2.9 Double Facade

Double facades can be used to control environmental noise break-in without the need for acoustic attenuation. When using a double facade, air enters the building through conventional open windows. The acoustic protection is achieved by acoustically screening these windows by means of a secondary facade. Air enters the void between the two facades via a gap at the bottom of the outer, secondary facade. The edges of the secondary facade are typically taken back to the primary building envelope. Attenuation may be required at the air inlet between the two facades. The advantage of this type of facade is the fact that simple openable windows can be used. It is also possible to form buildings with an interesting and unique appearance.
The drawbacks are clearly cost and space and for these reasons this type of noise control measure is somewhat uncommon.
It is also important to note that the acoustic separation between two rooms can be compromised when windows are open. Acoustic splitters or absorption may be required.

2.10 Secondary Facades - External Ventilation Shafts

An alternative to secondary facades is to use external chimneys. This scheme uses very similar principles to that of a double facade; the difference being that the external chimneys are only used over the ventilation openings. This arrangement clearly has cost and space saving advantages over that of double façades.

A second architectural advantage is that it is possible to provide an animated facade. Forming the chimneys from glass or other translucent materials, allows interesting designs in the form of graphics to be incorporated within the chimneys, adding further interest to the facade of the building.

One of the drawbacks of this design is that acoustic treatment may be required within the chimneys to prevent the spread of sound along its length. This may be required to maintain the acoustic separation across floors. If this is the case, acoustic art work could be used to give the architectural design and also to provide the acoustic absorption within the chimneys.

1.2 Acoustics and Vented Facades

When naturally ventilating a building on a noisy or moderately noisy site the acoustic design of the facade becomes fundamental. The ability to provide high levels of sound resistance within a limited depth is often a requirement for an attenuator in the facade of a building. Due to the honeycomb structure and the performance of the acoustic foam, the NAT Vent Attenuator provides an exceptionally slim line attenuator with an outstanding acoustic performance. The size and depth of the NAT Vent Attenuator is dependent upon two main factors;

1 The free/open area specified by the M&E consultant/engineer. This governs the required face area of the attenuator and its percentage free area. A large face area will transfer a greater level of sound into a room, hence the depth of the attenuator is required to be lengthened such to resist the increased level of sound power.

2 The second factor affecting the depth of the attenuator is the required level difference between the environmental noise and the required internal noise level. The greater the difference, the longer the attenuator. To design and test the NAT Vent Attenuator, MACH Products has an in-house calibrated test rig to BS EN ISO 7235:2003.  This test facility enables MACH Acoustics to design and test a range of options during Design Stages C, D and E of a project.  The NAT Vent Attenuator is then manufactured to fit into a given location, as well as meeting the buildings ventilation and acoustic requirements.

See Chapter 2 and our website, www.machproducts.co.uk for further details.