Noise Control Terminology and Definitions
- Absorption Coefficient:
- The absorption coefficient of a material
or sound absorbing device is the ratio of the sound
absorbed to the sound incident on the material or device.
- Acoustical Material:
- A material used to alter a sound field.
The material may be used to absorb, damp or block acoustical
energy.
- Airborne Noise:
- A condition when sound waves are being
carried by the atmosphere.
- Ambient Noise:
- All the sounds from many sources associated
with a given environment.
- Anechoic Room:
- A test chamber which has a lining of
absorbent acoustical material to eliminate all sound reflections.
It is most often used to determine the sound radiation
characteristics of equipment.
- Damping:
- The process of dissipating mechanical vibratory
energy into heat. In noise control, a damping material is
usually applied to a vibrating surface to reduce the noise radiating
from that surface.
- Dissipative Silencer:
- A device inserted into an air duct or
opening to reduce noise transmitted through the duct or
opening. Noise reduction is accomplished through the use of
internal sound absorbing materials.
- Flanking Transmission:
- Noise that reaches an observer by
paths around or over an acoustical barrier.
- Frequency Spectrum:
- A graph or plot of the sound pressure
level in each band from a set of octave or 1/3 octave bands.
- Insertion Loss:
- The reduction of sound power level attained
by inserting a silencer or muffler in an acoustic transmission
system (see ASTM E-477).
- Loudness:
- Loudness is the subjective human definition of the
intensity of a sound. Human reaction to sound is highly
dependent on the sound pressure and frequency.
- Mass Law:
- A rule for estimating the transmission loss of a
barrier in its mass controlled region. The rule states that transmission
loss increases/decreases 6 dB for each doubling/halving
of either frequency or barrier surface density.
- Noise:
- Any undesired sound.
- Noise Reduction (NR):
- The reduction in sound pressure level
caused by making some alteration to a sound field.
- Noise Reduction Coefficient (NRC):
- A single number rating
which is the average of the sound absorption coefficients in the
octave bands centered at 250, 500, 1000 and 2000 Hz expressed
to the nearest integral multiple of 0.05 (see ASTM C-423).
- Octave Band (O.B.):
- A range of frequencies where the highest
frequency of the band is double the lowest frequency of
the band. The band is usually specified by the center frequency,
i.e., 31.5, 63, 125, 250, 500 Hz, etc.
- Radiation:
- The process whereby structure-borne vibration is
converted into airborne sound.
- Reverberation:
- Reverberation is the echoing of previously
generated sound caused by reflection of acoustic waves from
the surface of enclosed spaces.
- Reverberation Room:
- A test chamber so designed that the
reverberant sound field within the room has an intensity that
is approximately the same in all directions and at every point.
It is commonly used to measure sound absorption, ASTM
C-423 and transmission loss, ASTM E-90.
- Sabin:
- The unit of measure of sound absorption. The number
of square feet of sound absorbing material multiplied by the
material absorption coefficient.
- Sound:
- Pressure waves that are traveling in the air or other
elastic materials.
- Sound Absorption:
- The acoustical process whereby sound
energy is dissipated as heat rather than reflected back to the
environment.
- Sound Level Meter:
- An instrument used to measure sound
pressure level. Sound level meters are commonly either Type
1, precision instruments, or Type 2, general purpose instruments.
Both types can have weighting and filter networks to
provide dB readings by octave band in the A, B, or C scales.
- Sound Power Level (Lw):
- A measure of the total airborne
acoustic power generated by a noise source, expressed on a
decibel scale referenced to some standard (usually 10-12 watts).
- Sound Pressure Level (Lp):
- A measure of the air pressure
change caused by a sound wave, expressed on a decibel scale
referenced to 20µPa.
- Sound Transmission Class (STC):
- A single number rating
derived from measured values of transmission loss in accordance
with ASTM 413. The rating provides an estimate of the
performance of a barrier in certain common noise attenuation
applications.
- Structure-borne Noise:
- Mechanical vibration in a structure
which can ultimately become audible sound. Until such time
as radiation occurs, these vibrations are inaudible and of little
concern.
- Transmission Loss (TL):
- The reduction in sound power that is
caused by placing a wall or barrier between the source and
receiver. Transmission loss is expressed in decibels.
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