Design considerations
Effective Sound Transmission Loss and how to improve it
It is vital to distinguish between sound transmission loss and sound absorption . Sound absorbing materials control sound from within spaces and work by allowing sound to pass through them quite easily. They are generally porous and absorb sound as a result of many interactions. Conversely, a material or system, that provides a good sound transmission loss is usually non-porous and a good reflector of sound.
For significant noise reduction between rooms, the walls or floor, separating them must transmit only a fraction of the sound energy striking them. The ratio of the sound energy striking the wall to the sound energy transmitted through the structure is called transmission loss and expressed in decibels.
The sound transmission loss of a basic construction can be improved by applying the following:
- increasing mass
- breaking the sound vibration path
- increasing damping
- increasing cavity depth
- providing cavity absorption
Increasing Mass
Heavier materials block sound better than light materials. For example, adding another layer of sound block plasterboard provides increased sound transmission loss.
As a general rule, every doubling of the weight of the wall increases sound transmission loss by an additional 5-6dB. Heavier walls, however, are not the most economical or practical solution to sound control.
It should be noted that lightweight blockwork has poorer sound insulation values at low frequencies because of its porous nature. These types of blocks must be plastered to achieve satisfactory sound insulation.
Plasterboard must be butt jointed at the studs only. Layers should be 'staggered' so that the joints do not 'overlap'. All joints must be sealed with plaster at each stage. All edge peripheries must be sealed, with mastic, at each stage.
Breaking Vibration Paths
Walls transmit sound most effectively when they can transmit vibrations from one wall face to the other through structural elements such as the studs. Whatever can be done to interfere with the transmission of vibration between one wall surface and the other will help reduce sound transmission through them.
Metal studs are more resilient than wood studs and reduce the transmission of vibrations between one wall surface and the other. In wood stud constructions resilient metal bars can be used between the plasterboard and the studs to break the vibration path.
Although the rules for maximising sound control are relatively simple, applying them is not.
The theory underlying these rules is not well understood and the opportunities for error are high.
Cavity Absorption
The sound transmission loss of a wall can also be increased by filling the wall cavity with sound absorbing materials such as rockwool or fibreglass insulation.
Care must be taken to avoid bridging any cavities in such a structure. Cavity infill has the added advantage of keeping the cavity clean of mortar droppings. It takes only 1 lump of mortar, bridging the cavity, to ruin the elaborate precautions taken to optimise the sound insulation in the first place.
Perimeter Sealing
An air-tight seal should be used around the perimeter of the wall or ceiling to effect a proper acoustic seal. This is critical.
DO NOT under-estimate the amount of sealing mastic you will need for an acoustic partition !
A wall with a potential STC of 55 or 60, which has a hole of only 0.001% of the total wall area, is reduced to an effective STC of 50. As the hole area increases, the rating is eventually determined entirely by the hole area.
Such tiny leaks can easily occur at the periphery of walls and floors, where caulking is absent or improperly done, or where a hole is made to add some service such as electricity or plumbing. To reduce sound leaks, all holes and fissures in a party wall or floor must be thoroughly caulked, all windows and doors must be tightly weatherstripped,and holes for services properly repaired. As a rule of thumb, if a construction is not watertight, then there is a good chance it is not 'sound-tight'.
