We’ve Sounded the Alarm: Now What?

In 1990, a fire aboard a passenger ferry sailing in Norwegian waters took the lives of 158 people. In response, two Scandinavian universities began research on methods to decrease the amount of time needed for evacuation and increase the use of exit paths available. The goal was to lessen occupant evacuation time and increase the efficiency of a structure’s exits.

Occupants unfamiliar with a building, such as in The Station nightclub fire in Rhode Island, instinctively attempt to exit the way they came in. In the 1942 fire at The Coconut Grove nightclub, many victims were found piled atop each other at the front entrance as they tried to escape.

Researchers hypothesized that those who couldn’t locate an exit path or see one might be able to hear where it was. Trials and technical work began around 1980 to determine just how the human ear pinpoints the origin of sounds. When providing an audible ‘sign’ for locating exits during an emergency, there was up to a 75 percent decrease in the amount of evacuation time test subjects needed to navigate complicated hallways filled with smoke.

Directional sounders
Fire bells, chimes and horns provide a tonal signal, while directional sounders provide a burst of broadband sound. The bursts of white noise produced by these directional sounders contain almost all the discernable frequencies the human ear is capable of hearing. This means that the directional sound contains all the “binaural localization clues” the brain needs to locate the source of the sound and the exit. In the 2007 edition of NFPA 72, part of section states that if/when these devices are used the sound they generate must be “within the effective frequency ranges of the interaural time difference (ITD), interaural level or intensity difference (ILD or IID) and anatomical transfer function or head-related transfer function (ATF or HRTF) localization cues. The signal penetrates both the ambient noise and the fire alarm signal.

One experiment on a cruise ship employing these directional sounders proved that even those test subjects that did not have the sounders’ function explained to them prior to the test instinctively migrated toward the pulsing white noise. 

But how did they know which way the sound was coming from? In Fig. A, Mr. Green can tell the location of the exit sounder because of the ITD cues. This happens when the lower frequency sound reaches his right ear before his left or “shadowed ear.” His brain interprets the sound’s arrival time difference and determines the direction he needs to head is to his right. For ITD cues to work best, a portion of the white noise must be in the lower frequencies. However, Mr. Blue (Fig. A) is hearing the sound level (volume) equally in both ears. Because the brain can best determine loudness in the medium to higher frequencies (above 3000Hz) Mr. Blue determines the sounder is directly in front of him, a phenomenon also known as the Interaural Intensity Difference (IID), or Interaural Level Difference (ILD). NFPA 72 2007, section also mentions a third way humans obtain “localization cues” which is known as the Head-Related Transfer Function (HRTF), or Anatomical Transfer Function (ATF). The “transfer function” refers to the transforming effect the head, torso and external ear has on sound as it travels to the ear. HRTF works independently to reinforce the ITD and IID localization cues. It is the HTRF phenomenon that allows Mr. Blue to determine that the sounder is directly in front, rather than overhead or behind him.

In the Coconut Grove fire, it was the audible directional signal given by a waiter who said “this way out” that led some to the closer, uncrowded exit and to safety. New technology that can save lives should be embraced by those of us in the business of installing life-safety systems.