Page 192 - The Master Handbook Of Acoustics

167
CONTROL OF INTERFERING NOISE

80 dB 80 dB
Outside Outside
noise 45 dB T.L. noise
level level
35 dB 60 dB T.L.
In studio
20 dB
In studio

T.L. is transmission loss
FIGURE 8-1
The difference between the outside noise level and the desired noise level inside deter-
mines the required transmission loss of the wall.

Airborne Noise
A heavy metal plate with holes to the extent of 13 percent of the total
area can transmit as much as 97 percent of the sound impinging on it.
The amount of sound that can pass through a small crack or aperture
in an otherwise solid wall is astounding. A crack under a door or
loosely fitting electrical service box can compromise the insulating
properties of an otherwise excellent structure. Air-tightness is espe-
cially necessary to insulate against airborne noises.

Noise Carried by Structure

Unwanted sounds can invade an enclosure by mechanical transmis-
sion through solid structural members of wood, steel, concrete, or
masonry. Air conditioner noises can be transmitted to a room by the air
in the ducts, by the metal of the ducts themselves, or both. Water pipes
and plumbing fixtures have excellent sound-carrying capabilities.
It is very difficult to make a solid structure vibrate by airborne
noise falling upon it because of the inefficient transfer of energy from
tenuous air to a dense solid. On the other hand, a motor bolted to a
floor, a slammed door, or an office machine on a table with legs on the
bare floor can cause the structure to vibrate very significantly. These
vibrations can travel great distances through solid structure with little
loss. With wood, concrete, or brick beams, longitudinal vibrations are