of its own accord.
These resonances lead to uneven frequency response, the greatest problems being in
the 60–150 Hz range for a typical domestic living room. (At lower frequencies in larger
rooms.)
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