Smoke Alarms and Adults who are Hard of Hearing

Editor: I'm happy to see that the issue of smoke alarms that will awaken hard of hearing folks is finally getting some attention. There's lots of misinformation out there, and some of it could cost people their lives. Here's the Executive Summary of a recent study that explores this topic. The entire report is available at http://www.nfpa.org/assets/files//PDF/Research/hardofhearing&alarms.pdf

Note that the Darrow Company  provides an alarm that outputs the recommended 520 Hz squarewave. [Full Disclosure: The Darrow Company advertises with Hearing Loss Web.]

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August 2007

As it has been shown that smoke alarms save lives (e.g. Ahrens, 2004) and that being asleep is a strong risk factor for fire fatality (e.g. Thomas and Brennan, 2002, Brennan 1998), the ability of different sections of the population to wake to their smoke alarm is an issue of importance.

Among the people at risk of not waking to an auditory smoke alarm are the more than 34.5 million people in the US who are hard of hearing. In some cases such people purchase alternative alerting devices which may send a visual signal (e.g. a strobe light) or a tactile signal which vibrates. Bed shakers and pillow shakers have become available for people with hearing impairment and may be used with an alarm clock or for emergency notification. Some specialist alerting devices allow the option of a loud low frequency sound.

Standards exist for the intensity of a strobe light for emergency notification (NFPA 72, 2002), with an intensity of 177 or 110 candela (cd) specified, depending on placement. However, several studies of their efficacy in alerting sleepers suggest that only about a third of hard of hearing people or people with normal hearing will awaken to strobe lights of similar intensities (Bowman et al. 1995: Du Bois et al. 2005).

For bed shakers UL 1971 (1991) provides a standard but the specifications relate to a shaker of different shape to most shakers currently sold. The bed shakers tested in sleep studies report waking rates of 70-100% when using shakers at "off the shelf" intensities (Underwriters Laboratories, 1991; Murphy et al. 1995; Du Bois et al. 2005). The British Standard, BS 5446-3 (2005) relates to a smoke alarm "kit" for people with hearing impairment which combines the normal UK smoke alarm with a vibration pad (a bed shaker) and a flashing light. The minimum intensity specified for the flashing light is quite low (15 cd). For the bed shaker frequency ranges, pulse patterns and displacement specifications are provided, along with a standardized testing procedure. However, there are no published sleep studies which test the specified kit or report the intensity of the bed shakers as compared to the BS5446-3.

This study set out to test the waking effectiveness of several different auditory signals, a bed shaker, a pillow shaker and a strobe light in a sample of hard of hearing people. A range of different intensity levels were tested for each signal. Each device was tested separately. Participants were 38 volunteers aged 18-77 years (16 males, 22 females) with an average hearing loss of 25-70 dB in both ears (i.e. mild to moderately severe hearing loss). No deaf individuals participated. Each participant was exposed to a range of signals across two non-consecutive nights during slow wave sleep (the deeper part of sleep, stages 3 and 4). Six signals were tested: • 400 Hz square wave signal in T-3 pulse • 520 Hz square wave signal in T-3 pulse1 • 3100 Hz pure tone in T-3 pulse (the current smoke alarm) • Bed shaker (under mattress) in T-3 pulse • Pillow shaker in T-3 pulse • Strobe light in T-3 pulse (modified)

The first two signals used in the sleep study (as listed above) were chosen from a larger set of audible sounds that were used in the awake portion of the study. These first two audible signals had the lowest average hearing threshold from a set of eight different auditory signals. Each signal was presented for 30 seconds, followed by a short period without a signal (30 -70 seconds). After this pause the signal was presented at a higher intensity level and this continued until a range of intensities had been presented, or the participant awoke.

In addition, a questionnaire was administered to all volunteers for the study (n=44). This asked a variety of questions about their hearing, use of specialised alerting devices and their confidence in their ability to hear different alerting sounds in their home (e.g. telephone, fire alarm) both during the day and night.

The main conclusions from this study are:

1. Under the testing conditions a 520 Hz square wave T-3 sound was the single most effective signal, awakening 92% of hard of hearing participants when presented at or below 75 dBA for 30 seconds and awakening 100% at 95 dBA. Both the 520 Hz square wave and the 400 Hz square wave were significantly more effective than the 3100 Hz pure tone T-3 sound, which awoke 56% at or below 75 dBA. In addition the 520 Hz square wave signal yielded the lowest hearing threshold when awake for this sample of people who were hard of hearing, from a set of eight alternative sounds with a range of pitch and patterns.

2. Under the testing conditions the bed shaker and pillow shaker devices, presented alone, awoke 80-83% of the hard of hearing participants at the intensity level as purchased (vibrating in intermittent pulses).

3. Those hard of hearing participants who were aged 60 years or more were less likely to awaken to the bed shaker than those aged below 60 years. No age group differences were found for any other signal.

4. Strobe lights, presented alone, were not an effective means of waking this population, with only 27% waking to the lowest strobe light intensity, which was more intense than that required by the standard (NFPA 72, 2002).

5. There was tentative evidence that people may respond differently to different types of signals, suggesting that a bedroom alarm "kit" that combined two types of sensory signals (i.e. an auditory signal plus a tactile signal) may be more effective than one signal.

6. The results in this study are likely to be overestimations of the proportion of the hard of hearing population who may awaken to these signals in an unprimed, unscreened population, especially from deep sleep. Thus extrapolations of absolute intensities and percentages awoken in the study to the field should be made with caution.

7. It was found that, when a signal was presented at a level that caused awakening, most people awoke to the signal within the first 10 seconds of the signal being on. Thus it seems highly probable that a T-3 signal that is alternatively ON for about 10-15 seconds and OFF for a certain period of time (possibly of the same duration) will be more effective than a continuous sounding T-3 signal.

8. Questionnaire responses indicated a high level of misplaced complacency among people who are hard of hearing in terms of their need for specialist alerting devices. In view of this, and the fact that many people are not aware of their hearing loss, it is desirable that any standard audible smoke alarm for the general population emit a signal that maximises the chances of awakening for hard of hearing people (provided such a signal presents no increased risk to other sections of the population).

Recommendations:

1. The technical feasibility of replacing the current high frequency smoke alarm T- 3 signal with a low frequency square wave T-3 signal (with a fundamental frequency of 520 Hz or thereabouts4) for the entire population should be investigated as a matter of priority.

2. A suitable ON duration of such a T-3 signal appears likely to be in the range of 10-15 seconds, with the OFF duration tentatively suggested to be of similar duration but further research is required to determine this.

3. For this population of people with mild to moderately severe hearing loss the single best emergency alerting device is a low frequency square wave auditory signal and this is superior to bed shakers, pillow shakers and strobe lights, presented alone. Ideally this square wave signal should be as loud as possible. There is tentative evidence that combining a low frequency square wave with a tactile device may provide additional waking effectiveness.

4. Any recommendations for the use of strobe lights, presented alone, as an emergency alarm to awaken sleepers who are hard of hearing or of normal hearing should be withdrawn as soon as possible.

5. Further study should be undertaken with people with hearing loss ranging from moderate to profound (i.e. including deaf people) to determine the best signals, or combination of signals, that will reliably awaken this population from deep sleep. This should include bed shakers, pillow shakers, low frequency square waves (beneficial for those with residual hearing) and could include strobe lights. In such research it would also be of interest to test bed shakers (vibrating in intermittent pulses) in an under-the-pillow placement.

6. Research on the efficacy of a range of different signals and signal combinations in different populations (e.g. with and without hearing loss) should also be conducted in a large number of home environments where the participants were not primed to expect a signal during the night and unscreened for factors such as medication or prior alcohol consumption.