By John Murray
John Murray gives an overview of the 2010 NFPA72 code changes and how they applied to one of his jobs.
Brenda asked that I blog on the new SAC site and I was, of course, flattered to be asked. I thought it would be best to keep to audio topics, avoiding other topics best left to other dissemination vehicles, and to focus on audio particulars I am involved in or thinking about currently.
With that in mind, I would like to encourage comments about the new 2010 NFPA 72 requirements for intelligible evacuation systems. Primarily Chapter 24, Emergency Communications Systems and Annex D.
Recently I have been involved in measuring STI in a large airport. The first big issue I encountered is the fact that existing large, distributed sound systems are seldom in top condition in terms of gain structure for consistent level and proper equalization in all areas. So, if you are planning to measure intelligibility in a large existing system, you should plan on spending a lot of time getting the system into top calibration first. Variations in area levels and poor equalization will undermine any attempt to collect a definitive intelligibility rating for the space.
I have to make my measurements at night when there are no travelers to trip over mic stands, etc. As a result, the ambient noise is far lower than it would be during actual emergency conditions.
The new code discusses the effect of background noise on the resulting STI. In the code, it is recommended that in areas where the background noise exceeds 90 dBA, that perhaps a visual signal for evacuation information should be used. It is also mentioned that a 15 dB S/N ratio between the announcement level and the background noise should be maintained. In most of the literature I have read, this 15-dB S/N ratio roughly equates to an 80% articulation of sentences deemed good enough for understanding the message.
In Don & Carolyn’s first “yellow book” (Sound System Engineering), there is an old chart that defines a 15-dB S/N ratio or better as the threshold to 15% Alcons or less. According to the chart, it takes a 25-dB S/N ratio to remove any detrimental effect of ambient noise whatsoever.
So, from that standpoint, the emergency message should be at least 15 dBA louder than the ambient level at the time of an emergency, and even this presupposes an RT60 of roughly 0.6 seconds for the room. I have measured the ambient level in this airport’s concourses with normal traffic at 76 dBA. This dictates a message level of at least 91 dBA, and says nothing of what an excited crowd in an emergency might require for adequate message levels, which would drive the message level needed to 95 or 100 dBA.
I invite anyone to comment on this blog, particularly those with some experience in STI for evacuation systems like Peter Mapp. Also, if anyone has a chart or formula for adding a higher noise level to STI results measured at a lower noise level, I would be most interested in using it.