August 13, 2007,
According to the July 2007 study, "Waking Effectiveness of Alarms for Adults who are Hard of Hearing," the typical audible signal used by smoke alarms failed to wake up 43 percent of tested subjects with mild to moderately severe hearing loss despite the fact that all were able to hear the 3100 Hz tone when awake. Strobe lights woke up only 27 percent of the hard of hearing subjects. In contrast, a specific audible multiple frequency signal consisting of a 520 Hz square wave successfully
alerted 92 percent of the subjects at the benchmark level of 75 dBA and alerted 100 percent at 95 dBA.
The study, authored by Dorothy Bruck and Ian Thomas of Victoria University, Australia,
estimated at least 34.5 million people in the United States have partial hearing
loss and projected that this number would increase due to the aging of the baby boomer
The Hearing Loss Association of America (HLAA) has long suspected that people have
died in fires because they could not hear or wake up to high-frequency smoke alarms,
but government investigations of fire fatalities have not inquired into whether the
victims had hearing loss. Neither audible smoke alarms nor strobe lights were specifically
tested with hard of hearing people during stages of deep sleep until the twenty-first
century. The findings of this study indicate that millions of people with hearing
loss will not be wakened from deep sleep by audible alerts which use only one tone
in the high frequencies rather than a range of frequencies beginning at approximately
"This study shows there is a critical need for emergency warning systems to be redesigned
or supplemented as soon as technically feasible, said Terry Portis, executive director
of the Hearing Loss Association of America. "Millions of people do not and will not
know that they will not wake up to the high-pitched tones used by most emergency
alerts. We call upon manufacturers of emergency alerting equipment, such as smoke
alarms, carbon monoxide alarms, and weather radios, to provide solutions that recognize
this reality as soon as possible."
The study evaluated the performance of six different signals for waking up hard of
hearing people from deep sleep and found the 520 Hz square wave to be the most effective
of all the signals. Bed and pillow shakers awoke 80 percent of the subjects at benchmark
levels, awaking the majority very quickly, but did not wake up 100 percent of the
subjects even at higher levels of intensity.
For people with more severe hearing loss, the authors recommended studying the effectiveness
of two or more different signals, such as a 520 Hz square wave audible signal with
a tactile alert and/or a strobe light.
"Even though strobe lights may not be effective by themselves at waking up hard of
hearing people from deep sleep, it’s important to remember that strobe lights are
still needed for alerting deaf people when they are awake and are not in contact
with a tactile alerting device," said Dana Mulvany, member of the Technical Panel
for the research project.
"Presently, there are no known emergency alerting products on the market incorporating
a range of tones and also including a low frequency near 500 Hz," Mulvany said. "Other
research has already shown that the 520 Hz square wave is superior to the 3100 Hz
tone for waking up other vulnerable populations, such as elderly people and people
under the influence of alcohol. All manufacturers of emergency warning devices, including
smoke alarms, carbon monoxide alarms and weather radios, should explore methods of
providing variations of the 520 Hz square wave as an audible alert so that people
with unknown, temporary or permanent partial hearing loss can be awoken quickly from
deep sleep. Manufacturers also need to specify the frequency response of their audible
alerts so that customers of all kinds can make informed decisions about their purchases."
Brenda Battat, associate executive director of HLAA, said: "Operators of hotels,
motels, college dormitories and many other facilities with sleeping areas must ensure
that they provide equally effective communication access for people with hearing
loss to the building alarm system, which is required by Title II and III of the Americans
with Disabilities Act (ADA). Many deaf or hard of hearing guests have been provided
portable alerting systems with a bed shaker, but these systems have typically not
provided effective alerting when the building alarm system is triggered and have
only been activated by smoke within the guest’s own room, which is too late for optimal
notification. We urge people with hearing loss to file complaints under the Americans
with Disabilities Act if they are not provided effective alerts to the building alarm
system by ADA-covered entities."
Source: Hearing Loss Association of America