Types of sound
Researchers in sound transmission use three definitions to describe how sound travels in a building.
- Airborne transmission: sound in a room generates pressure waves which induce vibration in a wall.
- Impact transmission: sound created by an object striking the dividing wall – shoes on a hardwood floor would be an example of this.
- Flanking transmission: the resulting vibrations of a noise are transmitted by a building component.
Air tightness in a building will significantly r educe airborne transmission. This is why energy efficient homes are quieter. A Code for Sustainable Homes Level 3 home with less than 3 air changes per hour air leakage should be very quiet. Sound must have a medium to move through and, if the building is airtight, air can’t be that medium.
Generally speaking, added mass will also reduce the transmission of outside noise. This is why there is sometimes concern about timber frame houses being noisier. It isn’t true, however. The additional cavity insulation in a timber frame wall will reduce outside noise considerably.
Preventing impact transmission
As with fire safety, the reduction of sound transmission has more to do with how you design the house rather than what the house is made of. The best strategy is isolation – finding a way to decouple the two sides of walls or floors from each other. Resilient metal channels can be used to attach wallboard to timber frame assemblies. These absorb sound. A double layer of gypsum board adds considerable mass and is very effective in most cases. Sound absorbing insulation will also reduce impact transmission.
Flanking transmission is especially a problem in multi-unit residential buildings where sound can travel via ceilings, floors, structural joints and plumbing chases. In some cases shared components, such as floorboards, concrete block walls or concrete floors can make flanking problems worse. The key to stopping flanking transmission is isolation. There should be an air gap in a partition wall, so the wall is not shared between units. Remember that flanking transmission is not sound passing through the wall, it is sound that causes the wall to vibrate, and it becomes a sound transmitter.