Purr
Purring is a low-frequency (25 to 150 Hz, domestic cats ~ 25 Hz), uniformly vibrating noise that cats produce in certain situations. In general, it signaled well-being, but also brought forth in stressful situations.
Sound production
About the emergence of purring there are several hypotheses; finally resolved, however, the mechanisms are not yet.
Occurrence
Cats purr after birth, as soon as they are weaned from their mother for the first time. The mother purrs during the lactation period and so calms the boy and himself The purr indicates to the mother that the kittens are well supplied with milk and thus satisfied.
However, cats purr even in stressful situations, such as pain. It is believed that this is its own settling. A basic matching all situations in which cats purr is that they are voted peacefully.