Recent high-profile emergencies, including natural and man-made disasters, have increased the demand for mass notification systems (MNS). Today’s threats have reinforced the growing need to provide clear, concise and intelligible voice messages that communicate how people should respond in an emergency.
In order to properly plan, design and measure intelligibility, it is important for security dealers and integrators to first understand emerging codes and standards, space requirements and influential factors.
Intelligibility Codes & Requirements
Intelligibility is the degree to which people understand what is being communicated. The National Fire Protection Association’s (NFPA) latest national fire alarm and signaling code standards, NFPA 72 2013, defines intelligible as “capable of being understood; comprehensible; clear.” If a voice message is not understood by occupants in a facility, then the MNS has failed and may have caused more harm than good.
According to Chapter 24 of the standard on emergency communications systems (ECS), the loudspeaker layout of an MNS must be designed to ensure both intelligibility and audibility. In the 2013 edition, updates were made to clarify intelligibility requirements. Previously, codes specified intelligibility must be verified via quantitative test methods or other methods acceptable to the Authority Having Jurisdiction (AHJ). The new codes state that voice communications must be verified as being distinguishable and understandable and a simple listen test, or a qualitative assessment, is permitted by code.
While the NFPA does not require quantitative testing measurements for intelligibility, it is permitted. In fact, Department of Defense facilities are required to test voice intelligibility using Common Intelligibility Scale (CIS) or Sound Transmission Index (STI). The U.S. Army and Air Force require a minimum CIS score of 0.80. The U.S. Navy and Marine Corps require a minimum CIS score of 0.7.
Introduced in the NFPA 72 2010 edition, Acoustically Distinguishable Space (ADS) is also an important term to understand when designing for intelligibility. Defined by NFPA, an ADS is an ECS zone, subdivision, physically defined space or section of a room that might be distinguished from other spaces due to different acoustical, environmental or use characteristics, such as ambient sound pressure level.
All parts of a building intended to have occupant notification are subdivided in an ADS. As determined by the system designer during the planning and design phase of the ECS, each ADS must be identified as either requiring intelligibility or not requiring intelligibility. Updates for 2013 include new sections that require system designers to not only identify, but also document which locations will have audible notification and which spaces will not.
Factors That Affect Intelligibility
Designing an ADS for intelligible voice can be impacted by many variables, including background noise, frequency response, space configuration, acoustical properties of construction materials, distortion and bandwidth, and speaker characteristics. Here are some of the key factors that affect intelligibility:
Signal-to-Noise Ratio compares the sound level output from the speaker to ambient noise in the room. It is the obscuring of voice due to background noise — the higher the ratio, the greater the intelligibility. In Chapter 18, NFPA recommends an average of 15 decibels (dB) over ambient noise. A level higher than 15 dB results in reduced returns in terms of improving intelligibility.
Frequency Response: Most fire alarm system speakers produce frequency responses of 400 hertz (Hz) to 4 kilohertz (KHz). Commercial sound systems can usually distribute frequency responses of 125 Hz to 12.5 KHz, and professional sound systems can deliver frequency responses of 20 Hz to 20 KHz. Since humans can hear from approximately 50 Hz up to 22 kHz, the wider the frequency response of a speaker, the better it is at reproducing the frequencies in the original signal; thus, the chance is increased that the message will be understood.