FM Issue: Sounds Great
By Daniel J. O’Connor, P.E., Fellow SFPE
Published in the May 2012 issue of Today’s Facility Manager
Directional sound is a relatively new technology for improving the use of exits and helping people find their way to a refuge area or some other means of egress during an emergency. It is well suited for adverse conditions like smoke or darkness.
Short bursts of broadband sound are introduced in a frequency range that is distinct from simultaneously operating fire alarm sounders like bells or horns. These pulses of sound, which make use of the human ability to localize sound sources, are produced by electronic signaling devices installed as part of a building’s fire alarm. They provide additional sound cues that don’t conflict with the traditional notification systems.
Improving the wayfinding to exits in buildings can be important for a variety of reasons. Although exit signage is the traditionally acceptable method of guiding occupants to exits, many building designers tend to minimize signage (just enough to meet codes) in preference to architectural aesthetics, while other buildings (such as retail environments) have such visually cluttered spaces that exit signs are lost and are not readily spotted.
Spaces with relatively low ceiling heights, such as parking garages, result in line of sight issues that make it difficult to observe exits signs easily from a distance. Also, it has been shown that signs located above exit doors and along egress routes, as required by building and life safety codes, do not always help occupants locate exits during an emergency.
When Exits Really Matter
The observed behavior of those who ignore essential safety information—even during life and death instances—has been explained in terms of an established psychological concept known as learned irrelevance. This concept considers that exit signs have a learned irrelevance which occurs when a person is continually exposed to a stimulus (like an exit sign) but rarely needs to react to it because it is always present.
Fortunately, directional sound technology can overcome these circumstances in a very simple manner. By taking advantage of the human sense of hearing and combining it with broadband, this technology keenly provides cues to the location of the sound.
To understand broadband noise, facility managers (fms) just need to consider the sound of a cell phone ringing in a room full of people. This is a narrow band sound; when the cell phone rings, people in the crowd are often confounded trying to locate whose phone it is and often think it may be theirs.
Broadband noise uses a multi-sound and frequency spectrum that eliminates the confusion that otherwise occurs with narrow band sound signals. A significant positive feature of broadband noise is that its wide sound spectrum works very well for those who have hearing impairments.
The Benefits Of Broadband
To understand the technical benefits of broadband sound, it is important for fms to gain a basic understanding of the area of study known as psychoacoustics. Human hearing as a mechanism for locating sound sources has been studied in great detail, and there is much scientific literature to explain the basis for human sound localization. A few key factors are recognized to account for the human ability to pinpoint with surprising accuracy a given sound—especially if it is broadband sound.
The anatomy of the pinna (or outer ear) and the fact that ears are situated on each side of the head allow for subtle sound differences that provide the binaural cues important for locating a sound source. These binaural (two ear) cues are:
- Interaural time differences (ITD);
- Interaural intensity differences (IID); and
- Head related transfer function (HRTF).
Figure 1 (below) illustrates the impact of IID. A stationary observer is presented pure tones of sound to one side. A difference in intensity occurs at the two ears, as one ear is shadowed by the head. The shadow is a function of frequency, and a significant difference in loudness is perceived by the two ears.
The figure indicates that at very low frequencies, there is no difference and no impact from the shadow of the head. However, at frequencies above 5000 Hz, a difference in loudness between the two ears is as great as 30 db. With complex sounds (e.g. speech, music, broadband sound) there will be not only this difference in loudness/intensity but also a change in the sound spectrum as high frequency components are shown to be lost to the ear on the far side of the head.
Setting A Sound Precedent
While binaural cues provide complimentary and redundant means for locating sound, there is a fourth psycho-acoustic phenomenon that assures that too many sound waves resulting from highly reverberant spaces do not cause confusion. This is attributable to the “precedence effect” of human hearing.
The ear is capable of discerning and fixating on the first sound received (a line of sight direct signal) and disregarding later signals (by way of reflected sound). The acoustical signal arriving first at the ear suppresses the human ability to hear other signals (including reverberation) that arrive up to about 40 milliseconds after the initial signal.
A pulsing broadband directional signal makes good use of the precedence effect and can compensate for less than optimum listening conditions. Even in highly reverberant spaces (where every surface was sound reflective), test subjects typically have no problem determining the location of directional sounders.
A example installation of directional sounders is illustrated for the floor of a building in Figure 2:
and at a doorway in Figure 3:
Finding A Place For Directional Sound
The intent of directional sound is to provide sound cues to assist occupants in more easily locating the direction to a nearby exit or area of refuge when occupants are moving through the means of egress during an evacuation. A variety of facilities have adopted this technology as an improvement for their patrons and staff. Examples include schools, museums, the downtown Chicago Macy’s Department store, and the Munich International Airport.
When new fire safety technologies appear, the potential benefit of the technology will be a function of the effectiveness of existing technologies—whether or not the new technology can fulfill a need or provide a safety enhancement at reasonable cost. And since sound has the advantage of penetrating in many directions—such as around corners—there is an inherent flexibility and efficiency in using directional sound compared to line of sight and other traditional methods for marking exits.