An Intelligent Approach to Intelligibility
NFPA 72-2010 includes a section outlining important new requirements for voice evacuation systems. Better understand the concept of intelligibility, learn about the code changes and how they apply to notification systems.
4 Steps to Applying the Code
Figure 2 illustrates the process involved in following NFPA 72-2010 intelligibility requirements. In Step 1, the process starts with determining the different ADSs within an occupancy. In Step 2, designers determine if intelligibility will be required. An example of where intelligibility would not be required is an area deemed impossible. If not, the process is complete. If intelligibility is required, designers move on to Step 3.
In the new Chapter 18 for Notification Appliances, NFPA 72 states that within the ADS, where intelligibility is required, voice communications systems shall reproduce prerecorded, synthesized, or live messages with voice intelligibility (18.4.10). In each of these defined spaces, measuring for intelligibility may or may not be required.
For example, in some areas, such as hotel rooms and some office spaces, the code may require intelligibility but not testing or verification. In these cases, it is adequate to ensure those conducting the testing can hear and understand the messages in these small spaces. The code assumes that occupants will also be able to understand these messages. In the cases when measurement is not required, the process is complete.
If measurement is required, then Step 4 is deciding what type of measurement will be performed.
When intelligibility is required and must be measured, quantitative methods using an intelligibility meter are typically used. Intelligibility meters, which measure either STI or CIS scores, are the most accurate and practical means of conducting intelligibility testing.
There are also a number of subjective test methods that use a group of people who listen to a passage that is spoken or played over the speaker system. The group is then asked to recall how much of the passage they understood to determine the intelligibility score. The two main types of tests done in this manner are the Modified Rhyme Test and Phonetically Balanced word score. To get more detailed information on intelligibility testing, review Annex D in NFPA 72-2010.
In most cases, however, the more practical method is to use a meter. Following are the basic steps of performing intelligibility testing using a meter:
- Calibrate the meter: At the beginning of each testing period the meter should be calibrated using the instructions supplied by the manufacturer.
- Measure ambient dB
- Measure dB with test tone: Run the intelligibility test tone and ensure that the dB reading with the tone is at least 15dB over ambient.
- Set the meter for intelligibility testing: Check to ensure that the meter is set properly. Most intelligibility meters also do other types of acoustical testing.
- Choose a scale: Make sure that the proper testing scale is selected.
- Run the intelligibility test
- Record the intelligibility score: Note the score and move on to your next test area. Many meters can save test data internally to be downloaded to a computer at a later time.
Christa Poss is Marketing Manager, Audible Visible Business Unit, for St. Charles, Ill.-based System Sensor (systemsensor.com).
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