The ambulance siren: a classic attention-grabber. Mike Steinhoff/CC BY 2.0

When the cockpit recorder transcript from Air France Flight 447 was leaked to the public in 2011, many startling details emerged. The plane, which crashed into the Atlantic Ocean on June 1, 2009, killing all 228 people on board, had been under the control of pilots who were communicating poorly and not realizing one another’s mistakes. The plane’s speed slowed to dangerous levels, activating the stall alarm—the one, in the words of Popular Mechanics, “designed to be impossible to ignore.” It blared the word “Stall!” 75 times.

Everyone present ignored it. Within four minutes, the plane had hit the water.

Alarm sounds are engineered to elicit particular responses in humans. And yet, sometimes, humans choose not to respond, having decided that the situation is not urgent enough or that the sound is a false alarm. Audio alarm designers seek to avoid this by designing sounds that have an intuitive meaning and precisely reflect the level of urgency. But what makes an “awooga” sound more or less urgent than a “ding”? And how do you create an alarm noise that’s annoying enough to get someone’s attention, but not so annoying that said person disables the alarm?

Auditory alarm designers like Carryl Baldwin face these questions regularly. Baldwin, a human factors psychologist, constructs sounds in a lab, tests those sounds on human subjects to see if they are communicating the intended meaning, and ensures they are appropriate for use as alarms in household, aviation, medical, and automotive settings.

Alarms in the cockpit can conflict with one another. Joao Carlos Medau/CC BY 2.0

Of course, the door alarm that LG uses in its fridges to convey “Your food needs to stay cold, silly” should be different than the alarm that signals your commercial jet is about to crash. But what factors go into the design of these sounds?

One of the main considerations is the annoyance factor. To test for annoyance in the lab, says Baldwin, “we’ll construct sounds and we’ll look at all of the different acoustic parameters, so we might vary, for instance, intensity, frequency, the number of harmonics, how fast it ramps up and down, the temporal characteristics—like whether it’s going d-d-d-d-d-duh rapidly or duhhhh-duhhhhh-duhhhh.”

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.

One factor that the design of individual alarms doesn’t take into account is how that sound will mesh with the other audio in its intended environment. This is especially relevant in the medical and aviation fields, in which multiple alarms and alerts may be pinging and screeching at the same time—and indicating widely varying levels of danger.

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.”

“Oh, that vitals monitoring thing? The alarm goes off all the time, just ignore it.” Quinn Dombrowski/CC BY-SA 2.0

These noises, says Baldwin, are “having a negative impact on the stress levels of both the medical providers and the patients, from having these constant annoying alarms going off in the background.”

Audio warnings that go off simultaneously in a cockpit can have a similar effect on pilots. The paper Auditory Warning Sounds in the Work Environment offers this worrying tale from a pilot:

I was flying in a Jetstream at night when my peaceful reverie was shattered by the stall audio warning, the stick shaker, and several warning lights. The effect was exactly what was NOT intended; I was frightened numb for several seconds and [took hands off] instruments trying to work out how to cancel the audio/visual assault rather than taking what should be instinctive actions.

The current way to prevent such a confusing cacophony in the workplace is via “alarm management,” a process that looks at the full line-up of possible alert sounds within one environment and how they combine. The phrase “alarm philosophy” is also bandied about in these circles—it involves prioritizing alarms to ensure that the sounds that signal a potentially life-threatening moment are the most attention-getting.

They may be annoying, says Baldwin, “but that doesn’t matter in a time-critical life-or-death situation. We don’t really care if it annoys you a little bit as long as it gets your attention—and gets you to hopefully avoid that plane crashing.”