More than nine in 10 of the hearing-impaired participants—92%—who participated in a study on smoke detector effectiveness at Victoria University, Melbourne, Australia, were awakened by a low-sound frequency alarm. In comparison, 44% were brought out of sleep by an alarm sounding at the more commonly used higher-sound frequencies.

While high-frequency sounds are effective when people are awake, many people do not hear them while asleep and this can cost lives, according to researcher Michelle Ball, coauthor with Dorothy Bruck of Optimizing Emergency Awakening to Audible Smoke Alarms: An Update.

Ball and her colleagues investigated which sound frequencies were most effective in alerting sleeping people. They found that a signal in the range between 400 to 520 Hz was most effective, especially in waking hearing-impaired sleepers.

Ball says 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, which may explain why square sounds are more effective in alerting sleepers.

Also, hearing-impaired people are typically able to hear low-frequency sounds better than high frequencies, according to Ball.

SOURCE: Victoria University