Ninety-two percent, or more than nine in 10 of the hearing impaired participants who took part in an Australian study were awoken by a low sound frequency smoke alarm. In comparison, 44 percent were awoken by an alarm sounding at the commonly used higher sound frequencies.
Lower frequencies more effective
Usually, smoke alarms sound in high sound frequencies. High frequency sounds are great when people are awake. But many people do not hear them while asleep and this can cost lives, according to Michelle Ball, a researcher at Victoria University, Melbourne, Australia.
Ball and her colleagues investigated which sound frequencies are most effective in alerting sleeping people. They found that a signal in the range between 400 and 520 Hz was most effective, especially in waking hearing impaired sleepers.
Square versus pure toner
Ball explained that lower frequency alarm tones, which are square, are more effective than higher frequency pure alarm tones. A square tone is composed of several frequencies, each of which activates a different part of the basilar membrane in the cochlear. This may explain why square sounds are more effective in alerting sleepers.
In addition, hearing impaired people are typically able to hear low frequency sounds better than high frequencies.
Source: The Australian Broadcasting Corporation, www.abc.net.au