Franz de Waal's Inner Ape

Scientists used to consider the frequency band of 500 hertz and below in the human voice as meaningless noise, because when a voice is filtered, removing all higher frequencies, ne hears nothing but a low-pitched hum. All words are lost. But then it was found that this low hum is an unconscious social instrument. It is different for each person, but in the course of a conversation people tend to converge. They settle on a single hum, and it is always the lower status person who does the adjusting