MIL-HDBK-1191
Class (IIC) rating system. In terms of criteria, the values are roughly
analogous to the STC ratings (that is, if an adjacency requires an STC 40 wall
or floor construction, then a similar adjacency probably requires an IIC 40
rating for impact noise).
23.2.5.1
Carpet. With structural concrete floor systems, most structure-
borne impact noise can be alleviated with the use of carpet, even a thin
industrial carpet. If hard flooring is necessary, try to keep heavily
trafficked areas directly above other less sensitive areas that are also
heavily trafficked.
23.2.5.2
Carts. Noise sources, such as rolling carts, should have soft
rubber wheels to minimize impact noise generation.
23.2.6
Speech Privacy. The speech privacy obtained between spaces
depends upon both the sound isolation provided by the intervening construction
systems STC and the ambient noise level NC of that particular receiver space.
Recommended ambient noise levels and STC ratings are listed, room by room, in
Appendix "A". It is important to provide the proper degree of speech privacy
between acoustically sensitive spaces, such as doctors' offices and
examination/treatment rooms. The approximate degree of speech privacy between
spaces can be estimated using the following relationships:
Speech Privacy Rating = STC+ NC
Speech Privacy Rating
Approximate degree of Speech
Privacy
55
none
60 to 70
normal
65 to 75
Moderate
70 to 80
Confidential
>80
Complete
The recommended ambient level in an examination room is NC-30 to NC-35, and
the recommended isolation between an examination room and private office is
STC 45. The sum is 75 to 80 which will provide a moderate-to-confidential
degree of speech privacy between the two spaces. If, however, the
wall/ceiling construction provided only STC 25 due to an accordion type
folding door, or if the background sound level were only NC-20 to NC-25 while
the ceiling CAC were only CAC 35, then the STC + NC total would be 55 to 60,
which provides a none-to-normal degree of speech privacy. This degree of
speech privacy is not acceptable for an examination room.
23.2.6.1
Sound Masking. Speech Privacy Ratings can be boosted when STC
ratings are low or background NC levels are too quiet by the addition of
background sound masking. This approach introduces low level broadband
inconspicuous background sound, like air conditioning noise, at levels around
NC-40. This masking sound covers over the intrusion of a neighbor's
intelligible speech. The electronic sound is distributed through loudspeakers
hidden above the ceiling.
23-6
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