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.

2.3 Locating Inlet Vents and Cross Vents as a Noise Control Measure

Locating Air Inlet Vents
The orientation of a building has a significant impact upon noise levels at the different facades of the building. It is often the case that facades on the opposite side of a building to a significant noise source will be considerably lower than those on the noisy side of the building.
By orientating the building and by placing non-critical spaces on the noisy side of a building, it is possible to form a good acoustic buffer. In these instances, cross vent can be used where the air intake is placed on the quiet side of the building. Cross ventilation to an atrium or circulation zone is then used to provide the air extract. Alternatively, single sided ventilation could be used for sensitive spaces on the quiet side of a building.
1 shows sound levels around a building in the form of a noise map. The classroom on the far side of building to the A40 are vented using openable windows.

1

Cross Vent to Assist with the Prevention of Noise Break-in
In instances where a building is located on an exceptionally noisy site, cross ventilation can improve the feasibility of natural ventilation. Cross ventilation has an important advantage over single sided ventilation, in that air inlet vents can be between 25% to 75% smaller than those required for single sided ventilation. This significant reduction in vent size helps considerably in preventing noise break-in, as smaller vents restrict the passage of sound into a building.

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.

2.5 Window Details

The details used to incorporate the attenuation box into a seat or play box are very similar to those used when incorporating the NAT Vent Box into the facades of buildings. As noted, it is often easier to extend out the facade line of single storey buildings such to accommodate deep attenuators. This in turn means that it is seen as possible to provide natural ventilation irrespective of noise levels. The illustration below 1 was used to control noise break-in to a sensitive office space in close proximity to a major motorway.

Installation of the Attenuator
Forming the NAT Vent Attenuator by tessellated, W-shaped foam blocks, means that this product can easily be dropped into a timber enclosure or metal duct work. The W-shaped tiles compress and can be cut to any size; hence these units are extremely easy to accommodate into the facade of a building 2

2.6 Window Systems and Curtain Walling

The thermal damper is one of the main factors affecting the cost and depth of the NAT Vent Box. Replacing the damper with an openable or motorised vent/window, eliminates both the thermal damper and weather louvre from the box make up. This typically reduces costs by ≈50% and can reduce its depth by ≈225mm.
Facade and window manufacturers can easily accommodate openable vents in curtain walling or window frames. Placing the NAT Vent Attenuator directly behind an open vent, provides a simple, cost effective design solution for preventing noise break-in.

High level air inlet
In the case where noise levels are exceptionally high, for example due to motorway noise, flight paths or inner city noise, the depth of the attenuator is required to be increased. The additional depth of the NAT Vent Box can be accommodated by using a high level bulkhead 11.

2.8 Internal Sliding Doors and Windows

The presented illustrations show alternative arrangements 9 – 13. In this case, the NAT Vent Attenuator is placed on the outside of a thermally insulated door or sliding window. The key advantage of this scheme is that it again eliminates the need for a thermal damper. Additionally, it can often be easier to accommodate the NAT Vent Box outside of the thermal line.

2.13 Screening under overhangs and above roof

The scheme below provides three design options incorporating acoustic screens into the facade of a development. In these instances, the air inlet vents are acoustically screened by baffles which break the line of sight to a given noise source. The acoustic screens are, in this instance, created by extending parts of the facade or adding panels to the facade such to cover the air inlet vents.

Option 1 - Overlapping Façades
With a perpendicular air inlet to the facade, this design provides an ideal screen to a noise source propagating from the left-hand side of the building.

Option 2 - Solar Shading and Acoustic Screening
Here a solid transparent screen incorporated into the solar shading, provides acoustic screening to a noise source directly in front of the building.

Option 3 - Photovoltaics used as Acoustic Screens
Photovoltaics provide acoustic screens in this instance. The photovoltaics are used to provide solar shading, power and acoustic attenuation, all within the building’s facade. Off-setting the photovoltaics and placing the air vents directly behind these panels provide high levels of acoustic resistance.