Filter
Top Products
5-5
There is significant explanatory information in Annex A, recently revised for the 2002 edition:
From NFPA 72, 2002 Edition:
A.7.4.1.4.
The designer of an intelligible voice/alarm system should possess skills sufficient to properly
design a voice/alarm system for the occupancy to be protected. System designs for many smaller
occupancies can be accomplished satisfactorily, if not optimally, based upon experience with the
performance of other systems in similar spaces. For existing construction, relatively simple
acoustic measurements combined with knowledge of the chosen loudspeaker’s performance
characteristics can frequently result in satisfactory performance using mathematical formulas
developed for the purpose.
For occupancies that do not yet exist, the designer should have an understanding of the acoustic
characteristics of the architectural design, as well as the acoustic performance properties of
available loudspeakers. Architecturally, this includes the physical size and shape of the space, as
well as the acoustic properties of the walls, floors, ceilings, and interior furnishings. A proper
design analysis can sometimes reveal that an intelligible system is not achievable unless some
features of the architectural design are changed. The designer should be prepared to defend such
conclusions and, if necessary, refuse to certify the installation of such a system. While “hand
calculations” and experience work well for simpler installations, more complex designs are
frequently better and more cost-effectively analyzed using one of a number of readily available
computer-based design programs.
The designer and the authority having jurisdiction should both be aware that the acoustic
performance parameters of the chosen loudspeakers, as well as their placement in the structure,
play a major role in determining how many devices are necessary for adequate intelligibility. The
numerical count of devices for a given design and protected space cannot, by itself, be used to
determine the adequacy of the design. Sometimes, the acoustic problems of certain placement
constraints can be satisfactorily overcome through the careful selection of loudspeakers with the
requisite performance characteristics, rather than by increasing their number.
There might be applications where not all spaces will require intelligible voice signaling. For
example, in a residential occupancy such as an apartment, the authority having jurisdiction and
the designer might agree to a system that achieves the required audibility throughout the
apartment, but does not result in intelligible voice signaling in the bedrooms. The system would be
sufficient to awaken and alert. However, intelligibility might not be achieved in the bedrooms with
the doors closed and the sounder in the adjacent hallway or room. In some cases this can require
that messages repeat a sufficient number of times to ensure that occupants can reach a location
where the system is sufficiently intelligible to be understood. Systems that use tone signaling in
some areas and voice signaling in other areas would not require voice intelligibility in those areas
only covered by the tone.
Reverberation times can drop significantly with the presence of plants, furniture, carpeting etc.
If the Certificate of Occupancy (CO) requires intelligibility testing before signing off on the
installation, certain factors should be considered:
• Consider designing to the worst case conditions. This is usually an unfurnished room,
the addition of absorbing materials enhance intelligibility.
• Audibility measurements taken in an unfurnished area can drop by as much as 3 dB with the
addition of absorptive materials.
• For large areas where the reverberation times are expected to drop dramatically, request a
delay in intelligibility testing until furnishings are added.
Intelligibility, Continued
Intelligibility,
(continued)
Intelligibility
Certification