|Directional Sound Performance Testing|
Issue 68: Directional Sound Performance Testing
By Scott Lang
After a fire has been detected in a building and its occupants have been notified, those individuals need to quickly find their way to an exit or area of refuge. Typically, fire safety professionals rely on visible exit markings and pre-planning to identify egress paths. As buildings and occupancies have grown in size and complexity, achieving an egress design goal has become a complex and expensive undertaking.
Directional sound is one method that can enhance the ability of building occupants to find the closest safe exit during an emergency. Adding sound to the visible indicators that are already required helps to overcome the limitations of relying only on one exit marking method.
Codes and Standards
In 2007, the NFPA 721 technical committee for notification appliances recognized the need to create a new category of notification appliance called "Exit Marking Audible Notification Appliances.” These devices are defined as notification appliances that mark building exits and areas of refuge by the sense of hearing for the purpose of evacuation or relocation. Beyond the definition, requirements were also added to Chapter 18 (Notification Appliances) to detail minimum device sound pressure level requirements and how directional sounders must be installed. NFPA 72 does not require the use of directional sounders.
Despite recognition of the technology in NFPA 72, there is currently no industry standard that can be used to measure the effectiveness of audible notification appliances in creating a sound that will direct building occupants to an exit. However, there is a publicly available specification developed by the British Standards Institute known as PAS 41.2 PAS 41 describes the construction and performance requirements for directional sounders. It also proposes a method to quantify the ability of a person to locate the sound source using subject-based testing similar to that used to measure intelligibility.
PAS 41 describes a localization test in which a subject is seated in a fixed position encircled by 36 identical notification appliances. The sounders are mounted about 1.2 meters from the ground and 3 meters away from the subject. The subject must sit facing the front unit and not move his or her head while the testing is conducted. A random unit is then activated and the subject, while using a record diagram as a guide (see Figure 1), must visualize which sounder is on and record the test number on the sheet.
This test is repeated 18 times for each subject. A group of ten subjects must correctly identify which sounder activated at least 75 percent of the time for the sounder to be considered "directional.”
In order to get a better understanding of the requirements of PAS 41 and assess its applicability as a quantitative measurement of directionality, experiments were conducted with dozens of volunteers with self-reported normal hearing to perform the localizing tests. A number of test modifications were also evaluated with the goal of improving the test method and gauging the overall viability of the document. Both directional and standard audible notification appliances (including temporal-3 and whoop tones) were used in the tests.
In phase 1, the testing was performed exactly as it was described in PAS 41. The following notification appliances were used in the testing: a directional sounder, a chime set to whoop tone, a standard 2-4 kHz temporal-3 sounder, and a 3 kHz pure tone mini horn. Table 1 shows the percentage of correct location identifications made by the group as a whole as well as the minimum and maximum percentage of correct identifications by any one subject. The results show that none of the notification appliances would pass the 75% threshold to be considered "directional” according to PAS 41.
Table 1: Percentage of correct identifications of sound during localization test
Table 2: Percentage of correct identifications in original and modified test method
Table 3: Results for directional sound and whoop tone with and without competing sounds.
In terms of further testing in this area, an assessment as to the repeatability and reproducibility of the test method needs to be conducted. Additional sounds could also be tested to assess their localization potential.
Scott Lang is with System Sensor
2nd Quarter 2011 - After the Alarm Sounds: Historical, Present and Future Perspectives - Jason D. Averill, Erica D. Kuliogowski, and Richard D. Peacock, National Institute of Standards and Technology
2nd Quarter 2011 - Signaling Strategies for Means of Egress – NEMA
Summer 2006 - Visual Signaling in Large Spaces – NEMA
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