Which sounds drive our ears absolutely bonkers?
A British study rated the screechy scrape of a sharp knife along the surface of a ridged metal bottle as the most unpleasant sound. It earned the top spot in a recent ranking of 75 different sounds.
A steel fork scraping along a glass finished a close second among the nastiest noises heard by 50 college students during the research. And the annoying squeal of chalk on a blackboard was the third most awful sound in this small experiment , in which listeners rated each sound on a nine-point scale from 0 for least unpleasant to 9 for most unpleasant.
The study volunteers all had good hearing and used headphones to listen to a variety of natural and man-made sound recordings, which ranged from a cat screaming and a clarinet squeaking to a fire alarm blaring and an engine revving.
Interestingly, the always cringe-inducing sound of fingernails on a blackboard came in fifth on the rankings -- behind a ruler on a bottle but ahead of a woman's scream.
"It was a bit surprising that a female scream and a baby's cry were rated lower than the scratchy sounds, such as a knife on a bottle," says study author Sukhbinder Kumar, Ph.D., a neuroscientist at Newcastle University Medical School in the U.K.
He says the reason the scratchy sounds may have scored higher may have to do with the acoustic structure of sounds that result from scraping actions. Scratchy noises, like knife on glass, may be the most unpleasant because more of the sound's energy is concentrated in the frequency band of 2000 to 5000 Hz, where our ears are most sensitive. (Screams also have energy in the same frequency band, but its concentration is less.)
In a new study published in The Journal of Neuroscience, Kumar and his colleagues built upon this earlier research to understand the brain mechanism behind why people perceive certain sounds to be so unpleasant.
They asked 13 healthy adults to listen to the same sounds as they previously tested while in a functional MRI machine. Volunteers rated the sounds on a scale from 1 (least unpleasant) to 5 (most unpleasant).
This time, study participants rated chalk on a blackboard at the worst sound while they found bubbling water to be the most soothing. Kumar suspects that the noisy and confining quarters of the MRI machine and the smaller number of participants may have contributed to the slight differences in results between this research and the earlier study.
The brain scans allowed the scientists to see the interaction between the sensory (auditory) part of the brain, involved in hearing, and the emotional part of the brain (the amygdala) in response to the various noises.
What they found was that the emotional part of the brain modulates, or changes, the activity of the sensory part of the brain based on the emotional value of the sound. Kumar says that if a sound is "highly unpleasant," which may have the potential to cause harm, the emotional part of the brain heightens up the response of the sensory cortex, which provokes a negative reaction to its perceived unpleasantness.
Kumar suggests that these new findings may help shed light on the brain's reaction to disorders such as migraine and tinnitus (ringing in the ears), in which people are super sensitive to the unpleasant aspects of sound.
Want to test your reaction to some of the sounds used in both studies? Listen to some of the sounds here:
The 10 worst sounds based on both studies were:
6. Female scream
7. Angle grinder tool used on a hollow metal pipe
8. Bicycle breaks squealing
9. Baby crying
10. Electric drill running at a range of speeds
The 4 least unpleasant sounds were:
4. Water flowing