A knife scraping against a glass bottle is the most unpleasant sound for most human beings, researchers from the Newcastle University and Wellcome Trust Centre for Neuroimaging at UCL, both in England, reported in the Journal of Neuroscience.

When activity between the auditory and emotional parts of the brain increases, it may cause a sensation of revulsion or disgust at a sound, as typically occurs to most of us when we hear somebody scrape chalk or their nails on a blackboard.

When we hear unpleasant sounds, the auditory cortex and the amygdala interact more intensely and process the negative emotions. The amygdala is a small almond shaped part of the brain that processes our emotions and aggression. It also controls fear responses and forms emotional memories.

The scientists used brain imaging to see what goes on in the brain when we are exposed to unpleasant sounds. The imaging showed that when we hear something we don’t like, the amygdala becomes much more active. They believe it processes the data from the auditory nerve in such a way as to provoke a negative reaction.

Study author, Dr Sukhbinder Kumar, said, “it appears there is something very primitive kicking in. It’s a possible distress signal from the amygdala to the auditory cortex.”

Study leader, Professor Tim Griffiths, of Newcastle University, and colleagues used fMRI (functional magnetic resonance imaging) to find out how the brains of 13 participants responded to a variety of different sounds.

The volunteers listened to the sounds while inside the scanner and rated them from most unpleasant to the most pleasing (least unpleasant). The sound of a knife scraping against a bottle was the most hated sound, while bubbling water was rated the most pleasing. The scientists studied the brain responses to each type of noise.

There appeared to be a correlation between the type of sound the participants heard and the levels of activity in the amygdala and the auditory cortex – the activity varied according to the ratings of the sounds. The higher the activity, the greater the revulsion.

The amygdala, which is the emotional part of the brain, appears to take charge and modulate the activity of the auditory part of the brain, making our perception of a very disagreeable noise feel even worse, compared to nice sounds, such as bubbling water or applause.

After analyzing all the sounds the participants liked and hated, and placing them in order of unpleasantness, the researchers found that disagreeable sounds tend to be between the frequency range of about 2,000 to 5,000 Hz.

Dr. Kumar said:

“This is the frequency range where our ears are most sensitive. Although there’s still much debate as to why our ears are most sensitive in this range, it does include sounds of screams which we find intrinsically unpleasant.”

If we can better understand what is going on in the brain when we are exposed to sounds, we might have a greater insight into what makes some have lower sound tolerance, as is the case with many people with autism, hyperacusis (a reduction of normal tolerance for everyday sounds), and misophonia (a hatred of sound).

Scientists have known for a long time that long-term exposure to certain sounds can affect our mental and physical health. Arab architects have for centuries designed buildings with fountains and the sounds of bubbling water. An article published in the European Heart Journal revealed that long-term exposure to the sound of traffic increases our risk of stroke.

Professor Griffiths said:

“This work sheds new light on the interaction of the amygdala and the auditory cortex. 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.”

Would you like to test your own reactions to these sounds? Here are the five most unpleasant sounds from the study…

1. The worst sound – a knife scraping against a bottle.

2. The second worst sound – a fork on a glass.

3. The third worst – chalk on a blackboard.

4. A ruler on a bottle.

5. Nails on a blackboard.

The next five worst were:

  • 6. Female scream.
  • 7. Anglegrinder.
  • 8. Brakes on a cycle squealing.
  • 9. Baby crying.
  • 10. Electric drill.

The most pleasing sounds (least unpleasant) were:

  • 1. Applause.
  • 2. Baby laughing.
  • 3. Thunder.
  • 4. Water flowing.

To ease your nerves after listening to the unpleasant sounds, and to put a smile back on your face, our final sound clip is of a baby laughing (number two in the list of most pleasing sounds):

Written by Christian Nordqvist