MIL-HDBK-1191
APPENDIX E:
E.1. Introduction. This appendix covers general acoustics information,
designed to help a design professional or facility program manager in
understanding in more detail, individual aspects not covered in section 23 and
Appendix A.
E.2
Definitions.
E.2.1.
A-weighted decibels (dBA): The A-weighted scale of a sound level
meter measures decibels in a manner that discriminates against lower
frequencies in the same manner as does human hearing. Therefore, sound
measured in dBA is a fair measure of how loud we perceive a source.
E.2.2.
CAC (Ceiling Attenuation Class): CAC values measure the amount of
sound that is blocked by an acoustic tile ceiling for the sound path that goes
from one room, through its acoustic tile ceiling, into a standard plenum, and
back through the acoustic tile ceiling into a neighboring room. The CAC value
applies just to this path through the plenum, and is analogous to the STC
rating for a wall. Higher values indicate a greater ability to block sound.
E.2.3
Coefficient of absorption. All materials absorb some sound, and this
percentage of sound is measured (in laboratory tests) as a coefficient of
absorption. Coefficients of absorption range in value from close to 0 (no
absorption) to nearly 1.0 (100 % efficient); these coefficients vary as a
function of frequency. Materials that are most efficient at absorbing sound
include soft porous "fuzzy" materials such as glass fiber, mineral wool,
carpet, curtains, acoustic tile, and other specialty materials. Materials
that depend on their porosity to absorb sound should not be painted in a way
which will clog their pores, and thus degrade their acoustical performance.
E.2.4.
Decibels (dB). Sound energy is measured in decibels (dB), which
corresponds to loudness. The decibel scale ranges from 0 dB (threshold of
hearing) to over 100 dB (painful and injurious to one's health). Decibels are
a logarithmic scale, which means that you can not add decibels directly (50 dB
+ 50 dB equals 53 dB, and does not equal 100 dB). Discussions that follow
will avoid detailed calculations or technical analysis.
E.2.5.
Frequency (Hz). The frequency of vibrations for a sound source is
measured in cycles per second, or Hertz (Hz), which corresponds to pitch.
Human hearing responds to sound from 20 Hz (very low tones) to 20,000 Hz (very
high tones). Frequencies of sound relate to types of noise sources (e.g.,
diesel engines produce low frequency sound, human speech carries
intelligibility at higher frequencies), sound paths (some materials and
constructions are better at blocking or absorbing sound at certain frequencies
than at other frequencies), and the receivers (humans are most sensitive to
sounds at mid- to high-frequencies of 500 Hz and above).
E.2.6.
IIC (Impact Isolation Class): IIC is a single number rating system
for the ability of a floor/ceiling construction system to reduce the noise of
impact or structure-borne energy. Higher values indicate a greater ability to
reduce impact noise.
E.2.7.
NC (Noise Criteria level): NC is a single number rating system for
level and spectrum of steady-state background noise levels in buildings, as
determined by the noise of mechanical systems. Minimum and maximum ratings per
room are listed in Appendix "A".
23-19
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