This won't answer your question but does it really matter as long as the alarm can be heard by all in attendance? How about 5 decibel more than any sound level present, if more than that is required then consider a system of lights etc. (visual).

From a legal / Code of practice point of view the guidance would be along the lines of the RRFSO act , HSG 168 and the joint code of practice for prevention of fires on construction sites . To the best of my knowledge there is not a level or number given in any only a requirement to provide suitable and suffficient means of raising the alarm. This is where the good old chestnut of risk assessment comes into play and would be asked for in the event of a fire . Id look at the requirements when undertaking the fire risk assessment as in the worst case (loudest ) scenario and base the requirements on that . From experience using the battery powered alarms they are adequate 9 times out of 10 but also as the site develops so quikcly , partitions etc it needs constant monitoring . hope this helps , Phil Roche

The art is to put sufficient numbers in to ensure that they can be heard throughout the structure, just like a fixed system the number varies according to the building layout and size. If somebody claims it cannot be heard then more are needed.

After many dsicussion with the HSE on this matter (timber frame construction) they stated that it has to be suitable and sufficient so that when the alarm is raised it can be heard by thoses who are located within the building and the surrounding area to allow evacuation, note that these alarms may need to be linked so that they provide sufficient coverage. We ended up using the howler system as it can be linked and easily installed/moved nad adapted as required.