The faster an alarm goes, the more urgent it tends to sound. And in terms of pitch, alarms start high. Most adults can hear sounds between 20 Hz and 20,000 Hz—Baldwin uses 1,000 Hz as a base frequency, which is at the bottom of the range of human speech. Above 20,000 Hz, she says, an alarm “starts sounding not really urgent, but like a squeak.”
Harmonics are also important. To be perceived as urgent, an alarm needs to have two or more notes rather than being a pure tone, “otherwise it can sound almost angelic and soothing,” says Baldwin. “It needs to be more complex and kind of harsh.” An example of this harshness is the alarm sound that plays on TVs across the U.S. as part of the Emergency Alert System. The discordant noise is synonymous with impending doom.
After the alarm designers create a range of sounds in the lab, says Baldwin, they will test the annoyance factor of these sounds in a process called “psychophysical matching, or psychophysical ratings.” Yes, this involves subjecting human beings to a bunch of irritating sounds. Participants determine how annoying the sounds are by sorting them into categories ranking them on a scale of one to 100.
Then there’s more testing. “If it’s a medical alarm, for instance, we’ll start using that sound and then we’ll maybe measure people’s physiological response to it—does their heart rate go up, does their skin conductance level go down, what happens to their brain activity,” says Baldwin. Skin conductance measures how much the sound affects the body—skin gets better at conducting electricity when the body is physiologically aroused.
An effective audio alarm is one in which the annoyance factor and perceived urgency of the sound is matched to the hazard level—a soft little chime for the fridge door, say, and a “BREHHHHK BREHHHHK BREHHHHK” for a plane in a tailspin. “We want it to be detectable, so to get your attention, but for you to recognize what it means right away,” says Baldwin.
It turns out this is a problem in hospitals:
In hospitals in particular, there are “so many nuisance alarms going off all the time, that people—nurses, doctors—just tune them out,” says Baldwin. “They don’t even hear them anymore.” The statistics say that most of these alarms are not indications of peril. A 2012 review of medical audio alarms found that in one intensive therapy unit, “of 1455 soundings of alarms, only eight were associated with potentially life-threatening problems.”