MIL-HDBK-1191
E.2.8.
NIC (Noise Isolation Class): NIC is the single number rating based
on field tests of how well all inter-connected constructions around a room
block sound. NIC will often be less than the STC rating for the same
construction by about 4 to 8 decibels. The NIC ratings include the
contribution of all sound paths between adjacent spaces (including doors,
ceilings, windows, etc.). Higher values indicate a greater ability to block
sound.
E.2.9.
NR (Noise Reduction): NR is another measure of all the sound
transfer between two spaces, by way of multiple paths (such as walls, floors,
doors, ceilings, windows, etc.) The NR is the difference in A-weighted sound
levels (dBA) from source to receiver. Higher values indicate a greater
ability to block sound.
E.2.10. NRC (Noise Reduction Coefficient): NRC is a measure of the sound
absorption of a material within a space. It is the average of absorption
coefficients of the mid-frequencies that are most typical of general office
and speech use. NRC values range from 0 to 1, with the value being rounded to
the nearest .05 value. Higher values indicate a greater ability to absorb
sound.
E.2.11. Source/Path/Receiver. Every acoustics problem and issue can be
analyzed by looking at the separate elements that comprise the
source/path/receiver outline. The source may be a neighbor talking,
mechanical equipment, a vibrating pump, music from a stereo, or outside
traffic. The path may be the building envelope, the intervening construction
between two spaces and the multiple paths by which sound may travel, the air
in a room, the building structure (in the case of structure-borne
transmission), or several of these elements together. The receiver is the
human occupant (patient, office worker, neighbor) whose health and welfare are
the goal of the acoustical design.
E.2.12. STC (Sound Transmission Class): STC is the single number rating
based on laboratory tests of how well a particular construction type blocks
sound. STC values are determined from TL data (see below). Higher values
indicate a greater ability to block sound.
E.2.13. TL (Transmission Loss): The ability of materials to block sound is
measured in a laboratory as the Transmission Loss, TL. TL covers a wide range
of discrete octave band or one-third octave band frequencies. A higher TL
means that less sound is transmitted through the construction, and hence
provides better sound isolation. TL is mainly useful in order to derive the
single number STC value for a material (see above).
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