Have you ever wondered why some sounds are so pleasant and others annoying (think nails on a chalkboard)? Recent research examined that question by modeling the neural signaling of the auditory pathway (http://physicsworld.com/cws/article/news/47171). The findings were very interesting and have physics principles at their core.
The spacing of action potential signals was the dependent factor on the harmony of a certain sound frequency. The authors explained that pitch was the ability to differentiate between sounds of two different frequencies and that the intervals of firing determine if a chord is harmonious or disharmonious. Sensory neurons in the periphery form synapses on what is called an interneuron. The way that these signals add up in space and time determine if a sound has a regular pattern of firing or an irregular pattern of firing. Constructive and destructive interference determine how the sound waves sum up together. Constructive interference produces a larger amplitude than either wave component alone and destructive interference produces a lower amplitude. Sound waves are longitudinal waves and, therefore, the way they sum up is by adding together the pressure differentials they form in the air. At the end of the article, the authors mention entropy, a measure of disorder. Low entropy patterns of signal firing were associated with harmonious sounds while the opposite was true for disharmonious sounds. This makes sense because there are many more ways that the waves can be added together to form disharmony than harmony. Overall, this article presents an interesting finding as to what types of sounds (literally) sync in the human mind.