The ear-splitting screech of a knife on a glass bottle has been identified as the worst sound to the human ear by scientists who studied the brain's response to unpleasant noises.
People who listened to a series of 74 recordings while having their brain activity measured by an MRI scanner rated the sound of a fork on a glass as the second worst noise, followed by chalk on a blackboard.
The scans revealed that unpleasant sounds provoked a stronger response in the brain than pleasant ones such as the noise of blubbing water.
While sounds are processed in the brain's auditory cortex, uncomfortable noises activate the amygdala, a separate brain region which processes emotions.
When we hear sounds like nails on a blackboard – another of the worst-rated recordings – the amygdala takes charge of activity in the auditory cortex, making us more sensitive to the noise.
The researchers studied a group of 13 volunteers and found that sounds with a frequency of between 2,000 and 5,000 Hz, the range at which our ears are the most sensitive, were the hardest to bear.
Although it remains unclear why our ears are most sensitive to this type of sound, researchers noted that screams, which we naturally find uncomfortable, fall within the same range.
Dr Sukhbinder Kumar of Newcastle University, author of the study, which was published in the Journal of Neuroscience, said: "It appears there is something very primitive kicking in. It’s a possible distress signal from the amygdala to the auditory cortex."
His colleague Prof Tim Griffiths added: "This might be a new inroad into emotional disorders and disorders like tinnitus and migraine, in which there seems to be heightened perception of the unpleasant aspects of sounds.”
