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.

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.