Scientists have spent a lot of time over the last 100 years trying to determine what exactly it is about the sound of fingernails on a chalkboard that makes it so difficult to bear. A new study blames psychology and the design of our ear canals.
Past research on the subject has suggested that the sound is acoustically similar to the warning call of a primate, but that theory was discredited after it was found that monkeys responded to amplitude-matched white noise and other high-pitched noises, and humans did not. Another study conducted in 1986, manipulated a recording of the sound of blackboard scraping and found that the medium-pitched frequencies are the noises that bother us so much, as opposed to the higher-frequency pitches (as scientists thought before). That work won author Randolph Blake an Ig Nobel Prize in 2006.
The Study
The latest study, organized by musicologists Michael Oehler from Macromedia University for Media and Communication in Cologne, Germany, and Christoph Reuter from the University of Vienna, examined other sounds that generate similar reactions from humans – the sound of chalk on slate, the squeak of styrofoam, a plate being scraped by a fork, and the sounds of fingernails on the chalkboard.
Some participants in the study were actually told the source of the sound, while others were told that the odd noises were part of a contemporary music composition. Researchers then asked the participants to rank which noises were the worst, and also monitored the patients for physical signs of distress – heart rate, blood pressure and the electrical conductivity of skin. They found that disturbing sounds do cause a very measurable physical reaction, with skin conductivity changing drastically, and that the frequencies involved with the annoying noises also lay in the range of human speech – between 2,000 and 4,000 Hz. Removing those frequencies from the sound made them much more pleasant on the ears. But, removing the scraping part of the sound made very little difference at all.
The Pain is Genuine
A very strong psychological indicator was identified. If the listeners were aware that the sound was fingernails on the chalkboard, they rated it as more unpleasant than if they thought it came from a musical composition. Even when they did think it was from music however, their skin conductivity still changed by the same amount, indicating that the physical part of the response stayed the same.
That physical change is probably because of the shape of the human ear canal, which past studies have shown can amplify frequencies in the range of 2,000 to 4,000 Hz. The scientists believe that when the sound of nails on a chalkboard is generated, the sound is amplified within our ears to a painful degree.
The next step for the team will be to take a deeper look at the parameters of unpleasant noises, with the eventual goal of trying to cover up those sounds within factory equipment, vacuum cleaners or construction equipment.