Can You Hear Me Now? Audibility, Intelligibility and Visibility of Notification Appliances

Published On
Oct 30, 2018

In my August article, I explored smoke detector applications. This month, I examine notification appliances and discuss the concepts of audibility, intelligibility and visibility.

NFPA 72 2016, National Fire Alarm and Signaling Code, provides all the requirements for notification appliance performance. First let’s discuss the concepts of public mode and private mode signaling.

Public mode signaling simply means that, when any initiating device actuates an alarm signal, all the notification appliances operate throughout the building or protected premises. Essentially our goal with public mode signaling is to simultaneously notify all the occupants of the building to evacuate.

Fire alarm systems use private mode signaling when we intentionally do not want to notify all occupants of an alarm but wish to notify those personnel who will respond to the alarm location to assist with a carefully pre-planned evacuation. Hospitals, for example, are a perfect application of private mode signaling. Obviously, most of the patients would have difficulty in responding to an evacuation alarm without assistance. Private mode signaling enables the system to notify those personnel—nurses, aids, technicians and similar individuals—who would respond to the alarm and determine what actions to take to ensure patient safety, including offering assistance to those needing help in evacuation.

The types of audible notification appliances vary from place to place. Chimes, bells, horns, sirens and other similar appliances may serve well in many occupancies. Increasingly, however, most systems employ speakers to produce audible evacuation signals, including the sound of a human voice giving very specific instructions. Emergency voice/alarm communication systems (EVACS) and mass notification systems (MNS) obviously use speakers to produce audible voice instructions for evacuation, relocation or other specified instructions.

The code requires all audible notification appliances to meet the performance goal of a sound pressure level of 15 dBA above all ambient sounds—normal sounds in a protected premises—measured with a calibrated sound pressure level meter. In other words, a designer cannot simply start placing audible notification appliances on a drawing and expect to meet the performance requirements of the Code. In addition, without knowing the construction of the hallway and entrance doors to each occupant’s space and the number of rooms in that space, a designer should never assume hallway-located audible notification appliances would meet the code-required sound pressure levels in those spaces.

One of the most common mistakes I see when designers lay out audible notification appliances occurs when they automatically make each notification appliance a combination audible/visible unit. A careful reading of Chapter 18, Section 18.5 (public mode) in the code, covering visible appliance performance requirements, would show that a design must place visible appliances into spaces based on their candela rating and the size of the space. In many cases, a space does not require a combination A/V unit. A designer who pays attention to the code requirements for both audible and visible appliances will invariably save the client costs, which of course makes a quote more competitive.

I always recommend designers lay out the audible notification appliances first and then lay out the visible appliances. Where the design places both types of appliances together, the design can appropriately use a combination unit. Because of their differing performance requirements, one type of appliance will often outnumber the other. However, if a designer uses all combination units, the design will likely prove deficient in one or more areas, or the cost of installing unnecessary units will unnecessarily increase the installation cost.

A designer also needs to exercise care regarding notification appliances intended for commercial residential occupancies. For example, the performance requirements change for audible appliances located in or near bedrooms. The performance for all bedrooms requires a minimum of 75 dBA measured at the pillow level. Any residential space designated for the deaf or hard of hearing individuals must have visible appliances in all separate rooms within that space, including the bathrooms. Again, the candela rating for each visible notification appliance (strobe) will depend on the size of the space.

In addition to meeting audibility and visibility requirements for notification appliances, any fire alarm system described in NFPA 72 2016, Chapter 24, Emergency Communications Systems, must also meet an audible intelligibility requirement. Contractors often misunderstand intelligibility requirements, but they need not do so.

Think about it this way. When a system must deliver a voice message through speakers, those who hear the message must clearly understand the message. A number of factors in the construction and configuration of the space can cause a distortion of the audible signal: too much reverberation, unique ambient noise, too many sound-absorbing materials in the space, and so forth.

Likewise, by analyzing the construction materials in the space, the dimensions of the space, the configuration of fixtures in the space, and other relevant factors, a designer can place the audible notification appliances (speakers) to ensure the likelihood of meeting the intelligibility goal. Then, acceptance testing using an intelligibility test instrument can confirm the efficacy of the design.

Of course, the design must still meet the code's audibility requirements. Often designers make the mistake of increasing the tap setting on a speaker to obtain the desired sound level to pass the audibility acceptance test. Although driving a speaker at a higher wattage may accomplish the audibility needed for the space, it may also overdrive the speaker and provide a garbled message that inhibits the intelligibility needed to understand the message.

To understand intelligibility, the designer must know more about the space in which the placement of the speakers will occur. Is the space “hard” or “soft”? Areas with marble or granite surface, those with high ceilings, and those spaces with high ambient noise levels will prove more difficult to design for intelligibility. Hard spaces will reverberate the sound. For example, a hotel lobby with high ceilings and glass and marble or tile will need special speakers.

The designer should investigate online sources to become familiar with the various speakers and their operation to better understand how to ensure the design will meet the code's intelligibility. In addition, when placing speakers in a space, the designer should use more speakers, equally spaced at a lower output, to ensure intelligibility.

Annex A for Chapter 18 and 24 and Annex D of the code will provide a fair amount of guidance regarding intelligibility. If a designer feels unsure about how to proceed with a design to meet the intelligibility requirements, I suggest a partnership with a trusted and qualified sound and communications systems company. As an example, to reinforce these concepts, the next time you attend a meeting or seminar in a hotel meeting room, count the sound system speakers and then count the fire alarm speakers. You will most likely find the sound systems speakers outnumber the fire alarm speakers four to one.

One final note on testing and measuring intelligibility: Chapter 14 of the code states the following:

“ Intelligibility shall not be required to be determined through quantitative measurements.

" Quantitative measurements as described in Annex D shall be permitted but shall not be required”

A contractor can inspect and test intelligibility relatively easily. Play a message through all the speakers and walk each area to ensure the message remains clear and understandable.

For all the information that you, as a contractor, need to understand audibility, visibility, and intelligibility, simply make a thorough reading of NFPA 72 2016, Chapters 18 and 24, along with their associated Annex A material. A building owner installs voice communications systems, whether an in-building fire EVACS or an MNS, to provide the correct information clearly to the occupants. Ensure the design accomplishes the goal of ensuring the occupants can both hear and understand the messages your client intends the system to convey.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist

Wayne D. Moore, a licensed fire protection engineer, frequent speaker, writer and expert in the life safety field, has been a principal member and chair of NFPA 72, Chapter 24, as well as a former principal member of NFPA 909 and NFPA 914. He is the...

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