The most common passive attenuators are either circular fan duct attenuators or straight rectangular duct attenuators, in either case with galvanised steel casings and mineral wool infill. However, attenuators can have various features that may be appropriate for different applications.
Flange/Spigot Connections: An attenuator is usually part of a duct system or is connected directly to other equipment. It is important to ensure that the connections on the attenuator are compatible with everything else. Typically, this may be a flanged—such as Rolled Steel Angle (RSA), Doby or Mez—or a spigot connection, although in some cases the end of the attenuator may need to be tapped for bolting directly to another flange.
Painted Finish: Most duct attenuators have self-colour galvanised sheet steel casings. However, they can be coloured where required either by using a colour coated steel such as Plastisol, or by painting the attenuator. A painted finish may also be appropriate to provide increased corrosion protection in more hostile environments.
Perforated Metal Facings: These protect the acoustically absorptive infill against mechanical damage, which may occur through several causes, including incorrect handling. If the infill becomes damaged it may erode over time resulting in fibres migrating to other parts of the system and a reduction in attenuator performance.
Aerodynamic Fairings: These reduce the resistance imposed by the attenuator but increase the length and can slightly reduce the attenuation performance.
Scrim Inter Layer: In addition to a perforated metal facing, an acoustically transparent scrim inter layer should be used between the acoustically absorptive infill and perforated metal facing. This will stop fibre migration that can otherwise gradually occur through the relatively large holes in the perforated metal. For some applications, a Melinex inter layer can be used instead but this is not acoustically transparent and it reduces the high frequency attenuation performance.
Removable Splitters: These may be appropriate for cleaning, such as kitchen hood extract attenuators, or possibly to provide access for maintenance where space is very restricted.
Different Material Construction: Material such as stainless steel or plastic can be used to improve resistance to corrosion in hostile environments.