Answer: The sounds which constitute the ‘voice’ of animal (including man) are produced by vibration of the vocal cords within the larynx as air passes over them. The larynx, or voice box, in the throat is part of the respiratory tract along which air passes to and from the lungs. The shorter the vocal cords, because the greater the tension in them, the higher pitch of the sound. The normal cat’s miaow is produced in this way.
However, the purring noise of my members of the cat family, including our domestic cat’s actually consists of rapidly recurring bursts of sound, through again originating in the larynx. Each separate sound is due to a sudden release of air pressure following separation of the right and left vocal sounds. The laryngeal muscles are briefly stimulated by never impulses between twenty and thirty times per seconds. Each time this happens, the vocal cords come together so that the glottis (the opening between the cords) is closed, and as a result air pressure beings to builds up. When the muscle stimulation ceases the vocal cords suddenly part and the air pressure is releases producing a sound.
These sequence of events occurs during both breathing in(inspiration) and berating out (expiration), through there is a brief pause, detectable by a listener, between each phase. In addition, during inspiration, to prevent the air pressure being dissipated, the diaphragm does not contract continuously (which is usual) but is succession of short bursts which alternate with the contractions of the laryngeal muscles.
The intensity of the purring sound depends upon the degree to which the cat is being stimulated, for example by stroking or being spoken to. Often purring occurs at a low level in the presence of human and is barely audible, though finger placed over throat can detect vibrations in the larynx.
An alternative theory for the mechanism of purring has been put forward, but appears unlikely. This suggests that purring is due to an increase in the velocity and turbulence of blood flow in the posterior vena cava, which is major vein retuning blood from the hind part of the body. The turbulence creates a vibration in the wall of this blood vessel as it passes through the chest, and this vibration is then transmitted through the air passages to the head. However, whereas there is scientific evidence for the explanation given either, this one appears to be purely hypothetical.
