TM 5-805-4/AFJMAN 32-1090
GLOSSARY
Absorption
Frequency (Hz)
Conversion of acoustic energy to heat energy or
The number of cycles occurring per second.
another form of energy within the medium of
(Hertz is a unit of frequency, defined as one cycle
sound-absorbing materials.
per second).
Noise
Absorption Coefficient
Any unwanted sound that can produce undesir-
The ratio of sound energy absorbed by the
able effects or reactions in humans.
acoustical material to that absorbed by a perfect
absorptive material. It is expressed as a decimal
Noise Criteria (NC)
fraction.
Octave band curves used to define acceptable
levels of mechanical equipment noise in occupied
Average Sound Level and Average SPL
spaces. Superseded by the Room Criteria (RC).
The arithmetic average of several related sound
levels (or SPL in a specified frequency band)
Noise Isolation Class (NIC)
measured at different positions or different times,
A single-number rating derived from measured
or both.
values of noise reduction, as though they were
values of transmission loss, in accordance with
A-Weighting (dBA)
E-413. It provides an estimate of the sound isola-
A frequency response characteristic incorporated
tion between two enclosed spaces that are acousti-
in sound-level meters and similar instrumentation.
cally connected by one or more paths.
The A-weighted scale response de-emphasizes the
lower frequencies and is therefore similar to the
Octave Band
human hearing.
A range of frequencies whose upper band limit
frequency is nominally twice the lower band limit
Background Noise
frequency.
The total noise produced by all other sources
associated with a given environment in the vicin-
Octave-Band Sound Level
ity of a specific sound source of interest, and
The integrated sound pressure level of only
includes any Residual Noise.
those sin-wave Pressure components in a specified
octave band, for a noise or sound having a wide
Decibel (dB)
spectrum.
A unit for expressing the relative power level
Residual Noise
difference between acoustical or electrical signals.
The measured sound level which represents the
It is ten times the common logarithm of the ratio
summation of the sound from all the discrete
of two related quantities that are proportional to
sources affecting a given site at a given time,
power.
exclusive of the Background Noise or the sound
Field Sound Transmission Class (FSTC)
from a Specific Sound Source of interest. In acous-
tics, residual noise often is defined as the sound
A single-number rating derived from measured
level exceeding 90% of a noise monitoring period.
values of field sound transmission loss in accord-
ance with ASTM E-413, "Rating Sound Insula-
Room Criteria (RC)
tion", and ASTM E-336, "Measurement of Air-
Octave band criteria used to evaluate acceptable
borne Sound Insulation in Buildings". It provides
levels of mechanical equipment noise in occupied
an estimate of the performance of actual partitions
spaces.
in place and takes into account acoustical room
effects.
Sound Power level (Lw or PWL)
Ten times the common logarithm of the ratio of
Field Sound Transmission (FSTL)
the total acoustic power radiated by a sound
The sound loss through a partition installed in a
source to a reference power. A reference power of a
building, in a Loss specified frequency band. It is
picowatt or 10-12 watt is conventionally used.
the ratio of the airborne sound power incident on
Sound Pressure Level (Lp or SPL)
the partition to the sound power transmitted by
the partition and radiated on the other side,
Ten times the common logarithm to the base 10
expressed in decibels.
of the ratio of the mean square sound pressure to
Glossary-l
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