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.

1.2 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.