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Fire fatality statistics show that people are often asleep when they die in a fire, even where there is a functioning smoke alarm.

Dr Michelle Ball of Victoria University says this suggests that alarms capable of waking people need to be different from those alerting people who are already awake.

Frequency matters
The standard smoke alarm involves a 75-decibel repeated pattern of groups of three beeps called the “T-3 pattern”.

The smoke alarm standard does not specify what frequency should be used, but most fire alarms use a high pitched pure tone of 3000 Hertz.

Ball and colleagues tried to find a signal that was more effective at waking people up when there is a fire.

They studied the ability of variously pitched T-3 signals to wake up children and adults of different ages.

They included adults who had been drinking, were sober or had mild to moderate hearing loss in their research.

To their surprise the researchers found that the signals that were most effective at waking people up were 400 and 520 Hertz.

“The lower pitched signals were superior to all other signals that we tested,” says Ball.

The 520 Hertz signal was successful in waking all sober young adults whereas 21% of them had slept through the current high pitched alarm.

It’s recommended you do your own research which may also include finding out from a licensed smoke alarm expert which are the current smoke alarms on the market and what independent journals say about them.

Of the hearing-impaired adults, 56% slept through the high pitched alarm but only 8% slept through the low pitched alarm.

Of course both a crying baby and a screeching alarm are designed to assault our auditory processing system – one designed by “God or evolution”, the other by human beings. But in this article we’re talking about the science of waking us up. Should alarms trigger freezing water to also splash on our faces? Or our lovers’ voices to yell out “honey, I’m in the mood”.

Complex sound

Unlike the pure tone of the current high-pitched alarm, the lower-pitched alarm was a square wave signal, which carries a number of frequencies along with the dominant frequency.

“A pure wave is like striking one note on a piano and a square wave is like playing several notes at once,” says Ball.

She says this complex sound actually seems louder than the equivalent pure tone, even though a sound level meter would not detect a difference in volume.

The different peaks of the square wave signal activate different parts of the basilar membrane in the cochlear, says Ball.

It seems that when you are asleep this complex sound is more likely to wake you up than a higher-pitched pure tone, she says.

Ball also says that the lower tones are more likely to be heard by those who are hearing impaired since the ability to hear extreme frequencies, including 3000 Hertz, are more likely to be lost first.

As a result of the team’s research Ball says experts are now lobbying for standards to specify the use of the lower-pitched signal.

A paper on the research has been accepted for publication in the Journal of Sleep Research.