Noise Control Product Types
ABSORBERS
 Use: To reduce noise reflection. To dissipate noise energy.
Physical Properties: Porous, fibrous and sometimes covered
with protective membranes. Noise enters the absorber and is
partly dissipated (absorbed) within the material. Some is
transmitted. Some is reflected. Absorber performance is
expressed as a decimal value. A perfect absorber is rated at
1.00. The higher the decimal value the more effective the
absorber will be.
Effectiveness is expressed as NRC (Noise Reduction
Coefficient).
NRC: Percentage of acoustical energy absorbed calculated as
an average of laboratory test data at several frequencies.
|
Noise Reduction Coefficients of Materials | NRC |
| Brick, unglazed | .05 |
| Concrete block | .05 |
| 1/8" pile Carpet | .15 |
| 5/16" pile Carpet and foam | .35 |
| Concrete floor | .00 |
| Plaster, smooth finish | .05 |
| Plywood paneling, 1/4" thick | .10 |
| Water surface (as in swimming pool) | .00 |
| 1" thick fiberglass curtain | .70 |
| 3" thick "SONEX" wedge foam | .86 |
| 4" thick smooth surface foam | .89 |
| 4" thick metal panel | .95 |
BARRIERS
 Use: To block transmission of noise.
Physical Properties: Non-porous, high density and
usually non-fibrous. Barriers are generally flexible or
damped. The noise is blocked, reflected and re-routed
in another direction. Barrier materials are tested and
rated for their Sound Transmission Loss capability. The
number is stated in dB and the higher number signifies
the better barrier.
Effectiveness is expressed as STC (Sound Transmission Class).
STC: Single number rating derived from decibel loss data at
several frequencies.
|
Sound Transmission Class of Materials | STC |
| 1 lb. density barrier material | 26 |
| 1 lb. density transparent curtain | 26 |
| 5/8" Gypsum wallboard | 30 |
| 3/16" Steel wall | 31 |
| 2" fiberglass curtain with 1 lb. barrier | 29 |
| 2" thick metal panel (solid and perforated) | 35 |
| 4" thick metal panel (solid and perforated) | 41 |
| 12" thick concrete | 53 |
| 3/8" plasterboard | 26 |
| 22 gauge steel | 25 |
| Solid core wood door, closed | 27 |
| Concrete block wall, unpainted | 44 |
COMPOSITES
Use: To block the transmission of noise and reduce reflections
from the barrier.
Physical Properties: Consists usually of a layer of porous material
and a layer of dense material. The composite material will
have a performance capability as an absorber and as a barrier.
Septum barriers are sandwiched between two absorber layers.
Effectiveness is a combination of STC and NRC ratings. |  |
DAMPING
Use: To reduce noise radiated from vibrating surfaces.
Physical Properties: Viscoelastic. Damping coatings take
many forms. There are mastics for spraying, troweling, etc.
and there are tapes and sheets with pressure sensitive adhesive.
Damping treatments are sometimes combined with
absorbers.
Effectiveness is expressed as a "loss factor" which is the
damping/stiffness ratio of a material. |  |
DECOUPLED
COMPOSITES
Use: To enhance the performance of the composite material
when applied to the inside of an existing barrier. Decoupling
creates an air space between the existing barrier and the
septum composite barrier boosting transmission loss beyond
what could be expected with direct attachment.
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DIFFUSION
Use: To reflect sound waves off convexly curved or uneven
surfaces for the purpose of evenly distributing and blending
the sound over a broad area. In critical listening environments
diffusion can eliminate sharp echoes without eliminating
the sound by absorbing it.
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ELECTRONIC
Use: To cancel unwanted noise energy through destructive
interference by electronically generating a 180° out
of phase anti-noise which is equal and opposite in phase
and amplitude.
Physical Properties: Equipment includes an input
microphone, controller/amplifier, speakers and an error
microphone. Works best with noise propagated in a
confined/closed loop space such as a pipe or duct. Works
best where the noise source is repetitive and not random.
Works best in low frequencies up to about 500 Hz.
Effectiveness is expressed as Dynamic Insertion Loss (DIL)
for active/electronic mufflers and silencers.
DIL: The noise reduction of sound power level attained by
inserting a silencer or muffler in a pipe or duct transmission
system under air flow conditions.
Technological Advances: Electronic or active noise and
vibration technologies are emerging from research and
development to production applications. Recent advances
include cancellation headsets, mufflers for automotive and
industrial and a variety of consumer appliances. |  |
FLOW CONTROL
Use: To reduce flow/fluid-borne noise transmission
traveling through pipes and ducts connected to air/fluid
control devices, equipment and systems. Mufflers or
silencers use absorptive and reactive designs to allow
air passage while attenuating the noise. Fluid-borne
flow systems may be air, gases or steam.
Physical Properties: Internal geometry of the flow
control device dictates the overall noise reduction that
can be achieved and the resultant pressure loss of the
system. Absorptive designs can vary the insulation
thickness and density in the wall cavity as well as the
distance between internal baffles (passage width).
Reactive designs can vary the flow control device internal
chamber length and volume as well as the number
of interconnected chambers and the size and length of
choke tubes connecting the chambers.
Effectiveness is expressed as Dynamic Insertion Loss
(DIL) for mufflers and silencers.
DIL: The noise reduction of sound power level attained
by inserting a silencer or muffler in a pipe or duct transmission
system under air flow conditions. |  |
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