period three
Acoustical Analysis
notes
Absorption
| absorbed | |
incident Wi | sound energy | |
Wa | ||
sound energy | ||
Wr | Wt | |
reflected | ||
sound energy | transmitted | |
| sound energy |
Figure 49
Absorptive materials work by converting acoustical energy into heat energy. The absorbed energy (Wa) is the portion of the incident sound energy (Wi) that is neither transmitted through the material nor reflected off the material.
The absorptivity of a material depends on several factors, including thickness, frequency of the sound, and whether there is a reflective surface located behind the absorptive material. Materials that are porous (such as open cell foam) or fibrous (such as fiberglass insulation) are more absorptive than materials that are smooth and dense (such as sheet metal or gypsum board). Increasing the thickness of a material, and installing a reflective surface behind the material, both increase its absorptivity. It is also important to note that absorption is dependent on frequency.
The absorptivity of a material is typically described in terms of an absorption coefficient. The absorption coefficient is the ratio of sound energy absorbed by the material to the sound energy incident upon the surface of the material. Preferably, absorption coefficients are reported for each octave band, but may also be expressed in terms of a single Noise Reduction Coefficient (NRC). The NRC is simply the arithmetic average of the absorption coefficients for the 250, 500, 1,000 and 2,000 Hz octave bands.
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