Unlike airborne noise, mechanical vibrations pass quite readily through rigid structures. Thus the bouncing of a golf ball on a concrete floor may be clearly audible to people several floors below whereas the noise from a loud vacuum cleaner would not. This is because there are direct pathways for the impact noise to travel quickly around the building. The only way to tackle impact noise problems is by preventing them in the first place, or providing resilient joints between rigid building elements to obstruct these pathways.

Constructional Consequences

Doors

The insulation provided by a door does not follow the predictions of the mass law for two basic reasons:

1. There are nearly always small gaps between the door and door frame;
2. The size of the door is usually much smaller than the wall in which it is placed so the resonant frequencies of the door occur at a higher frequency.

Therefore, when high insulation is required, the edges of the door should be sealed very carefully with gaskets of felt or rubber.

Values of insulation greater than those predicted by the mass law can be obtained through the use of double doors. For double doors separated by at least 8cm, the average insulation is usually 5 dB greater. If the two doors are separated by a short passageway, and possibly lined with absorbent material, then the increase can be as much as 10-12 dB.

Outer Walls and Windows

Most of the noises that disturb people occur out of doors. The noise from such sources enter buildings through the outer walls, the windows and the roof. Many modern buildings have roofs made of concrete (or other comparably heavy materials) so these do not pose as much of a problem as the outer walls and windows.

The insulation of outer walls is usually (but not always) determined by the insulation of the windows. When a high degree of insulation is required, it is essential that fixed windows be used (i.e.: not openable), which may mean the use of mechanical ventilation. The insulation of windows is slightly more difficult to estimate than a solid wall because it is more dependant on the window's dimensions and coincidence plays a much more important role.

The insulation curve of a single glazed window is typically marked by a deep trough in the mid-frequency range, at which the ear is most sensitive. Whilst not a major problem with traffic noise (given the predominance of low frequencies), aircraft noise shows up a window's poor insulation properties at higher frequencies.

As with a wall, an improvement in the insulation of a window can be obtained through the use of double-leaf construction. As the mass of each glass pane is relatively low, the thickness of the air gap should be quite large in order to act as a spring, reducing the resonant frequency.

In some countries double glazing is often used for thermal reasons before acoustic. Unfortunately, such constructions usually have a very narrow cavity (only 10-12 mm). Consequently the two panes are closely coupled and the resonant frequency is around 300Hz.

For increased insulation, the gap should be at least 75 mm (preferably 100 mm), with sound absorbent material placed around the perimeter and different thicknesses used in each layer (even angled slightly differently).

Given local conditions, it is often more appropriate to provide natural ventilation. This can cause problems when there are high levels of background noise immediately outside a window. To overcome this, acoustic baffles can be used as shown below.

Figure 1 - Acoustic baffles used to reduce noise whilst still allowing for natural ventilation.

Floating Floors

As well as normal transmission, floors require special consideration due to impact noise. Often a floor that is a good sound insulator (say a concrete slab) may be unacceptable when considering the transmission of impact noise. An obvious solution to the problem of impact insulation is to cover the floor with a resilient layer such as carpet or rubber tiling. Such floor coverings are most effective in reducing the higher frequencies of the impact noise but may conflict with other considerations such as durability and resistance to chemical attack.

To overcome this, it is possible to use a floor composed of a hard upper layer and a resilient lower layer which rests on the structural floor. In such a construction it is vitally important that the resilient layer is not bridged at any point. This includes service pipes and conduits.

Floating Rooms

When very high insulation values are required, a discontinuous construction may be considered. This means that the entire room is completely separated from the main structure of the building, supported only by vibration-isolating mountings. Once again, small inadvertent sound bridges can undo a lot of money spent isolating a room. Therefore great care must be taken not to bridge the gap between the two rooms.

Ceilings

Ceilings can only really assist in the insulation of impact noise from above. This can be achieved to some degree using either a false or suspended ceiling. A false ceiling is one which is independent of the structure above it, supported by side walls whilst a suspended ceiling is hung from the structure by wires or resilient hangers.

Given that most proprietary ceiling systems are lightweight and modular, they provide flanking paths around each panel and suffer from a lack of mass. They can be made much more effective with the use of sealants and heavier materials, however, the additional structure required for support may make such a construction uneconomical.