3.4 Fire and Air Return Paths

Fire Separation
The spread of fire is an important consideration; as such the NAT Vent Attenuator can be made from Class O Foam. Additionally, it is common to install either an intumescent fire damper 13 or smoke damper into the corridor wall adjacent to the NAT Vent Attenuator. The requirement for fire/smoke dampers is dependent on the level of fire compartmentalisation of the building.

Air Return Path
Several methods can be used to provide air return paths. These include atriums, light shafts, ventilation shafts and stair cores. Stair cores were successfully used as an air return path as part of the ventilation scheme for the refurbishment of a Bristol University Education Building. In this development, smoke dampers were proposed to be incorporated within the walls to the stair cores. A cost saving was provided by allowing the flow of air through fire doors held back on magnetic holders. In the case of a fire, the magnet released the fire door, providing the required protection to the stair cores.

2.2 NAT Vent Box

The key elements making up the Nat Vent Box are shown in the illustration to the left. Air is brought in through the external weather louvre 11, then through a volume control damper 10. The volume control damper can either be used to seal the building or manage/control the fl ow of air into the building. This damper can be manually operated by means of a handle or Teleflex cable. Motorised thermal dampers can be used in combination with a BMS system. A second important function of the thermal damper is to maintain the thermal line/thermal performance of the building envelope when shut.

The noise reduction across this unit is achieved through the attenuator 9. The attenuator is incorporated between the thermal damper and an internal louvre 8. The attenuator is formed by stacking W-shaped tiles which creates a honeycomb structure, restricting the passage of sound. Air however, is still allowed to fl ow through the central paths of the honeycomb body. The difference between the external noise levels and the required internal noise levels, governs the depth of the attenuator 9, the greater the difference the deeper the attenuator.

Finally, the internal louvre can be manufactured from wooden slats,  perforated metal, or a conventional internal louvre can be used.

2.4 Bench Seating

Adding attenuation to the vented facade of a single storey building is comparatively easier than a multistory building, since it is often possible to extend the building envelope, accommodating the additional depth of the NAT Vent Boxes.

In the case of single storey buildings, it is common to maintain a simple vertical thermal line, by placing the thermal damper into the line of the facade 4. The acoustic attenuation in this instance is placed on the external side of the building. This design approach has been implemented by MACH Acoustics on several projects.

The NAT Vent Box has been installed under bench seating, flower boxes, play boxes, small steps in the facade and other elements. These units have all been used to hide and accommodate the additional facade depth often required when naturally ventilating a building on a particularly noisy site.

A second advantage of single storey buildings is the potential to incorporate the NAT Vent Box above or within roof lines, above corridors, over storage rooms and other areas.

2.12 Solar Shading and Acoustic Screening to Open Windows

‘Acoustic Scaled Models of Vented Facades’ is a technology which has been developed by MACH Acoustics. This technology enables the effects of acoustic screens attached directly to the facade of a building to be assessed. Scaled models are typically used to assess the acoustics of auditoriums during the design stages. For major concert halls, a scaled model of the auditorium is built such to assess its acoustic performance and characteristics. Scaled models are used due to their practical, accurate and cost effective nature. The same principles apply to the design of screened acoustic facades. MACH Acoustics has developed a method of assessing the acoustic resistance of screens attached directly to the facade of a building by means of scaled models.

The illustration below shows two design options where screened facades were proposed in order to add acoustic attenuation to a vented facade within an inner city office block. This method of noise control is simple, cost effective and provides the additional acoustic resistance such to prevent inner city noise being a nuisance within the office accommodation. Screened facades are also a good method of meeting the requirements set out by BREEAM. The drawback of this system is that these screens can only enhance the performance of an open-able window by around 5 to 7 dB, meaning that these facades can only be used when external noise levels are moderately high.

1.3 Acoustics and Cross Ventilation

It is generally accepted that cross ventilation is the most effective form of natural ventilation. Acoustics plays a key role in the design of a cross ventilated building as air must flow freely through the building whilst maintaining privacy across partitions. To allow cross ventilation and maintain privacy, cross talk attenuators are required within partitions adjacent to circulation spaces.

One of the key design benefits of the NAT Vent Attenuator is the simple implementation of cross ventilation through a corridor wall, while still maintaining the acoustic integrity of these partitions. Furthermore, this product enables cross ventilation to vertically stacked rooms, vented through a single stack. In other words, vertically stacked spaces no longer require independent chimneys to maintain the acoustic separation between rooms, resulting in a significant recovery of floor area and a considerable cost saving.

One of the drawbacks of ventilating through the corridor wall is the requirement for an exceptionally large bulk head, such to accommodate large, heavy attenuators. The NAT Vent has been designed to provide exceptional levels of cross talk separation. MACH Acoustics has undertaken extensive research to understand the required levels of acoustic separation across these partitions. Depending upon the air flow and the required level of acoustic separation, the NAT Vent can be as slim as 600mm deep. In some instances, this is required to be increased to 1200mm, depending on the required acoustic performance.