Understanding 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 |
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: Visco-elastic. 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.