Glossary of Common Acoustical Terms
The energy of sound waves being taken in
(entering) the surface of any material rather than being bounced off or
reflected; usually given in octave bands. Materials are rated in terms of their ability to absorb sounds.
Louver: A specially built
louver using sound-attenuating baffles instead of single-thickness bent steel or
extruded aluminum blades.
which have high absorption coefficients.
The combination of all near and far sounds, none
of which is particularly dominant.
The maximum variation of any wave from its mean value.
Increasing a sound wave's amplitude increases its loudness.
The reduction of sound level per unit distance by
divergence, diffusion, absorption, or scattering. In acoustics, the diluting or holding back of the
energy of sound waves as they pass through a material. Materials are rated for
their ability to prevent sounds from traveling
A-weighted Sound Level:
The sound pressure level which has been filtered or weighted to quantitatively
reduce the effect of low frequency noise. It was designed to approximate the
response of the human ear to sound. A-weighted sound pressure level is measured
in decibels with a standard sound level meter which contains the "A" weighting
network. A-weighted decibels are abbreviated dBA and indicates approximate relative loudness.
Ceiling Attenuation Class:
Rates a ceiling's efficiency as a barrier to airborne
sound transmission between adjacent closed offices. Shown as a minimum value,
previously expressed as CSTC
(Ceiling Sound Transmission Class). A single-figure
rating derived from the normalized ceiling attenuation
Ceiling Sound Transmission:
In a suspended ceiling construction, the transmission of
sound between adjoining rooms by way of the path consisting of the ceiling of
each room and the continuous plenum over, and common to, both rooms.
Community Noise Equivalent Level; the average (on an energy basis)
noise level measured in A-weighted sound pressure level for a twenty-four (24)
hour period with different weighting factors for the noise levels occurring
during the day, evening, and nighttime periods.
The primary unit of sound measurement; used to
quantify both sound pressure level and sound power level. In acoustics,
equal to ten times the logarithm of the ratio of one sound and a lower-intensity
reference sound. One decibel indicates a difference of about 26% and is about
the smallest change the ear can detect. The dB level is a logarithm quantity;
the maximum normal level is approximately 120dB.
Field Impact Insulation Class (FIIC):
A single-number rating
for impact sound insulation, calculated from measured values of normalized
impact sound pressure levels.
Field STC (FSTC):
Sound Transmission Class based on field
measurements of sound transmission loss of a partition.
Path: An indirect sound transmission path, such as the
structure-borne path between two adjacent rooms, that bypasses a transmission
The number of oscillations per second; generally
expressed in hertz (Hz) or cycles per second (cps).
Frequency: The lowest frequency of a vibrating
One cycle per second.
Loss: The sound level reduction at a given location
due to the insertion of a noise control device, expressed in decibels.
Day-Night Average Sound Level - A-weighted
equivalent continuous sound exposure level for a 24-hour period with a 10 dB
adjustment added to the sound levels occurring during nighttime hours (10 p.m.
to 7 a.m.)
Equivalent sound level. The
level of a
steady state sound which has the same dBA weighted sound energy as that
contained in the actual time-varying sound being measured over a specific time
Undesirable sound that interferes with rest, sleep,
mental concentration, or speech communication.
Noise Criteria (NC):
A single-number noise rating system
published in 1957 to rate steady-state continuous noise in a room from all types
of equipment, including fans, mixing boxes, diffusers, etc.
Noise Isolation Class (NIC):
A single-number rating for noise
isolation calculated from measured values of noise reduction between two
A single-number rating
system used to compare the sound-absorbing characteristics of building
A range of frequencies whose upper frequency limit is
twice that of its lower frequency limit. For example, the 1000-hertz octave band
contains noise energy at all frequencies from 707 to 1414 hertz.
Octave Analysis: Spectrum analysis using filters
whose bandwidth is a fractional ratio of the center frequency of the filter. For
example, a 1/3 octave filter centered at 1000 Hz would have a bandwidth of 260
Hz (26% equals 1/3 octave). Bandwidth (relative to a normalized center frequency
of 1) is computed as 2(1/N)-1.
The typical bandwidths used (primarily for acoustical and vibration analysis)
are 1/1 octave, 1/3, 1/12, and 1/24 octave.
The natural oscillation of a construction assembly
or air column that persists after the shutoff of an outside excitation. The
ringing that you hear after hitting a bare round sheet metal duct is an example
reflected sound in a room after its source has stopped emitting sound.
for the sound pressure level in a room to decay to a value one millionth of its original intensity, or to
drop 60 decibels.
Room Criteria (RC):
A single-number noise rating system
developed in 1981 to diagnose and rate the HVAC noise exposure in a room. This
system is more effective than the Noise Criteria (NC) System in rating noise
with strong low-frequency content.
Single Event Noise Exposure Level;
also referred to as SEL; the
time-integrated A-weighted sound pressure level of a single aircraft flyover
(which exceeds a threshold noise level) which is expressed by the level of an
equivalent one-second duration reference signal. SENEL provides a measure which quantifies the effect of
duration and magnitude for a single event measured above a specified threshold.
The reduction in the intensity or in the sound pressure
level of sound which is transmitted from one point to another.
Cracks under doors, openings in a wall, pipe or wiring
holes, etc., which allow sound to escape through a structure from one room to
Meter: A meter, usually handheld, that is used to
measure sound pressure levels.
Level (LW or PWL): The level, in dB, at
which a source produces sound, usually given in octave bands.
Level (Lp or SPL): The level of sound
energy, measured in dB, at a specific location. The frequency range of the
measurement or calculation must be indicated along with the sound level in dB.
Class (STC): A single-number rating system used to
compare the sound-isolating characteristics of partitions used to separate
Assemblies are rated in terms of Sound Transmission
Classifications. An STC value of 20-25 would indicate that even low speech would
be audible in an adjoining room. An STC value of 50-60, on the other hand, would
indicate that loud sound would be heard only faintly or not at all.
Sound Transmission Loss:
The amount of sound lost as the sound travels through a
Sound that radiates from a construction
assembly after traveling through a building's structure in the form of