"We're mammals, but we're specifically primates," Buxton said. We humans, it appears, are biologically hard-wired to respond to noises that come out of nowhere because they can be very bad news. Meanwhile, the sounds of a helicopter and traffic, when reaching the level of a shout at 70 decibels, still did not wake participants as frequently as alarms, ringing phones and even relatively quiet human conversations, which again can feature that jarring, no-noise-to-peak-noise delivery. Even at low volumes of around 40 decibels - a whisper, essentially - alarms from hospital equipment aroused study participants from shallow sleep 90 percent of the time, and half the time from deep sleep. This key acoustic distinction between abrupt threat and gradual non-threat was borne out in a 2012 study by Buxton in a hospital setting. "With a scream or a shout, it's 'no noise' and then it goes directly to high pitch," Buxton said. That's in stark contrast to a scream or a ringing phone suddenly piercing a silence, reaching peak loudness almost instantly. "The type of noise defines if you will wake up or not, controlling for the volume, because the noise information is processed by our brain differently," Buxton said.įor instance, although the sounds of crashing waves can vary considerably in volume, with quiet intervals followed by crescendos, the waves' hubbub smoothly rises and falls in intensity.
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