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Sound Isolation Principles |
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Copyright 2006
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Principle 3 - Absorption Installing insulation in a wall or ceiling cavity increases the sound loss due by eliminating/removing/destroying some sound. Another benefit of insulation in a cavity is to lower the resonant frequency of decoupled walls. All that noted, insulation loses its effectiveness at very low frequencies. Don’t get too caught up in fretting over what insulation to choose, just make sure to use something. Fortunate ly, common fiberglass routinely used in construction has been shown to be as effective as any other insulation type, particularly at low frequencies. Absorption is most effective in decoupled or damped walls, in a conven tional 2x4 wall, sound can easily pass through the studs and doesn’t need to go through the insulation.
Principle 4 - Resonance This works AGAINST the good things done by principals #1, #2 and #3 above by making it very easy for sound to vibrate a wall. At resonance frequencies even a massive de-coupled wall with insulation will vibrate very easily (as we saw above in the decoupling section). Since a vibrating wall vibrates air on the other side, reso nance increases the ease with which sound is transmitted. There are two basic ways to deal with resonances: 1. Damp the resonance - There are visco-elastic damping compounds available as well as commercially available pre-damped drywall and flooring. 2. Move the resonance point - If we lower the resonance frequency of a wall by employing principals # 1, 2 and 3, we are less likely to encounter sound at those lower frequencies
Principle 5 - ConductionThe last important principle of sound isolation is conduction. Conduction plays a role in keeping common wood stud walls (not decoupled walls) from attaining a high level of performance. The drywall conducts vibration to the studs, which transfer it to the other side, keeping performance low. Outside of the performance of walls, conduction plays a large role in flanking noise – noise traveling from one room to an other by some path other than the direct. To reduce the conduction of a structure, you can either insert mechanical breaks (like cuts), or raise the damping of the structure (damping dissipates the energy as it travels, lowering conduction considerably, often to the point where it is irrelevant ).
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