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Directional Sound Performance Testing
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Issue 68: Directional Sound Performance Testing

By Scott Lang

Introduction

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.”


Figure 1: Record Diagram


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

Product

% Correct

Min. %

Max. %

Directional Sounder

36

6

61

Chime (Whoop)

18

0

44

Standard Sounder in T-3

19

6

39

Mini horn in T-3

7

0

22


After analyzing the results from phase 1 and discussing the test methodology with the test subjects, it was determined that many subjects had difficulty visualizing the correct location of the sounder that was being activated during the tests. The tests from phase 1 were repeated with the subject seated in a chair that could swivel. While each sound was played, the subject could rotate the chair to locate the sounder. The sounders were also numbered during this round of tests. Table 2 compares the results of the original test to the modified test method. The results show that the subjects’ ability to locate the correct sounder improve greatly if they are allowed to rotate to face the sound.

Table 2: Percentage of correct identifications in original and modified test method

Product

Percent Correct Phase 1

Percent Correct Phase 2

Directional Sounder

36

90

Chime (Whoop)

18

55

Standard Sounder in T-3

19

69

Mini horn in T-3

7

27


One concern that is often raised in discussing the use of exit marking notification appliances is the effect of competing sounds on the ability of people to localize the sound. In phase 3 of the testing, the test was repeated as it was conducted in phase two for the directional sounder and the whoop tone, but three competing sounds were added: a standard sounder, a 520 Hz sounder, and a mini horn. The additional sounder was wall mounted approximately 3.1 meters to the right of the test subject (near sounder number nine in Figure 1). It was continuously activated at about 87 dBA during the localization test. Table 3 shows that competing sounds do diminish the subjects’ ability to localize the sound.

Table 3: Results for directional sound and whoop tone with and without competing sounds.

Product

Competing Sound

Percent Correct

Directional Sounder

Standard sounder

87

Directional Sounder

520 Hz

77

Directional Sounder

Mini horn

78

Directional Sounder

None (Phase 2)

90

Whoop

Standard sounder

45

Whoop

520 Hz

44

Whoop

Mini horn

29

Whoop

None (Phase 2)

55


Recommendations

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.

Based on these tests, PAS 41 should be moved forward on the path to become a full standard with the changes that have been described.

Scott Lang is with System Sensor

  1. NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 2007.
  2. PAS 41, Directional Sounders - Requirements and Tests, British Standards Institution, London, 2003.

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