Carryl Baldwin, a professor of cognition and applied auditory research, designs and tests sounds for "use as alarms in household, aviation, medical, and automotive settings." Atlas Obscura explores the art and science of making sounds that convey a spectrum of urgency:
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.
"An Alarm Designer on How to Annoy People in the Most Effective Ways" (Atlas Obscura)