Key considerations for smoke shaft systems: 5. Noise

Posted by Paul Compton on 05/08/14 11:30

ColtshaftIn the fifth article in our series about designing smoke shaft systems, we discuss noise and attenuation requirements.


When considering noise, shafts can be split into two types, those with and those without day-to-day comfort ventilation provision.

With day-to-day ventilation:

In a way this is easiest to deal with; it will need to be quiet to be acceptable for normal use.

Without day-to-day ventilation:

When a system is only for emergency use there is always there is always the question: “do we need attenuation for regular testing?” I can’t give a generic answer to that, but I can say that many systems are not attenuated and testing is scheduled for times when a brief noise is likely to cause least nuisance.

Internal noise levels

With day-to-day ventilation:

For systems in day-to-day use a reasonably low noise level is needed. CIBSE Guide A, always a useful source of information, suggests an acceptable noise level is NR35-40 in a hotel corridor. This would seem to be reasonable for a residential corridor as well, so NR40 might be a sensible specification.

Without day-to-day ventilation:

For smoke control in an emergency there is no specific noise limit. In residential buildings there is usually no alarm system that occupants need to hear so a high noise level may not be a safety concern. In commercial buildings with an alarm system with sounders in the lobbies, the noise level should be at least 5dB below the alarm noise level to ensure that the alarm can be heard and recognised. Noise may also need to be limited to enable fire and rescue service and refuge communications. 

For testing there are 3 basic options (I’m ignoring the temptation to just not do much testing), schedule for minimum annoyance, add an attenuator or, if inverter control is provided, run the fan at a low speed for testing.

External noise levels

Buildings on noise sensitive sites will have external noise limits set as a planning condition. I’m not sure if it’s also a planning condition that these are locked in a basement vault so no-one can gain access to them, but it feels like it sometimes. The limits are usually given as separate day and night time values in dBA at a set location, usually outside the nearest neighbour’s window. If the building has multiple noise sources likely to run at the same time, the specification to each individual supplier should be set lower, typically by 5dBA, to allow for the additive effect.

With and without day-to-day ventilation:

For systems in day-to-day use the limits have to be met, no question. For emergency use the noise limits are regarded as irrelevant.  The big issue again is testing. Scheduling may help, but it only needs one neighbour to complain to make life difficult for everyone. The safe options are an attenuator or inverter control.

Noise control

Attenuators cause two main problems: space - they’re always bigger than you think - and air flow resistance. Add too much resistance and the fan selection will change for a noisier unit, which needs bigger attenuators ……

If only a small degree of attenuation is needed, up to say 15 to 20dBA, circular attenuators matching the fan diameter are ideal as they are neat and cause relatively low pressure drops.

If higher attenuation is needed then rectangular splitter attenuators will be needed and it might even be necessary to enclose the fan and its flexible connections to limit noise break out. In extreme cases a small AHU type casing may be beneficial or necessary.

Smoke shafts v pressurisation webinar

Watch out for further posts in this blog series, where we’ll look at other aspects of shaft system design. In the meantime, watch my CPD accredited webinar on smoke shafts v pressurisation, where I discuss:

  • Features and benefits of both systems

  • Common problems with both systems

  • How to choose which option is best for your project 

Watch the webinar

Paul Compton Paul Compton is Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.

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Topics: Smoke ventilation, Smoke shafts, Smoke Shaft Series