3.2 Size and Acoustic Performance Requirements

The length of the NAT Vent Attenuator is a function of the required acoustic performance and the free area requirements for ventilation. If a large free area is needed, the depth of the cross talk attenuator will need to be increased. This increase in depth is required to balance against the increase in sound transmission as a result of a larger face area.

The free area of the NAT Vent Attenuator is typically between 20% and 50%. The calculated pressure drop through this product is minimal due the low air speed experienced with natural ventilation. 20% free area attenuators are used in cases where there is a limited depth for the attenuator. The drawback of this configuration is that a large face area is required to maintain the same free area specified by the M&E engineer. In this instance, the cross talk attenuator typically runs the width of the classroom, office or medical room.

The acoustic performance of the NAT Vent Attenuator is rated between 34 dB Dne,w and 39 dB Dne,w. Through research, it is seen that cross talk attenuators with an acoustic resistance of 34 dB Dne,w provide an equal performance to that of a solid partition containing an acoustically rated door (30 dB Rw). BB93 requires 39 dB Dne,w across a vent within a corridor wall, due to this limitation of the door this is seen as an over specification.

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.

2.11 External Spaces as Secondary Facades

Secondary Façade as a Functional Space
Often there is a need for spaces such as cloakrooms, changing areas, walkways, balconies and other non-acoustically sensitive spaces to be located adjacent to a building. By making these spaces external and unheated, i.e. open covered spaces, it is possible to use these areas as a secondary facade . If required, additional acoustic protection can be added by means of placing an attenuator within the secondary facade 1. This attenuator could be located under benches, cupboards, shelving areas, raised areas etc. This would be an ideal way of preventing noise break-in from low flying aircraft, nearby rail lines, or a large main road such as a motorway.

Acoustic screening and ventilating from under a building
Acoustic screening is an effective method of controlling noise break-in to a building.
Illustration 2 shows how a large, suspended, raised (play) area was used to accommodate the fall in the land across a school site. This play area provided a highly effective screen to aircraft noise and potentially other major noise sources. The acoustic attenuation is provided as the vents under the deck have little or no visibility to the noise sources affecting the development. In simple terms, providing an air inlet under the building prevented noise ingress into the building.

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.