Of course, there are many factors that can impact the intelligibility of voice warning system such as the acoustical properties of a building and speaker placement.
"Through a lot of field testing, we know that our alerting devices need to have a certain distance between them," Kurtzrock said. "A typical alarm system is at say 90 dB (decibels). Our voice information is generally at around 75 dB and the reason for that is that the clarity... is very, very self-evident. We also use very sophisticated sound compression for the actual recording of the messages so that the clarity of the information coming out is predetermined. We know exactly how it will sound before the system goes live."
Kurtzrock said that the architecture of a building also plays a significant role in voice intelligibility.
"In some cases you have narrow hallways; you can also have tile floors as opposed to carpet. Carpeting picks up sound and it dulls the sound a bit, so you have to do a fair amount of testing in terms of the distribution," Kurtzrock said of his company's voice systems. "At a lot of places they just install loudspeakers in a hallway; one speaker in one hallway and another in the next and just assume that that's good enough."
Helping to establish standards for voice intelligibility, industry groups like the National Fire Protection Association (NFPA) are now addressing the technology in their building codes.
"NFPA 72 requires that Intelligibility be measured for both fire and MNS systems that provide voice signaling," said Derek Mathews, senior staff engineer, life safety & security at Underwriters Laboratories. "Annex D provides some guidance on the planning, design, installation, and testing of voice communication systems. Intelligibility is not being mandated by all authorities so some manufacturers have had to address it and some have not."
According to Mathews, the issue of voice intelligibility has been a part of code discussions even before mass notification systems were brought under the umbrella of life safety systems.
"Intelligibility was being discussed before MNS entered the life safety realm (NFPA 72) because fire alarm systems can also provide voice communication for either evacuation or relocation purposes," he explained. "Speech intelligibility is the same for both MNS and fire systems. Lots of research and studies have been conducted in order to fully understand Intelligibility. Currently, there is guidance in Annex D in NFPA 72 2010, for speech intelligibility."
As a manufacturer of speakers for mass notification systems, Christa Poss, marketing manager for the audible/visible business unit at System Sensor, says that they've focused their efforts on improving the quality of their speakers and providing installers with tools to help them optimize intelligibility.
"I think previously, there were so many unknowns about intelligibility and now there is just a lot more talk about it, people are getting educated on it and now it's a real issue because it's in the code," Poss said.
According to Poss, one tool that has been in use by the pro audio market since the 90s that can help voice notification system designers is a speaker modeling software called EASE from German-based ASMG Technologies.
"They've made a version specifically scaled for the life safety market. They took out a lot of the advanced functionality that we just wouldn't use in this market so a fire alarm designer can easily input the model of a room, put the acoustical properties of the space... where the speakers are placed and it's going to give you a prediction of your audibility and intelligibility," she said.
Mike Flannery, director of commercial hardware for ADT, says that from a design standpoint on interior systems, his company generally always tries to use a large number of speakers at lower volumes.
"In an interior space, with a fire alarm system, you're just trying to get the person's attention with a very loud blast of noise or voice, give them some very basic instructions, turn strobes on, etc., but with mass notification, the instructions may be more detailed," he explained.