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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: 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.

STC: Single number rating derived from decibel loss data at several frequencies.

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.

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