Sounds forming the voice of animal (including humans), originate from vibrations of voice ligaments in larynx, when air comes through them. A larynx is a part of a respiratory tract, through which an air passes in lungs and moves away. Shortening of ligaments, due to increase in tension causes a higher pitch of sound. And usual cat purring goes this way.
However purring of different representatives of the cat family, including our domestic cats consists of quick alternate impulses of sound, arising in larynx. Every individual sound is caused by quick alteration of air pressure after right and left voice ligaments are separated. Larynx muscles are stimulated frequently by nerve impulses coming from twenty to thirty times a second. Every moment this happens, voice ligaments close up, and, as a result, an air pressure rises. When stimulation of muscles stops, voice ligaments drift apart and make the released air to produce a sound.
During inhalation and exhalation (though there's a small interval between) the events are ordered as stated above, and if one lends his ear, he may able to make them out. Furthermore, during inhalation, in order to prevent the dispersion of air (that is, chaotic loss of air pressure) the diaphragm is constricted not permanently (that is usual) but in a series of consistent impulses, alternating with constrictions of throat muscles.
The intensity of purring - its' loudness - depends on the power of stimulation of animal, for example, on stroke, or talking with it. Often purring even in presence of a man is not loud, delicate. But if one to put his finger on a cat throat he'll easily feel larynx vibrations.
The alternative theory of mechanism of purring has been put forward, but it seems quite unreal. This theory assumes that purring is caused by the increase of speed and turbulence of blood stream in vein, that sends back blood from the back of body. Turbulence produces vibration inside blood vessel in the point where it crosses chest, and then this vibration pass inside head along respiratory tract. Yet this hypothesis needs extra research.
Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).